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DBIx::Class::Manual::CUseroContributed Perl DoDBIx::Class::Manual::Cookbook(3)

NAME
       DBIx::Class::Manual::Cookbook - Miscellaneous recipes

SEARCHING
   Paged results
       When you expect a large number of results, you can ask DBIx::Class for
       a paged resultset, which will fetch only a defined number of records at
       a time:

	 my $rs = $schema->resultset('Artist')->search(
	   undef,
	   {
	     page => 1,	 # page to return (defaults to 1)
	     rows => 10, # number of results per page
	   },
	 );

	 return $rs->all(); # all records for page 1

	 return $rs->page(2); # records for page 2

       You can get a Data::Page object for the resultset (suitable for use in
       e.g. a template) using the "pager" method:

	 return $rs->pager();

   Complex WHERE clauses
       Sometimes you need to formulate a query using specific operators:

	 my @albums = $schema->resultset('Album')->search({
	   artist => { 'like', '%Lamb%' },
	   title  => { 'like', '%Fear of Fours%' },
	 });

       This results in something like the following "WHERE" clause:

	 WHERE artist LIKE ? AND title LIKE ?

       And the following bind values for the placeholders: '%Lamb%', '%Fear of
       Fours%'.

       Other queries might require slightly more complex logic:

	 my @albums = $schema->resultset('Album')->search({
	   -or => [
	     -and => [
	       artist => { 'like', '%Smashing Pumpkins%' },
	       title  => 'Siamese Dream',
	     ],
	     artist => 'Starchildren',
	   ],
	 });

       This results in the following "WHERE" clause:

	 WHERE ( artist LIKE '%Smashing Pumpkins%' AND title = 'Siamese Dream' )
	   OR artist = 'Starchildren'

       For more information on generating complex queries, see "WHERE CLAUSES"
       in SQL::Abstract.

   Retrieve one and only one row from a resultset
       Sometimes you need only the first "top" row of a resultset. While this
       can be easily done with $rs->first, it is suboptimal, as a full blown
       cursor for the resultset will be created and then immediately destroyed
       after fetching the first row object.  $rs->single is designed
       specifically for this case - it will grab the first returned result
       without even instantiating a cursor.

       Before replacing all your calls to "first()" with "single()" please
       observe the following CAVEATS:

       ·   While single() takes a search condition just like search() does, it
	   does _not_ accept search attributes. However one can always chain a
	   single() to a search():

	     my $top_cd = $cd_rs->search({}, { order_by => 'rating' })->single;

       ·   Since single() is the engine behind find(), it is designed to fetch
	   a single row per database query. Thus a warning will be issued when
	   the underlying SELECT returns more than one row. Sometimes however
	   this usage is valid: i.e. we have an arbitrary number of cd's but
	   only one of them is at the top of the charts at any given time. If
	   you know what you are doing, you can silence the warning by
	   explicitly limiting the resultset size:

	     my $top_cd = $cd_rs->search ({}, { order_by => 'rating', rows => 1 })->single;

   Arbitrary SQL through a custom ResultSource
       Sometimes you have to run arbitrary SQL because your query is too
       complex (e.g. it contains Unions, Sub-Selects, Stored Procedures, etc.)
       or has to be optimized for your database in a special way, but you
       still want to get the results as a DBIx::Class::ResultSet.

       This is accomplished by defining a ResultSource::View for your query,
       almost like you would define a regular ResultSource.

	 package My::Schema::Result::UserFriendsComplex;
	 use strict;
	 use warnings;
	 use base qw/DBIx::Class::Core/;

	 __PACKAGE__->table_class('DBIx::Class::ResultSource::View');

	 # ->table, ->add_columns, etc.

	 # do not attempt to deploy() this view
	 __PACKAGE__->result_source_instance->is_virtual(1);

	 __PACKAGE__->result_source_instance->view_definition(q[
	   SELECT u.* FROM user u
	   INNER JOIN user_friends f ON u.id = f.user_id
	   WHERE f.friend_user_id = ?
	   UNION
	   SELECT u.* FROM user u
	   INNER JOIN user_friends f ON u.id = f.friend_user_id
	   WHERE f.user_id = ?
	 ]);

       Next, you can execute your complex query using bind parameters like
       this:

	 my $friends = $schema->resultset( 'UserFriendsComplex' )->search( {},
	   {
	     bind  => [ 12345, 12345 ]
	   }
	 );

       ... and you'll get back a perfect DBIx::Class::ResultSet (except, of
       course, that you cannot modify the rows it contains, e.g. cannot call
       "update", "delete", ...	on it).

       Note that you cannot have bind parameters unless is_virtual is set to
       true.

       ·   NOTE

	   If you're using the old deprecated "$rsrc_instance->name(\'( SELECT
	   ...')" method for custom SQL execution, you are highly encouraged
	   to update your code to use a virtual view as above. If you do not
	   want to change your code, and just want to suppress the deprecation
	   warning when you call "deploy" in DBIx::Class::Schema, add this
	   line to your source definition, so that "deploy" will exclude this
	   "table":

	     sub sqlt_deploy_hook { $_[1]->schema->drop_table ($_[1]) }

   Using specific columns
       When you only want specific columns from a table, you can use "columns"
       to specify which ones you need. This is useful to avoid loading columns
       with large amounts of data that you aren't about to use anyway:

	 my $rs = $schema->resultset('Artist')->search(
	   undef,
	   {
	     columns => [qw/ name /]
	   }
	 );

	 # Equivalent SQL:
	 # SELECT artist.name FROM artist

       This is a shortcut for "select" and "as", see below. "columns" cannot
       be used together with "select" and "as".

   Using database functions or stored procedures
       The combination of "select" and "as" can be used to return the result
       of a database function or stored procedure as a column value. You use
       "select" to specify the source for your column value (e.g. a column
       name, function, or stored procedure name). You then use "as" to set the
       column name you will use to access the returned value:

	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     select => [ 'name', { LENGTH => 'name' } ],
	     as	    => [qw/ name name_length /],
	   }
	 );

	 # Equivalent SQL:
	 # SELECT name name, LENGTH( name )
	 # FROM artist

       Note that the "as" attribute has absolutely nothing to do with the SQL
       syntax " SELECT foo AS bar " (see the documentation in "ATTRIBUTES" in
       DBIx::Class::ResultSet). You can control the "AS" part of the generated
       SQL via the "-as" field attribute as follows:

	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     join => 'cds',
	     distinct => 1,
	     '+select' => [ { count => 'cds.cdid', -as => 'amount_of_cds' } ],
	     '+as' => [qw/num_cds/],
	     order_by => { -desc => 'amount_of_cds' },
	   }
	 );

	 # Equivalent SQL
	 # SELECT me.artistid, me.name, me.rank, me.charfield, COUNT( cds.cdid ) AS amount_of_cds
	 #   FROM artist me LEFT JOIN cd cds ON cds.artist = me.artistid
	 # GROUP BY me.artistid, me.name, me.rank, me.charfield
	 # ORDER BY amount_of_cds DESC

       If your alias exists as a column in your base class (i.e. it was added
       with add_columns), you just access it as normal. Our "Artist" class has
       a "name" column, so we just use the "name" accessor:

	 my $artist = $rs->first();
	 my $name = $artist->name();

       If on the other hand the alias does not correspond to an existing
       column, you have to fetch the value using the "get_column" accessor:

	 my $name_length = $artist->get_column('name_length');

       If you don't like using "get_column", you can always create an accessor
       for any of your aliases using either of these:

	 # Define accessor manually:
	 sub name_length { shift->get_column('name_length'); }

	 # Or use DBIx::Class::AccessorGroup:
	 __PACKAGE__->mk_group_accessors('column' => 'name_length');

       See also "Using SQL functions on the left hand side of a comparison".

   SELECT DISTINCT with multiple columns
	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     columns => [ qw/artist_id name rank/ ],
	     distinct => 1
	   }
	 );

	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     columns => [ qw/artist_id name rank/ ],
	     group_by => [ qw/artist_id name rank/ ],
	   }
	 );

	 # Equivalent SQL:
	 # SELECT me.artist_id, me.name, me.rank
	 # FROM artist me
	 # GROUP BY artist_id, name, rank

   SELECT COUNT(DISTINCT colname)
	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     columns => [ qw/name/ ],
	     distinct => 1
	   }
	 );

	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     columns => [ qw/name/ ],
	     group_by => [ qw/name/ ],
	   }
	 );

	 my $count = $rs->count;

	 # Equivalent SQL:
	 # SELECT COUNT( * ) FROM (SELECT me.name FROM artist me GROUP BY me.name) me:

   Grouping results
       DBIx::Class supports "GROUP BY" as follows:

	 my $rs = $schema->resultset('Artist')->search(
	   {},
	   {
	     join     => [qw/ cds /],
	     select   => [ 'name', { count => 'cds.id' } ],
	     as	      => [qw/ name cd_count /],
	     group_by => [qw/ name /]
	   }
	 );

	 # Equivalent SQL:
	 # SELECT name, COUNT( cd.id ) FROM artist
	 # LEFT JOIN cd ON artist.id = cd.artist
	 # GROUP BY name

       Please see "ATTRIBUTES" in DBIx::Class::ResultSet documentation if you
       are in any way unsure about the use of the attributes above (" join ",
       " select ", " as " and " group_by ").

   Subqueries
       You can write subqueries relatively easily in DBIC.

	 my $inside_rs = $schema->resultset('Artist')->search({
	   name => [ 'Billy Joel', 'Brittany Spears' ],
	 });

	 my $rs = $schema->resultset('CD')->search({
	   artist_id => { -in => $inside_rs->get_column('id')->as_query },
	 });

       The usual operators ( '=', '!=', -in, -not_in, etc.) are supported.

       NOTE: You have to explicitly use '=' when doing an equality comparison.
       The following will not work:

	 my $rs = $schema->resultset('CD')->search({
	   artist_id => $inside_rs->get_column('id')->as_query,	 # does NOT work
	 });

       Support

       Subqueries are supported in the where clause (first hashref), and in
       the from, select, and +select attributes.

       Correlated subqueries

	 my $cdrs = $schema->resultset('CD');
	 my $rs = $cdrs->search({
	   year => {
	     '=' => $cdrs->search(
	       { artist_id => { '=' => { -ident => 'me.artist_id' } } },
	       { alias => 'inner' }
	     )->get_column('year')->max_rs->as_query,
	   },
	 });

       That creates the following SQL:

	 SELECT me.cdid, me.artist, me.title, me.year, me.genreid, me.single_track
	   FROM cd me
	  WHERE year = (
	     SELECT MAX(inner.year)
	       FROM cd inner
	      WHERE artist_id = me.artist_id
	     )

   Predefined searches
       You can define frequently used searches as methods by subclassing
       DBIx::Class::ResultSet:

	 package My::DBIC::ResultSet::CD;
	 use strict;
	 use warnings;
	 use base 'DBIx::Class::ResultSet';

	 sub search_cds_ordered {
	     my ($self) = @_;

	     return $self->search(
		 {},
		 { order_by => 'name DESC' },
	     );
	 }

	 1;

       If you're using "load_namespaces" in DBIx::Class::Schema, simply place
       the file into the "ResultSet" directory next to your "Result"
       directory, and it will be automatically loaded.

       If however you are still using "load_classes" in DBIx::Class::Schema,
       first tell DBIx::Class to create an instance of the ResultSet class for
       you, in your My::DBIC::Schema::CD class:

	 # class definition as normal
	 use base 'DBIx::Class::Core';
	 __PACKAGE__->table('cd');

	 # tell DBIC to use the custom ResultSet class
	 __PACKAGE__->resultset_class('My::DBIC::ResultSet::CD');

       Note that "resultset_class" must be called after "load_components" and
       "table", or you will get errors about missing methods.

       Then call your new method in your code:

	  my $ordered_cds = $schema->resultset('CD')->search_cds_ordered();

   Using SQL functions on the left hand side of a comparison
       Using SQL functions on the left hand side of a comparison is generally
       not a good idea since it requires a scan of the entire table. (Unless
       your RDBMS supports indexes on expressions - including return values of
       functions - and you create an index on the return value of the function
       in question.) However, it can be accomplished with "DBIx::Class" when
       necessary by resorting to literal SQL:

	 $rs->search(\[ 'YEAR(date_of_birth) = ?', [ plain_value => 1979 ] ]);

	 # Equivalent SQL:
	 # SELECT * FROM employee WHERE YEAR(date_of_birth) = ?

	 $rs->search({ -and => [
	   name => 'Bob',
	   \[ 'YEAR(date_of_birth) = ?', [ plain_value => 1979 ] ],
	 ]});

	 # Equivalent SQL:
	 # SELECT * FROM employee WHERE name = ? AND YEAR(date_of_birth) = ?

       Note: the "plain_value" string in the "[ plain_value => 1979 ]" part
       should be either the same as the name of the column (do this if the
       type of the return value of the function is the same as the type of the
       column) or in the case of a function it's currently treated as a dummy
       string (it is a good idea to use "plain_value" or something similar to
       convey intent). The value is currently only significant when handling
       special column types (BLOBs, arrays, etc.), but this may change in the
       future.

       See also "Literal SQL with placeholders and bind values (subqueries)"
       in SQL::Abstract.

   Software Limits
       When your RDBMS does not have a working SQL limit mechanism (e.g.
       Sybase ASE) and GenericSubQ is either too slow or does not work at all,
       you can try the software_limit DBIx::Class::ResultSet attribute, which
       skips over records to simulate limits in the Perl layer.

       For example:

	 my $paged_rs = $rs->search({}, {
	   rows => 25,
	   page => 3,
	   order_by => [ 'me.last_name' ],
	   software_limit => 1,
	 });

       You can set it as a default for your schema by placing the following in
       your "Schema.pm":

	 __PACKAGE__->default_resultset_attributes({ software_limit => 1 });

       WARNING: If you are dealing with large resultsets and your DBI or
       ODBC/ADO driver does not have proper cursor support (i.e. it loads the
       whole resultset into memory) then this feature will be extremely slow
       and use huge amounts of memory at best, and may cause your process to
       run out of memory and cause instability on your server at worst,
       beware!

JOINS AND PREFETCHING
   Using joins and prefetch
       You can use the "join" attribute to allow searching on, or sorting your
       results by, one or more columns in a related table.

       This requires that you have defined the DBIx::Class::Relationship. For
       example :

	 My::Schema::CD->has_many( artists => 'My::Schema::Artist', 'artist_id');

       To return all CDs matching a particular artist name, you specify the
       name of the relationship ('artists'):

	 my $rs = $schema->resultset('CD')->search(
	   {
	     'artists.name' => 'Bob Marley'
	   },
	   {
	     join => 'artists', # join the artist table
	   }
	 );

	 # Equivalent SQL:
	 # SELECT cd.* FROM cd
	 # JOIN artist ON cd.artist = artist.id
	 # WHERE artist.name = 'Bob Marley'

       In that example both the join, and the condition use the relationship
       name rather than the table name (see DBIx::Class::Manual::Joining for
       more details on aliasing ).

       If required, you can now sort on any column in the related tables by
       including it in your "order_by" attribute, (again using the aliased
       relation name rather than table name) :

	 my $rs = $schema->resultset('CD')->search(
	   {
	     'artists.name' => 'Bob Marley'
	   },
	   {
	     join     => 'artists',
	     order_by => [qw/ artists.name /]
	   }
	 );

	 # Equivalent SQL:
	 # SELECT cd.* FROM cd
	 # JOIN artist ON cd.artist = artist.id
	 # WHERE artist.name = 'Bob Marley'
	 # ORDER BY artist.name

       Note that the "join" attribute should only be used when you need to
       search or sort using columns in a related table. Joining related tables
       when you only need columns from the main table will make performance
       worse!

       Now let's say you want to display a list of CDs, each with the name of
       the artist. The following will work fine:

	 while (my $cd = $rs->next) {
	   print "CD: " . $cd->title . ", Artist: " . $cd->artist->name;
	 }

       There is a problem however. We have searched both the "cd" and "artist"
       tables in our main query, but we have only returned data from the "cd"
       table. To get the artist name for any of the CD objects returned,
       DBIx::Class will go back to the database:

	 SELECT artist.* FROM artist WHERE artist.id = ?

       A statement like the one above will run for each and every CD returned
       by our main query. Five CDs, five extra queries. A hundred CDs, one
       hundred extra queries!

       Thankfully, DBIx::Class has a "prefetch" attribute to solve this
       problem.	 This allows you to fetch results from related tables in
       advance:

	 my $rs = $schema->resultset('CD')->search(
	   {
	     'artists.name' => 'Bob Marley'
	   },
	   {
	     join     => 'artists',
	     order_by => [qw/ artists.name /],
	     prefetch => 'artists' # return artist data too!
	   }
	 );

	 # Equivalent SQL (note SELECT from both "cd" and "artist"):
	 # SELECT cd.*, artist.* FROM cd
	 # JOIN artist ON cd.artist = artist.id
	 # WHERE artist.name = 'Bob Marley'
	 # ORDER BY artist.name

       The code to print the CD list remains the same:

	 while (my $cd = $rs->next) {
	   print "CD: " . $cd->title . ", Artist: " . $cd->artist->name;
	 }

       DBIx::Class has now prefetched all matching data from the "artist"
       table, so no additional SQL statements are executed. You now have a
       much more efficient query.

       Also note that "prefetch" should only be used when you know you will
       definitely use data from a related table. Pre-fetching related tables
       when you only need columns from the main table will make performance
       worse!

   Multiple joins
       In the examples above, the "join" attribute was a scalar.  If you pass
       an array reference instead, you can join to multiple tables.  In this
       example, we want to limit the search further, using "LinerNotes":

	 # Relationships defined elsewhere:
	 # CD->belongs_to('artist' => 'Artist');
	 # CD->has_one('liner_notes' => 'LinerNotes', 'cd');
	 my $rs = $schema->resultset('CD')->search(
	   {
	     'artist.name' => 'Bob Marley'
	     'liner_notes.notes' => { 'like', '%some text%' },
	   },
	   {
	     join     => [qw/ artist liner_notes /],
	     order_by => [qw/ artist.name /],
	   }
	 );

	 # Equivalent SQL:
	 # SELECT cd.*, artist.*, liner_notes.* FROM cd
	 # JOIN artist ON cd.artist = artist.id
	 # JOIN liner_notes ON cd.id = liner_notes.cd
	 # WHERE artist.name = 'Bob Marley'
	 # ORDER BY artist.name

   Multi-step joins
       Sometimes you want to join more than one relationship deep. In this
       example, we want to find all "Artist" objects who have "CD"s whose
       "LinerNotes" contain a specific string:

	 # Relationships defined elsewhere:
	 # Artist->has_many('cds' => 'CD', 'artist');
	 # CD->has_one('liner_notes' => 'LinerNotes', 'cd');

	 my $rs = $schema->resultset('Artist')->search(
	   {
	     'liner_notes.notes' => { 'like', '%some text%' },
	   },
	   {
	     join => {
	       'cds' => 'liner_notes'
	     }
	   }
	 );

	 # Equivalent SQL:
	 # SELECT artist.* FROM artist
	 # LEFT JOIN cd ON artist.id = cd.artist
	 # LEFT JOIN liner_notes ON cd.id = liner_notes.cd
	 # WHERE liner_notes.notes LIKE '%some text%'

       Joins can be nested to an arbitrary level. So if we decide later that
       we want to reduce the number of Artists returned based on who wrote the
       liner notes:

	 # Relationship defined elsewhere:
	 # LinerNotes->belongs_to('author' => 'Person');

	 my $rs = $schema->resultset('Artist')->search(
	   {
	     'liner_notes.notes' => { 'like', '%some text%' },
	     'author.name' => 'A. Writer'
	   },
	   {
	     join => {
	       'cds' => {
		 'liner_notes' => 'author'
	       }
	     }
	   }
	 );

	 # Equivalent SQL:
	 # SELECT artist.* FROM artist
	 # LEFT JOIN cd ON artist.id = cd.artist
	 # LEFT JOIN liner_notes ON cd.id = liner_notes.cd
	 # LEFT JOIN author ON author.id = liner_notes.author
	 # WHERE liner_notes.notes LIKE '%some text%'
	 # AND author.name = 'A. Writer'

   Multi-step and multiple joins
       With various combinations of array and hash references, you can join
       tables in any combination you desire.  For example, to join Artist to
       CD and Concert, and join CD to LinerNotes:

	 # Relationships defined elsewhere:
	 # Artist->has_many('concerts' => 'Concert', 'artist');

	 my $rs = $schema->resultset('Artist')->search(
	   { },
	   {
	     join => [
	       {
		 cds => 'liner_notes'
	       },
	       'concerts'
	     ],
	   }
	 );

	 # Equivalent SQL:
	 # SELECT artist.* FROM artist
	 # LEFT JOIN cd ON artist.id = cd.artist
	 # LEFT JOIN liner_notes ON cd.id = liner_notes.cd
	 # LEFT JOIN concert ON artist.id = concert.artist

   Multi-step prefetch
       "prefetch" can be nested more than one relationship deep using the same
       syntax as a multi-step join:

	 my $rs = $schema->resultset('Tag')->search(
	   {},
	   {
	     prefetch => {
	       cd => 'artist'
	     }
	   }
	 );

	 # Equivalent SQL:
	 # SELECT tag.*, cd.*, artist.* FROM tag
	 # JOIN cd ON tag.cd = cd.id
	 # JOIN artist ON cd.artist = artist.id

       Now accessing our "cd" and "artist" relationships does not need
       additional SQL statements:

	 my $tag = $rs->first;
	 print $tag->cd->artist->name;

ROW-LEVEL OPERATIONS
   Retrieving a row object's Schema
       It is possible to get a Schema object from a row object like so:

	 my $schema = $cd->result_source->schema;
	 # use the schema as normal:
	 my $artist_rs = $schema->resultset('Artist');

       This can be useful when you don't want to pass around a Schema object
       to every method.

   Getting the value of the primary key for the last database insert
       AKA getting last_insert_id

       Thanks to the core component PK::Auto, this is straightforward:

	 my $foo = $rs->create(\%blah);
	 # do more stuff
	 my $id = $foo->id; # foo->my_primary_key_field will also work.

       If you are not using autoincrementing primary keys, this will probably
       not work, but then you already know the value of the last primary key
       anyway.

   Stringification
       Employ the standard stringification technique by using the overload
       module.

       To make an object stringify itself as a single column, use something
       like this (replace "name" with the column/method of your choice):

	 use overload '""' => sub { shift->name}, fallback => 1;

       For more complex stringification, you can use an anonymous subroutine:

	 use overload '""' => sub { $_[0]->name . ", " .
				    $_[0]->address }, fallback => 1;

       Stringification Example

       Suppose we have two tables: "Product" and "Category". The table
       specifications are:

	 Product(id, Description, category)
	 Category(id, Description)

       "category" is a foreign key into the Category table.

       If you have a Product object $obj and write something like

	 print $obj->category

       things will not work as expected.

       To obtain, for example, the category description, you should add this
       method to the class defining the Category table:

	 use overload "" => sub {
	     my $self = shift;

	     return $self->Description;
	 }, fallback => 1;

   Want to know if find_or_create found or created a row?
       Just use "find_or_new" instead, then check "in_storage":

	 my $obj = $rs->find_or_new({ blah => 'blarg' });
	 unless ($obj->in_storage) {
	   $obj->insert;
	   # do whatever else you wanted if it was a new row
	 }

   Static sub-classing DBIx::Class result classes
       AKA adding additional relationships/methods/etc. to a model for a
       specific usage of the (shared) model.

       Schema definition

	   package My::App::Schema;

	   use base 'DBIx::Class::Schema';

	   # load subclassed classes from My::App::Schema::Result/ResultSet
	   __PACKAGE__->load_namespaces;

	   # load classes from shared model
	   load_classes({
	       'My::Shared::Model::Result' => [qw/
		   Foo
		   Bar
	       /]});

	   1;

       Result-Subclass definition

	   package My::App::Schema::Result::Baz;

	   use strict;
	   use warnings;
	   use base 'My::Shared::Model::Result::Baz';

	   # WARNING: Make sure you call table() again in your subclass,
	   # otherwise DBIx::Class::ResultSourceProxy::Table will not be called
	   # and the class name is not correctly registered as a source
	   __PACKAGE__->table('baz');

	   sub additional_method {
	       return "I'm an additional method only needed by this app";
	   }

	   1;

   Dynamic Sub-classing DBIx::Class proxy classes
       AKA multi-class object inflation from one table

       DBIx::Class classes are proxy classes, therefore some different
       techniques need to be employed for more than basic subclassing.	In
       this example we have a single user table that carries a boolean bit for
       admin.  We would like like to give the admin users objects
       (DBIx::Class::Row) the same methods as a regular user but also special
       admin only methods.  It doesn't make sense to create two separate
       proxy-class files for this.  We would be copying all the user methods
       into the Admin class.  There is a cleaner way to accomplish this.

       Overriding the "inflate_result" method within the User proxy-class
       gives us the effect we want.  This method is called by
       DBIx::Class::ResultSet when inflating a result from storage.  So we
       grab the object being returned, inspect the values we are looking for,
       bless it if it's an admin object, and then return it.  See the example
       below:

       Schema Definition

	   package My::Schema;

	   use base qw/DBIx::Class::Schema/;

	   __PACKAGE__->load_namespaces;

	   1;

       Proxy-Class definitions

	   package My::Schema::Result::User;

	   use strict;
	   use warnings;
	   use base qw/DBIx::Class::Core/;

	   ### Define what our admin class is, for ensure_class_loaded()
	   my $admin_class = __PACKAGE__ . '::Admin';

	   __PACKAGE__->table('users');

	   __PACKAGE__->add_columns(qw/user_id	 email	  password
				       firstname lastname active
				       admin/);

	   __PACKAGE__->set_primary_key('user_id');

	   sub inflate_result {
	       my $self = shift;
	       my $ret = $self->next::method(@_);
	       if( $ret->admin ) {### If this is an admin, rebless for extra functions
		   $self->ensure_class_loaded( $admin_class );
		   bless $ret, $admin_class;
	       }
	       return $ret;
	   }

	   sub hello {
	       print "I am a regular user.\n";
	       return ;
	   }

	   1;

	   package My::Schema::Result::User::Admin;

	   use strict;
	   use warnings;
	   use base qw/My::Schema::Result::User/;

	   # This line is important
	   __PACKAGE__->table('users');

	   sub hello
	   {
	       print "I am an admin.\n";
	       return;
	   }

	   sub do_admin_stuff
	   {
	       print "I am doing admin stuff\n";
	       return ;
	   }

	   1;

       Test File test.pl

	   use warnings;
	   use strict;
	   use My::Schema;

	   my $user_data = { email    => 'someguy@place.com',
			     password => 'pass1',
			     admin    => 0 };

	   my $admin_data = { email    => 'someadmin@adminplace.com',
			      password => 'pass2',
			      admin    => 1 };

	   my $schema = My::Schema->connection('dbi:Pg:dbname=test');

	   $schema->resultset('User')->create( $user_data );
	   $schema->resultset('User')->create( $admin_data );

	   ### Now we search for them
	   my $user = $schema->resultset('User')->single( $user_data );
	   my $admin = $schema->resultset('User')->single( $admin_data );

	   print ref $user, "\n";
	   print ref $admin, "\n";

	   print $user->password , "\n"; # pass1
	   print $admin->password , "\n";# pass2; inherited from User
	   print $user->hello , "\n";# I am a regular user.
	   print $admin->hello, "\n";# I am an admin.

	   ### The statement below will NOT print
	   print "I can do admin stuff\n" if $user->can('do_admin_stuff');
	   ### The statement below will print
	   print "I can do admin stuff\n" if $admin->can('do_admin_stuff');

       Alternatively you can use DBIx::Class::DynamicSubclass that implements
       exactly the above functionality.

   Skip row object creation for faster results
       DBIx::Class is not built for speed, it's built for convenience and ease
       of use, but sometimes you just need to get the data, and skip the fancy
       objects.

       To do this simply use DBIx::Class::ResultClass::HashRefInflator.

	my $rs = $schema->resultset('CD');

	$rs->result_class('DBIx::Class::ResultClass::HashRefInflator');

	my $hash_ref = $rs->find(1);

       Wasn't that easy?

       Beware, changing the Result class using "result_class" in
       DBIx::Class::ResultSet will replace any existing class completely
       including any special components loaded using load_components, eg
       DBIx::Class::InflateColumn::DateTime.

   Get raw data for blindingly fast results
       If the HashRefInflator solution above is not fast enough for you, you
       can use a DBIx::Class to return values exactly as they come out of the
       database with none of the convenience methods wrapped round them.

       This is used like so:

	 my $cursor = $rs->cursor
	 while (my @vals = $cursor->next) {
	     # use $val[0..n] here
	 }

       You will need to map the array offsets to particular columns (you can
       use the "select" in DBIx::Class::ResultSet attribute of "search" in
       DBIx::Class::ResultSet to force ordering).

RESULTSET OPERATIONS
   Getting Schema from a ResultSet
       To get the DBIx::Class::Schema object from a ResultSet, do the
       following:

	$rs->result_source->schema

   Getting Columns Of Data
       AKA Aggregating Data

       If you want to find the sum of a particular column there are several
       ways, the obvious one is to use search:

	 my $rs = $schema->resultset('Items')->search(
	   {},
	   {
	      select => [ { sum => 'Cost' } ],
	      as     => [ 'total_cost' ], # remember this 'as' is for DBIx::Class::ResultSet not SQL
	   }
	 );
	 my $tc = $rs->first->get_column('total_cost');

       Or, you can use the DBIx::Class::ResultSetColumn, which gets returned
       when you ask the "ResultSet" for a column using "get_column":

	 my $cost = $schema->resultset('Items')->get_column('Cost');
	 my $tc = $cost->sum;

       With this you can also do:

	 my $minvalue = $cost->min;
	 my $maxvalue = $cost->max;

       Or just iterate through the values of this column only:

	 while ( my $c = $cost->next ) {
	   print $c;
	 }

	 foreach my $c ($cost->all) {
	   print $c;
	 }

       "ResultSetColumn" only has a limited number of built-in functions. If
       you need one that it doesn't have, then you can use the "func" method
       instead:

	 my $avg = $cost->func('AVERAGE');

       This will cause the following SQL statement to be run:

	 SELECT AVERAGE(Cost) FROM Items me

       Which will of course only work if your database supports this function.
       See DBIx::Class::ResultSetColumn for more documentation.

   Creating a result set from a set of rows
       Sometimes you have a (set of) row objects that you want to put into a
       resultset without the need to hit the DB again. You can do that by
       using the set_cache method:

	my @uploadable_groups;
	while (my $group = $groups->next) {
	  if ($group->can_upload($self)) {
	    push @uploadable_groups, $group;
	  }
	}
	my $new_rs = $self->result_source->resultset;
	$new_rs->set_cache(\@uploadable_groups);
	return $new_rs;

USING RELATIONSHIPS
   Create a new row in a related table
	 my $author = $book->create_related('author', { name => 'Fred'});

   Search in a related table
       Only searches for books named 'Titanic' by the author in $author.

	 my $books_rs = $author->search_related('books', { name => 'Titanic' });

   Delete data in a related table
       Deletes only the book named Titanic by the author in $author.

	 $author->delete_related('books', { name => 'Titanic' });

   Ordering a relationship result set
       If you always want a relation to be ordered, you can specify this when
       you create the relationship.

       To order "$book->pages" by descending page_number, create the relation
       as follows:

	 __PACKAGE__->has_many('pages' => 'Page', 'book', { order_by => { -desc => 'page_number'} } );

   Filtering a relationship result set
       If you want to get a filtered result set, you can just add add to $attr
       as follows:

	__PACKAGE__->has_many('pages' => 'Page', 'book', { where => { scrap => 0 } } );

   Many-to-many relationship bridges
       This is straightforward using ManyToMany:

	 package My::User;
	 use base 'DBIx::Class::Core';
	 __PACKAGE__->table('user');
	 __PACKAGE__->add_columns(qw/id name/);
	 __PACKAGE__->set_primary_key('id');
	 __PACKAGE__->has_many('user_address' => 'My::UserAddress', 'user');
	 __PACKAGE__->many_to_many('addresses' => 'user_address', 'address');

	 package My::UserAddress;
	 use base 'DBIx::Class::Core';
	 __PACKAGE__->table('user_address');
	 __PACKAGE__->add_columns(qw/user address/);
	 __PACKAGE__->set_primary_key(qw/user address/);
	 __PACKAGE__->belongs_to('user' => 'My::User');
	 __PACKAGE__->belongs_to('address' => 'My::Address');

	 package My::Address;
	 use base 'DBIx::Class::Core';
	 __PACKAGE__->table('address');
	 __PACKAGE__->add_columns(qw/id street town area_code country/);
	 __PACKAGE__->set_primary_key('id');
	 __PACKAGE__->has_many('user_address' => 'My::UserAddress', 'address');
	 __PACKAGE__->many_to_many('users' => 'user_address', 'user');

	 $rs = $user->addresses(); # get all addresses for a user
	 $rs = $address->users(); # get all users for an address

	 my $address = $user->add_to_addresses(	   # returns a My::Address instance,
						   # NOT a My::UserAddress instance!
	   {
	     country => 'United Kingdom',
	     area_code => 'XYZ',
	     town => 'London',
	     street => 'Sesame',
	   }
	 );

   Relationships across DB schemas
       Mapping relationships across DB schemas is easy as long as the schemas
       themselves are all accessible via the same DBI connection. In most
       cases, this means that they are on the same database host as each other
       and your connecting database user has the proper permissions to them.

       To accomplish this one only needs to specify the DB schema name in the
       table declaration, like so...

	 package MyDatabase::Main::Artist;
	 use base qw/DBIx::Class::Core/;

	 __PACKAGE__->table('database1.artist'); # will use "database1.artist" in FROM clause

	 __PACKAGE__->add_columns(qw/ artist_id name /);
	 __PACKAGE__->set_primary_key('artist_id');
	 __PACKAGE__->has_many('cds' => 'MyDatabase::Main::Cd');

	 1;

       Whatever string you specify there will be used to build the "FROM"
       clause in SQL queries.

       The big drawback to this is you now have DB schema names hardcoded in
       your class files. This becomes especially troublesome if you have
       multiple instances of your application to support a change lifecycle
       (e.g. DEV, TEST, PROD) and the DB schemas are named based on the
       environment (e.g. database1_dev).

       However, one can dynamically "map" to the proper DB schema by
       overriding the connection method in your Schema class and building a
       renaming facility, like so:

	 package MyDatabase::Schema;
	 use Moose;

	 extends 'DBIx::Class::Schema';

	 around connection => sub {
	   my ( $inner, $self, $dsn, $username, $pass, $attr ) = ( shift, @_ );

	   my $postfix = delete $attr->{schema_name_postfix};

	   $inner->(@_);

	   if ( $postfix ) {
	       $self->append_db_name($postfix);
	   }
	 };

	 sub append_db_name {
	   my ( $self, $postfix ) = @_;

	   my @sources_with_db
	       = grep
		   { $_->name =~ /^\w+\./mx }
		   map
		       { $self->source($_) }
		       $self->sources;

	   foreach my $source (@sources_with_db) {
	       my $name = $source->name;
	       $name =~ s{^(\w+)\.}{${1}${postfix}\.}mx;

	       $source->name($name);
	   }
	 }

	 1;

       By overridding the connection method and extracting a custom option
       from the provided \%attr hashref one can then simply iterate over all
       the Schema's ResultSources, renaming them as needed.

       To use this facility, simply add or modify the \%attr hashref that is
       passed to connection, as follows:

	 my $schema
	   = MyDatabase::Schema->connect(
	     $dsn,
	     $user,
	     $pass,
	     {
	       schema_name_postfix => '_dev'
	       # ... Other options as desired ...
	     })

       Obviously, one could accomplish even more advanced mapping via a hash
       map or a callback routine.

TRANSACTIONS
   Transactions with txn_do
       As of version 0.04001, there is improved transaction support in
       DBIx::Class::Storage and DBIx::Class::Schema.  Here is an example of
       the recommended way to use it:

	 my $genus = $schema->resultset('Genus')->find(12);

	 my $coderef2 = sub {
	   $genus->extinct(1);
	   $genus->update;
	 };

	 my $coderef1 = sub {
	   $genus->add_to_species({ name => 'troglodyte' });
	   $genus->wings(2);
	   $genus->update;
	   $schema->txn_do($coderef2); # Can have a nested transaction. Only the outer will actualy commit
	   return $genus->species;
	 };

	 use Try::Tiny;
	 my $rs;
	 try {
	   $rs = $schema->txn_do($coderef1);
	 } catch {
	   # Transaction failed
	   die "the sky is falling!"	       #
	     if ($_ =~ /Rollback failed/);     # Rollback failed

	   deal_with_failed_transaction();
	 };

       Note: by default "txn_do" will re-run the coderef one more time if an
       error occurs due to client disconnection (e.g. the server is bounced).
       You need to make sure that your coderef can be invoked multiple times
       without terrible side effects.

       Nested transactions will work as expected. That is, only the outermost
       transaction will actually issue a commit to the $dbh, and a rollback at
       any level of any transaction will cause the entire nested transaction
       to fail.

   Nested transactions and auto-savepoints
       If savepoints are supported by your RDBMS, it is possible to achieve
       true nested transactions with minimal effort. To enable auto-savepoints
       via nested transactions, supply the "auto_savepoint = 1" connection
       attribute.

       Here is an example of true nested transactions. In the example, we
       start a big task which will create several rows. Generation of data for
       each row is a fragile operation and might fail. If we fail creating
       something, depending on the type of failure, we want to abort the whole
       task, or only skip the failed row.

	 my $schema = MySchema->connect("dbi:Pg:dbname=my_db");

	 # Start a transaction. Every database change from here on will only be
	 # committed into the database if the try block succeeds.
	 use Try::Tiny;
	 my $exception;
	 try {
	   $schema->txn_do(sub {
	     # SQL: BEGIN WORK;

	     my $job = $schema->resultset('Job')->create({ name=> 'big job' });
	     # SQL: INSERT INTO job ( name) VALUES ( 'big job' );

	     for (1..10) {

	       # Start a nested transaction, which in fact sets a savepoint.
	       try {
		 $schema->txn_do(sub {
		   # SQL: SAVEPOINT savepoint_0;

		   my $thing = $schema->resultset('Thing')->create({ job=>$job->id });
		   # SQL: INSERT INTO thing ( job) VALUES ( 1 );

		   if (rand > 0.8) {
		     # This will generate an error, thus setting $@

		     $thing->update({force_fail=>'foo'});
		     # SQL: UPDATE thing SET force_fail = 'foo'
		     #	    WHERE ( id = 42 );
		   }
		 });
	       } catch {
		 # SQL: ROLLBACK TO SAVEPOINT savepoint_0;

		 # There was an error while creating a $thing. Depending on the error
		 # we want to abort the whole transaction, or only rollback the
		 # changes related to the creation of this $thing

		 # Abort the whole job
		 if ($_ =~ /horrible_problem/) {
		   print "something horrible happend, aborting job!";
		   die $_;		  # rethrow error
		 }

		 # Ignore this $thing, report the error, and continue with the
		 # next $thing
		 print "Cannot create thing: $_";
	       }
	       # There was no error, so save all changes since the last
	       # savepoint.

	       # SQL: RELEASE SAVEPOINT savepoint_0;
	     }
	   });
	 } catch {
	   $exception = $_;
	 }

	 if ($caught) {
	   # There was an error while handling the $job. Rollback all changes
	   # since the transaction started, including the already committed
	   # ('released') savepoints. There will be neither a new $job nor any
	   # $thing entry in the database.

	   # SQL: ROLLBACK;

	   print "ERROR: $exception\n";
	 }
	 else {
	   # There was no error while handling the $job. Commit all changes.
	   # Only now other connections can see the newly created $job and
	   # @things.

	   # SQL: COMMIT;

	   print "Ok\n";
	 }

       In this example it might be hard to see where the rollbacks, releases
       and commits are happening, but it works just the same as for plain
       <txn_do>: If the "try"-block around "txn_do" fails, a rollback is
       issued. If the "try" succeeds, the transaction is committed (or the
       savepoint released).

       While you can get more fine-grained control using "svp_begin",
       "svp_release" and "svp_rollback", it is strongly recommended to use
       "txn_do" with coderefs.

   Simple Transactions with DBIx::Class::Storage::TxnScopeGuard
       An easy way to use transactions is with
       DBIx::Class::Storage::TxnScopeGuard. See "Automatically creating
       related objects" for an example.

       Note that unlike txn_do, TxnScopeGuard will only make sure the
       connection is alive when issuing the "BEGIN" statement. It will not
       (and really can not) retry if the server goes away mid-operations,
       unlike "txn_do".

SQL
   Creating Schemas From An Existing Database
       DBIx::Class::Schema::Loader will connect to a database and create a
       DBIx::Class::Schema and associated sources by examining the database.

       The recommend way of achieving this is to use the dbicdump utility or
       the Catalyst helper, as described in Manual::Intro.

       Alternatively, use the make_schema_at method:

	 perl -MDBIx::Class::Schema::Loader=make_schema_at,dump_to_dir:./lib \
	   -e 'make_schema_at("My::Schema", \
	   { db_schema => 'myschema', components => ["InflateColumn::DateTime"] }, \
	   [ "dbi:Pg:dbname=foo", "username", "password" ])'

       This will create a tree of files rooted at "./lib/My/Schema/"
       containing source definitions for all the tables found in the
       "myschema" schema in the "foo" database.

   Creating DDL SQL
       The following functionality requires you to have SQL::Translator (also
       known as "SQL Fairy") installed.

       To create a set of database-specific .sql files for the above schema:

	my $schema = My::Schema->connect($dsn);
	$schema->create_ddl_dir(['MySQL', 'SQLite', 'PostgreSQL'],
			       '0.1',
			       './dbscriptdir/'
			       );

       By default this will create schema files in the current directory, for
       MySQL, SQLite and PostgreSQL, using the $VERSION from your Schema.pm.

       To create a new database using the schema:

	my $schema = My::Schema->connect($dsn);
	$schema->deploy({ add_drop_table => 1});

       To import created .sql files using the mysql client:

	 mysql -h "host" -D "database" -u "user" -p < My_Schema_1.0_MySQL.sql

       To create "ALTER TABLE" conversion scripts to update a database to a
       newer version of your schema at a later point, first set a new $VERSION
       in your Schema file, then:

	my $schema = My::Schema->connect($dsn);
	$schema->create_ddl_dir(['MySQL', 'SQLite', 'PostgreSQL'],
				'0.2',
				'/dbscriptdir/',
				'0.1'
				);

       This will produce new database-specific .sql files for the new version
       of the schema, plus scripts to convert from version 0.1 to 0.2. This
       requires that the files for 0.1 as created above are available in the
       given directory to diff against.

   Select from dual
       Dummy tables are needed by some databases to allow calling functions or
       expressions that aren't based on table content, for examples of how
       this applies to various database types, see:
       http://troels.arvin.dk/db/rdbms/#other-dummy_table
       <http://troels.arvin.dk/db/rdbms/#other-dummy_table>.

       Note: If you're using Oracles dual table don't ever do anything other
       than a select, if you CRUD on your dual table you *will* break your
       database.

       Make a table class as you would for any other table

	 package MyAppDB::Dual;
	 use strict;
	 use warnings;
	 use base 'DBIx::Class::Core';
	 __PACKAGE__->table("Dual");
	 __PACKAGE__->add_columns(
	   "dummy",
	   { data_type => "VARCHAR2", is_nullable => 0, size => 1 },
	 );

       Once you've loaded your table class select from it using "select" and
       "as" instead of "columns"

	 my $rs = $schema->resultset('Dual')->search(undef,
	   { select => [ 'sydate' ],
	     as	    => [ 'now' ]
	   },
	 );

       All you have to do now is be careful how you access your resultset, the
       below will not work because there is no column called 'now' in the Dual
       table class

	 while (my $dual = $rs->next) {
	   print $dual->now."\n";
	 }
	 # Can't locate object method "now" via package "MyAppDB::Dual" at headshot.pl line 23.

       You could of course use 'dummy' in "as" instead of 'now', or
       "add_columns" to your Dual class for whatever you wanted to select from
       dual, but that's just silly, instead use "get_column"

	 while (my $dual = $rs->next) {
	   print $dual->get_column('now')."\n";
	 }

       Or use "cursor"

	 my $cursor = $rs->cursor;
	 while (my @vals = $cursor->next) {
	   print $vals[0]."\n";
	 }

       In case you're going to use this "trick" together with "deploy" in
       DBIx::Class::Schema or "create_ddl_dir" in DBIx::Class::Schema a table
       called "dual" will be created in your current schema. This would
       overlap "sys.dual" and you could not fetch "sysdate" or
       "sequence.nextval" anymore from dual. To avoid this problem, just tell
       SQL::Translator to not create table dual:

	   my $sqlt_args = {
	       add_drop_table => 1,
	       parser_args    => { sources => [ grep $_ ne 'Dual', schema->sources ] },
	   };
	   $schema->create_ddl_dir( [qw/Oracle/], undef, './sql', undef, $sqlt_args );

       Or use DBIx::Class::ResultClass::HashRefInflator

	 $rs->result_class('DBIx::Class::ResultClass::HashRefInflator');
	 while ( my $dual = $rs->next ) {
	   print $dual->{now}."\n";
	 }

       Here are some example "select" conditions to illustrate the different
       syntax you could use for doing stuff like
       "oracles.heavily(nested(functions_can('take', 'lots'), OF), 'args')"

	 # get a sequence value
	 select => [ 'A_SEQ.nextval' ],

	 # get create table sql
	 select => [ { 'dbms_metadata.get_ddl' => [ "'TABLE'", "'ARTIST'" ]} ],

	 # get a random num between 0 and 100
	 select => [ { "trunc" => [ { "dbms_random.value" => [0,100] } ]} ],

	 # what year is it?
	 select => [ { 'extract' => [ \'year from sysdate' ] } ],

	 # do some math
	 select => [ {'round' => [{'cos' => [ \'180 * 3.14159265359/180' ]}]}],

	 # which day of the week were you born on?
	 select => [{'to_char' => [{'to_date' => [ "'25-DEC-1980'", "'dd-mon-yyyy'" ]}, "'day'"]}],

	 # select 16 rows from dual
	 select	  => [ "'hello'" ],
	 as	  => [ 'world' ],
	 group_by => [ 'cube( 1, 2, 3, 4 )' ],

   Adding Indexes And Functions To Your SQL
       Often you will want indexes on columns on your table to speed up
       searching. To do this, create a method called "sqlt_deploy_hook" in the
       relevant source class (refer to the advanced callback system if you
       wish to share a hook between multiple sources):

	package My::Schema::Result::Artist;

	__PACKAGE__->table('artist');
	__PACKAGE__->add_columns(id => { ... }, name => { ... })

	sub sqlt_deploy_hook {
	  my ($self, $sqlt_table) = @_;

	  $sqlt_table->add_index(name => 'idx_name', fields => ['name']);
	}

	1;

       Sometimes you might want to change the index depending on the type of
       the database for which SQL is being generated:

	 my ($db_type = $sqlt_table->schema->translator->producer_type)
	   =~ s/^SQL::Translator::Producer:://;

       You can also add hooks to the schema level to stop certain tables being
       created:

	package My::Schema;

	...

	sub sqlt_deploy_hook {
	  my ($self, $sqlt_schema) = @_;

	  $sqlt_schema->drop_table('table_name');
	}

       You could also add views, procedures or triggers to the output using
       "add_view" in SQL::Translator::Schema, "add_procedure" in
       SQL::Translator::Schema or "add_trigger" in SQL::Translator::Schema.

   Schema versioning
       The following example shows simplistically how you might use
       DBIx::Class to deploy versioned schemas to your customers. The basic
       process is as follows:

       1.  Create a DBIx::Class schema

       2.  Save the schema

       3.  Deploy to customers

       4.  Modify schema to change functionality

       5.  Deploy update to customers

       Create a DBIx::Class schema

       This can either be done manually, or generated from an existing
       database as described under "Creating Schemas From An Existing
       Database"

       Save the schema

       Call "create_ddl_dir" in DBIx::Class::Schema as above under "Creating
       DDL SQL".

       Deploy to customers

       There are several ways you could deploy your schema. These are probably
       beyond the scope of this recipe, but might include:

       1.  Require customer to apply manually using their RDBMS.

       2.  Package along with your app, making database dump/schema
	   update/tests all part of your install.

       Modify the schema to change functionality

       As your application evolves, it may be necessary to modify your schema
       to change functionality. Once the changes are made to your schema in
       DBIx::Class, export the modified schema and the conversion scripts as
       in "Creating DDL SQL".

       Deploy update to customers

       Add the DBIx::Class::Schema::Versioned schema component to your Schema
       class. This will add a new table to your database called
       "dbix_class_schema_vesion" which will keep track of which version is
       installed and warn if the user tries to run a newer schema version than
       the database thinks it has.

       Alternatively, you can send the conversion SQL scripts to your
       customers as above.

   Setting quoting for the generated SQL
       If the database contains column names with spaces and/or reserved
       words, they need to be quoted in the SQL queries. This is done using:

	$schema->storage->sql_maker->quote_char([ qw/[ ]/] );
	$schema->storage->sql_maker->name_sep('.');

       The first sets the quote characters. Either a pair of matching
       brackets, or a """ or "'":

	$schema->storage->sql_maker->quote_char('"');

       Check the documentation of your database for the correct quote
       characters to use. "name_sep" needs to be set to allow the SQL
       generator to put the quotes the correct place, and defaults to "." if
       not supplied.

       In most cases you should set these as part of the arguments passed to
       "connect" in DBIx::Class::Schema:

	my $schema = My::Schema->connect(
	 'dbi:mysql:my_db',
	 'db_user',
	 'db_password',
	 {
	   quote_char => '"',
	   name_sep   => '.'
	 }
	)

       In some cases, quoting will be required for all users of a schema. To
       enforce this, you can also overload the "connection" method for your
       schema class:

	sub connection {
	    my $self = shift;
	    my $rv = $self->next::method( @_ );
	    $rv->storage->sql_maker->quote_char([ qw/[ ]/ ]);
	    $rv->storage->sql_maker->name_sep('.');
	    return $rv;
	}

   Working with PostgreSQL array types
       You can also assign values to PostgreSQL array columns by passing array
       references in the "\%columns" ("\%vals") hashref of the "create" in
       DBIx::Class::ResultSet and "update" in DBIx::Class::Row family of
       methods:

	 $resultset->create({
	   numbers => [1, 2, 3]
	 });

	 $row->update(
	   {
	     numbers => [1, 2, 3]
	   },
	   {
	     day => '2008-11-24'
	   }
	 );

       In conditions (e.g. "\%cond" in the "search" in DBIx::Class::ResultSet
       family of methods) you cannot directly use array references (since this
       is interpreted as a list of values to be "OR"ed), but you can use the
       following syntax to force passing them as bind values:

	 $resultset->search(
	   {
	     numbers => \[ '= ?', [numbers => [1, 2, 3]] ]
	   }
	 );

       See "array_datatypes" in SQL::Abstract and "Literal SQL with
       placeholders and bind values (subqueries)" in SQL::Abstract for more
       explanation. Note that DBIx::Class sets "bindtype" in SQL::Abstract to
       "columns", so you must pass the bind values (the "[1, 2, 3]" arrayref
       in the above example) wrapped in arrayrefs together with the column
       name, like this: "[column_name => value]".

   Formatting DateTime objects in queries
       To ensure "WHERE" conditions containing DateTime arguments are properly
       formatted to be understood by your RDBMS, you must use the "DateTime"
       formatter returned by "datetime_parser" in DBIx::Class::Storage::DBI to
       format any DateTime objects you pass to search conditions. Any Storage
       object attached to your Schema provides a correct "DateTime" formatter,
       so all you have to do is:

	 my $dtf = $schema->storage->datetime_parser;
	 my $rs = $schema->resultset('users')->search(
	   {
	     signup_date => {
	       -between => [
		 $dtf->format_datetime($dt_start),
		 $dtf->format_datetime($dt_end),
	       ],
	     }
	   },
	 );

       Without doing this the query will contain the simple stringification of
       the "DateTime" object, which almost never matches the RDBMS
       expectations.

       This kludge is necessary only for conditions passed to "search" in
       DBIx::Class::ResultSet, whereas create, find, "update" in
       DBIx::Class::Row (but not "update" in DBIx::Class::ResultSet) are all
       DBIx::Class::InflateColumn-aware and will do the right thing when
       supplied an inflated "DateTime" object.

   Using Unicode
       When using unicode character data there are two alternatives - either
       your database supports unicode characters (including setting the utf8
       flag on the returned string), or you need to encode/decode data
       appropriately each time a string field is inserted into or retrieved
       from the database. It is better to avoid encoding/decoding data and to
       use your database's own unicode capabilities if at all possible.

       The DBIx::Class::UTF8Columns component handles storing selected unicode
       columns in a database that does not directly support unicode. If used
       with a database that does correctly handle unicode then strange and
       unexpected data corrupt will occur.

       The Catalyst Wiki Unicode page at
       <http://wiki.catalystframework.org/wiki/tutorialsandhowtos/using_unicode>
       has additional information on the use of Unicode with Catalyst and
       DBIx::Class.

       The following databases do correctly handle unicode data:-

       MySQL

       MySQL supports unicode, and will correctly flag utf8 data from the
       database if the "mysql_enable_utf8" is set in the connect options.

	 my $schema = My::Schema->connection('dbi:mysql:dbname=test',
					     $user, $pass,
					     { mysql_enable_utf8 => 1} );

       When set, a data retrieved from a textual column type (char, varchar,
       etc) will have the UTF-8 flag turned on if necessary. This enables
       character semantics on that string. You will also need to ensure that
       your database / table / column is configured to use UTF8. See Chapter
       10 of the mysql manual for details.

       See DBD::mysql for further details.

       Oracle

       Information about Oracle support for unicode can be found in "Unicode"
       in DBD::Oracle.

       PostgreSQL

       PostgreSQL supports unicode if the character set is correctly set at
       database creation time. Additionally the "pg_enable_utf8" should be set
       to ensure unicode data is correctly marked.

	 my $schema = My::Schema->connection('dbi:Pg:dbname=test',
					     $user, $pass,
					     { pg_enable_utf8 => 1} );

       Further information can be found in DBD::Pg.

       SQLite

       SQLite version 3 and above natively use unicode internally. To
       correctly mark unicode strings taken from the database, the
       "sqlite_unicode" flag should be set at connect time (in versions of
       DBD::SQLite prior to 1.27 this attribute was named "unicode").

	 my $schema = My::Schema->connection('dbi:SQLite:/tmp/test.db',
					     '', '',
					     { sqlite_unicode => 1} );

BOOTSTRAPPING/MIGRATING
   Easy migration from class-based to schema-based setup
       You want to start using the schema-based approach to DBIx::Class (see
       "Setting it up manually" in DBIx::Class::Manual::Intro), but have an
       established class-based setup with lots of existing classes that you
       don't want to move by hand. Try this nifty script instead:

	 use MyDB;
	 use SQL::Translator;

	 my $schema = MyDB->schema_instance;

	 my $translator		  =  SQL::Translator->new(
	     debug		  => $debug	     ||	 0,
	     trace		  => $trace	     ||	 0,
	     no_comments	  => $no_comments    ||	 0,
	     show_warnings	  => $show_warnings  ||	 0,
	     add_drop_table	  => $add_drop_table ||	 0,
	     validate		  => $validate	     ||	 0,
	     parser_args	  => {
		'DBIx::Schema'	  => $schema,
				     },
	     producer_args   => {
		 'prefix'	  => 'My::Schema',
				},
	 );

	 $translator->parser('SQL::Translator::Parser::DBIx::Class');
	 $translator->producer('SQL::Translator::Producer::DBIx::Class::File');

	 my $output = $translator->translate(@args) or die
		 "Error: " . $translator->error;

	 print $output;

       You could use Module::Find to search for all subclasses in the MyDB::*
       namespace, which is currently left as an exercise for the reader.

OVERLOADING METHODS
       DBIx::Class uses the Class::C3 package, which provides for redispatch
       of method calls, useful for things like default values and triggers.
       You have to use calls to "next::method" to overload methods. More
       information on using Class::C3 with DBIx::Class can be found in
       DBIx::Class::Manual::Component.

   Setting default values for a row
       It's as simple as overriding the "new" method.  Note the use of
       "next::method".

	 sub new {
	   my ( $class, $attrs ) = @_;

	   $attrs->{foo} = 'bar' unless defined $attrs->{foo};

	   my $new = $class->next::method($attrs);

	   return $new;
	 }

       For more information about "next::method", look in the Class::C3
       documentation. See also DBIx::Class::Manual::Component for more ways to
       write your own base classes to do this.

       People looking for ways to do "triggers" with DBIx::Class are probably
       just looking for this.

   Changing one field whenever another changes
       For example, say that you have three columns, "id", "number", and
       "squared".  You would like to make changes to "number" and have
       "squared" be automagically set to the value of "number" squared.	 You
       can accomplish this by wrapping the "number" accessor with
       Class::Method::Modifiers:

	 around number => sub {
	   my ($orig, $self) = (shift, shift);

	   if (@_) {
	     my $value = $_[0];
	     $self->squared( $value * $value );
	   }

	   $self->$orig(@_);
	 }

       Note that the hard work is done by the call to "$self->$orig", which
       redispatches your call to store_column in the superclass(es).

       Generally, if this is a calculation your database can easily do, try
       and avoid storing the calculated value, it is safer to calculate when
       needed, than rely on the data being in sync.

   Automatically creating related objects
       You might have a class "Artist" which has many "CD"s.  Further, you
       want to create a "CD" object every time you insert an "Artist" object.
       You can accomplish this by overriding "insert" on your objects:

	 sub insert {
	   my ( $self, @args ) = @_;
	   $self->next::method(@args);
	   $self->create_related ('cds', \%initial_cd_data );
	   return $self;
	 }

       If you want to wrap the two inserts in a transaction (for consistency,
       an excellent idea), you can use the awesome
       DBIx::Class::Storage::TxnScopeGuard:

	 sub insert {
	   my ( $self, @args ) = @_;

	   my $guard = $self->result_source->schema->txn_scope_guard;

	   $self->next::method(@args);
	   $self->create_related ('cds', \%initial_cd_data );

	   $guard->commit;

	   return $self
	 }

   Wrapping/overloading a column accessor
       Problem:

       Say you have a table "Camera" and want to associate a description with
       each camera. For most cameras, you'll be able to generate the
       description from the other columns. However, in a few special cases you
       may want to associate a custom description with a camera.

       Solution:

       In your database schema, define a description field in the "Camera"
       table that can contain text and null values.

       In DBIC, we'll overload the column accessor to provide a sane default
       if no custom description is defined. The accessor will either return or
       generate the description, depending on whether the field is null or
       not.

       First, in your "Camera" schema class, define the description field as
       follows:

	 __PACKAGE__->add_columns(description => { accessor => '_description' });

       Next, we'll define the accessor-wrapper subroutine:

	 sub description {
	     my $self = shift;

	     # If there is an update to the column, we'll let the original accessor
	     # deal with it.
	     return $self->_description(@_) if @_;

	     # Fetch the column value.
	     my $description = $self->_description;

	     # If there's something in the description field, then just return that.
	     return $description if defined $description && length $descripton;

	     # Otherwise, generate a description.
	     return $self->generate_description;
	 }

DEBUGGING AND PROFILING
   DBIx::Class objects with Data::Dumper
       Data::Dumper can be a very useful tool for debugging, but sometimes it
       can be hard to find the pertinent data in all the data it can generate.
       Specifically, if one naively tries to use it like so,

	 use Data::Dumper;

	 my $cd = $schema->resultset('CD')->find(1);
	 print Dumper($cd);

       several pages worth of data from the CD object's schema and result
       source will be dumped to the screen. Since usually one is only
       interested in a few column values of the object, this is not very
       helpful.

       Luckily, it is possible to modify the data before Data::Dumper outputs
       it. Simply define a hook that Data::Dumper will call on the object
       before dumping it. For example,

	 package My::DB::CD;

	 sub _dumper_hook {
	   $_[0] = bless {
	     %{ $_[0] },
	     result_source => undef,
	   }, ref($_[0]);
	 }

	 [...]

	 use Data::Dumper;

	 local $Data::Dumper::Freezer = '_dumper_hook';

	 my $cd = $schema->resultset('CD')->find(1);
	 print Dumper($cd);
		# dumps $cd without its ResultSource

       If the structure of your schema is such that there is a common base
       class for all your table classes, simply put a method similar to
       "_dumper_hook" in the base class and set $Data::Dumper::Freezer to its
       name and Data::Dumper will automagically clean up your data before
       printing it. See "EXAMPLES" in Data::Dumper for more information.

   Profiling
       When you enable DBIx::Class::Storage's debugging it prints the SQL
       executed as well as notifications of query completion and transaction
       begin/commit.  If you'd like to profile the SQL you can subclass the
       DBIx::Class::Storage::Statistics class and write your own profiling
       mechanism:

	 package My::Profiler;
	 use strict;

	 use base 'DBIx::Class::Storage::Statistics';

	 use Time::HiRes qw(time);

	 my $start;

	 sub query_start {
	   my $self = shift();
	   my $sql = shift();
	   my @params = @_;

	   $self->print("Executing $sql: ".join(', ', @params)."\n");
	   $start = time();
	 }

	 sub query_end {
	   my $self = shift();
	   my $sql = shift();
	   my @params = @_;

	   my $elapsed = sprintf("%0.4f", time() - $start);
	   $self->print("Execution took $elapsed seconds.\n");
	   $start = undef;
	 }

	 1;

       You can then install that class as the debugging object:

	 __PACKAGE__->storage->debugobj(new My::Profiler());
	 __PACKAGE__->storage->debug(1);

       A more complicated example might involve storing each execution of SQL
       in an array:

	 sub query_end {
	   my $self = shift();
	   my $sql = shift();
	   my @params = @_;

	   my $elapsed = time() - $start;
	   push(@{ $calls{$sql} }, {
	       params => \@params,
	       elapsed => $elapsed
	   });
	 }

       You could then create average, high and low execution times for an SQL
       statement and dig down to see if certain parameters cause aberrant
       behavior.  You might want to check out DBIx::Class::QueryLog as well.

IMPROVING PERFORMANCE
       ·   Install Class::XSAccessor to speed up Class::Accessor::Grouped.

       ·   On Perl 5.8 install Class::C3::XS.

       ·   prefetch relationships, where possible. See "Using joins and
	   prefetch".

       ·   Use populate in void context to insert data when you don't need the
	   resulting DBIx::Class::Row objects, if possible, but see the
	   caveats.

	   When inserting many rows, for best results, populate a large number
	   of rows at a time, but not so large that the table is locked for an
	   unacceptably long time.

	   If using create instead, use a transaction and commit every "X"
	   rows; where "X" gives you the best performance without locking the
	   table for too long.

       ·   When selecting many rows, if you don't need full-blown
	   DBIx::Class::Row objects, consider using
	   DBIx::Class::ResultClass::HashRefInflator.

       ·   See also "STARTUP SPEED" and "MEMORY USAGE" in this document.

STARTUP SPEED
       DBIx::Class programs can have a significant startup delay as the ORM
       loads all the relevant classes. This section examines techniques for
       reducing the startup delay.

       These tips are are listed in order of decreasing effectiveness - so the
       first tip, if applicable, should have the greatest effect on your
       application.

   Statically Define Your Schema
       If you are using DBIx::Class::Schema::Loader to build the classes
       dynamically based on the database schema then there will be a
       significant startup delay.

       For production use a statically defined schema (which can be generated
       using DBIx::Class::Schema::Loader to dump the database schema once -
       see make_schema_at and dump_directory for more details on creating
       static schemas from a database).

   Move Common Startup into a Base Class
       Typically DBIx::Class result classes start off with

	   use base qw/DBIx::Class::Core/;
	   __PACKAGE__->load_components(qw/InflateColumn::DateTime/);

       If this preamble is moved into a common base class:-

	   package MyDBICbase;

	   use base qw/DBIx::Class::Core/;
	   __PACKAGE__->load_components(qw/InflateColumn::DateTime/);
	   1;

       and each result class then uses this as a base:-

	   use base qw/MyDBICbase/;

       then the load_components is only performed once, which can result in a
       considerable startup speedup for schemas with many classes.

   Explicitly List Schema Result Classes
       The schema class will normally contain

	   __PACKAGE__->load_classes();

       to load the result classes. This will use Module::Find to find and load
       the appropriate modules. Explicitly defining the classes you wish to
       load will remove the overhead of Module::Find and the related directory
       operations:

	   __PACKAGE__->load_classes(qw/ CD Artist Track /);

       If you are instead using the load_namespaces syntax to load the
       appropriate classes there is not a direct alternative avoiding
       Module::Find.

MEMORY USAGE
   Cached statements
       DBIx::Class normally caches all statements with prepare_cached().  This
       is normally a good idea, but if too many statements are cached, the
       database may use too much memory and may eventually run out and fail
       entirely.  If you suspect this may be the case, you may want to examine
       DBI's CachedKids hash:

	   # print all currently cached prepared statements
	   print for keys %{$schema->storage->dbh->{CachedKids}};
	   # get a count of currently cached prepared statements
	   my $count = scalar keys %{$schema->storage->dbh->{CachedKids}};

       If it's appropriate, you can simply clear these statements,
       automatically deallocating them in the database:

	   my $kids = $schema->storage->dbh->{CachedKids};
	   delete @{$kids}{keys %$kids} if scalar keys %$kids > 100;

       But what you probably want is to expire unused statements and not those
       that are used frequently.  You can accomplish this with Tie::Cache or
       Tie::Cache::LRU:

	   use Tie::Cache;
	   use DB::Main;
	   my $schema = DB::Main->connect($dbi_dsn, $user, $pass, {
	       on_connect_do => sub { tie %{shift->_dbh->{CachedKids}}, 'Tie::Cache', 100 },
	   });

perl v5.16.2			  2012-10-18  DBIx::Class::Manual::Cookbook(3)
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