Sometimes you need to formulate a query using specific operators:
- my @albums = MyApp::DB::Album->search({
+ my @albums = $schema->resultset('Album')->search({
artist => { 'like', '%Lamb%' },
title => { 'like', '%Fear of Fours%' },
});
Other queries might require slightly more complex logic:
- my @albums = MyApp::DB::Album->search({
+ my @albums = $schema->resultset('Album')->search({
-or => [
-and => [
artist => { 'like', '%Smashing Pumpkins%' },
=head2 Using joins and prefetch
-See L<DBIx::Class::ResultSet/ATTRIBUTES>.
+You can use the "join" attribute to allow searching on, or sorting your
+results by, one or more columns in a related table. To return
+all CDs matching a particular artist name:
+
+ my $rs = $schema->resultset('CD')->search(
+ {
+ 'artist.name' => 'Bob Marley'
+ },
+ {
+ join => [qw/artist/], # join the artist table
+ }
+ );
+
+ # equivalent SQL:
+ # SELECT cd.* FROM cd
+ # JOIN artist ON cd.artist = artist.id
+ # WHERE artist.name = 'Bob Marley'
+
+If required, you can now sort on any column in the related table(s) by
+including it in your "order_by" attribute:
+
+ my $rs = $schema->resultset('CD')->search(
+ {
+ 'artist.name' => 'Bob Marley'
+ },
+ {
+ join => [qw/ artist /],
+ order_by => [qw/ artist.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 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 a related table as well as the main table
+for your class:
+
+ my $rs = $schema->resultset('CD')->search(
+ {
+ 'artist.name' => 'Bob Marley'
+ },
+ {
+ join => [qw/ artist /],
+ order_by => [qw/ artist.name /],
+ prefetch => [qw/ artist /] # 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.
+
+Note that as of DBIx::Class 0.04, "prefetch" cannot be used with has_many
+relationships. You will get an error along the lines of "No accessor for
+prefetched ..." if you try.
+
+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!
+
+=head2 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 CDs whose LinerNotes contain a
+specific string:
+
+ # Artist->has_many('cds' => 'CD', 'artist');
+ # CD->has_one('liner_notes' => 'LinerNotes', 'cd');
+
+ $rs = $schema->resultset('Artist')->search(
+ {
+ 'liner_notes.notes' => { 'like', '%some text%' },
+ },
+ {
+ join => {
+ 'cds' => 'liner_notes'
+ }
+ }
+ );
+
+ # equivalent SQL
+ # SELECT artist.* FROM artist
+ # JOIN ( cd ON artist.id = cd.artist )
+ # 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:
+
+ # LinerNotes->has_one('author' => 'Person');
+
+ $rs = $schema->resultset('Artist')->search(
+ {
+ 'liner_notes.notes' => { 'like', '%some text%' },
+ 'author.name' => 'A. Writer'
+ },
+ {
+ join => {
+ 'cds' => {
+ 'liner_notes' => 'author'
+ }
+ }
+ }
+ );
+
+ # equivalent SQL
+ # SELECT artist.* FROM artist
+ # JOIN ( cd ON artist.id = cd.artist )
+ # JOIN ( liner_notes ON cd.id = liner_notes.cd )
+ # JOIN ( author ON author.id = liner_notes.author )
+ # WHERE liner_notes.notes LIKE '%some text%'
+ # AND author.name = 'A. Writer'
=head2 Transactions
=head2 Many-to-many relationships
-This is not as easy as it could be, but it's possible. Here's an
-example to illustrate:
-
- # Set up inherited connection information
- package MyApp::DBIC;
- use base qw/DBIx::Class/;
-
- __PACKAGE__->load_components(qw/PK::Auto::SQLite Core DB/);
- __PACKAGE__->connection(...);
-
- # Set up a class for the 'authors' table
- package MyApp::DBIC::Author;
- use base qw/MyApp::DBIC/;
-
- __PACKAGE__->table('authors');
- __PACKAGE__->add_columns(qw/authID first_name last_name/);
- __PACKAGE__->set_primary_key(qw/authID/);
-
- # Define relationship to the link table
- __PACKAGE__->has_many('b2a' => 'MyApp::DBIC::Book2Author', 'authID');
-
- # Create the accessor for books from the Author class
- sub books {
- my ($self) = @_;
- return MyApp::DBIC::Book->search(
- { 'b2a.authID' => $self->authID }, # WHERE clause
- { join => 'b2a' } # join condition (part of search attrs)
- # 'b2a' refers to the relationship named earlier in the Author class.
- # 'b2a.authID' refers to the authID column of the b2a relationship,
- # which becomes accessible in the search by being joined.
- );
- }
-
- # Define the link table class
- package MyApp::DBIC::Book2Author;
- use base qw/MyApp::DBIC/;
-
- __PACKAGE__->table('book2author');
- __PACKAGE__->add_columns(qw/bookID authID/);
- __PACKAGE__->set_primary_key(qw/bookID authID/);
-
- __PACKAGE__->belongs_to('authID' => 'MyApp::DBIC::Author');
- __PACKAGE__->belongs_to('bookID' => 'MyApp::DBIC::Book');
-
- package MyApp::DBIC::Book;
- use base qw/MyApp::DBIC/;
-
- __PACKAGE__->table('books');
- __PACKAGE__->add_columns(qw/bookID title edition isbn publisher year/);
- __PACKAGE__->set_primary_key(qw/bookID/);
-
- __PACKAGE__->has_many('b2a' => 'MyApp::DBIC::Book2Author', 'bookID');
-
-
- # Returns an author record where the bookID field of the
- # book2author table equals the bookID of the books (using the
- # bookID relationship table)
- sub authors {
- my ($self) = @_;
- return MyApp::DBIC::Author->search(
- { 'b2a.bookID' => $self->bookID }, # WHERE clause
- { join => 'b2a' } # JOIN condition
- );
- }
+This is straightforward using L<DBIx::Class::Relationship::ManyToMany>:
+
+ package My::DB;
+ # set up connection here...
+
+ package My::User;
+ use base 'My::DB';
+ __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 'My::DB';
+ __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 'My::DB';
+ __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
=head2 Setting default values
projects / dbsrgits/DBIx-Class-Historic.git / commitdiff