For more examples of using these attributes, see
L<DBIx::Class::Manual::Cookbook>.
-=head2 from
-
-=over 4
-
-=item Value: \@from_clause
-
-=back
-
-The C<from> attribute gives you manual control over the C<FROM> clause of SQL
-statements generated by L<DBIx::Class>, allowing you to express custom C<JOIN>
-clauses.
-
-NOTE: Use this on your own risk. This allows you to shoot off your foot!
-
-C<join> will usually do what you need and it is strongly recommended that you
-avoid using C<from> unless you cannot achieve the desired result using C<join>.
-And we really do mean "cannot", not just tried and failed. Attempting to use
-this because you're having problems with C<join> is like trying to use x86
-ASM because you've got a syntax error in your C. Trust us on this.
-
-Now, if you're still really, really sure you need to use this (and if you're
-not 100% sure, ask the mailing list first), here's an explanation of how this
-works.
-
-The syntax is as follows -
-
- [
- { <alias1> => <table1> },
- [
- { <alias2> => <table2>, -join_type => 'inner|left|right' },
- [], # nested JOIN (optional)
- { <table1.column1> => <table2.column2>, ... (more conditions) },
- ],
- # More of the above [ ] may follow for additional joins
- ]
-
- <table1> <alias1>
- JOIN
- <table2> <alias2>
- [JOIN ...]
- ON <table1.column1> = <table2.column2>
- <more joins may follow>
-
-An easy way to follow the examples below is to remember the following:
-
- Anything inside "[]" is a JOIN
- Anything inside "{}" is a condition for the enclosing JOIN
-
-The following examples utilize a "person" table in a family tree application.
-In order to express parent->child relationships, this table is self-joined:
-
- # Person->belongs_to('father' => 'Person');
- # Person->belongs_to('mother' => 'Person');
-
-C<from> can be used to nest joins. Here we return all children with a father,
-then search against all mothers of those children:
-
- $rs = $schema->resultset('Person')->search(
- undef,
- {
- alias => 'mother', # alias columns in accordance with "from"
- from => [
- { mother => 'person' },
- [
- [
- { child => 'person' },
- [
- { father => 'person' },
- { 'father.person_id' => 'child.father_id' }
- ]
- ],
- { 'mother.person_id' => 'child.mother_id' }
- ],
- ]
- },
- );
-
- # Equivalent SQL:
- # SELECT mother.* FROM person mother
- # JOIN (
- # person child
- # JOIN person father
- # ON ( father.person_id = child.father_id )
- # )
- # ON ( mother.person_id = child.mother_id )
-
-The type of any join can be controlled manually. To search against only people
-with a father in the person table, we could explicitly use C<INNER JOIN>:
-
- $rs = $schema->resultset('Person')->search(
- undef,
- {
- alias => 'child', # alias columns in accordance with "from"
- from => [
- { child => 'person' },
- [
- { father => 'person', -join_type => 'inner' },
- { 'father.id' => 'child.father_id' }
- ],
- ]
- },
- );
-
- # Equivalent SQL:
- # SELECT child.* FROM person child
- # INNER JOIN person father ON child.father_id = father.id
-
-You can select from a subquery by passing a resultset to from as follows.
-
- $schema->resultset('Artist')->search(
- undef,
- { alias => 'artist2',
- from => [ { artist2 => $artist_rs->as_query } ],
- } );
-
- # and you'll get sql like this..
- # SELECT artist2.artistid, artist2.name, artist2.rank, artist2.charfield FROM
- # ( SELECT me.artistid, me.name, me.rank, me.charfield FROM artists me ) artist2
-
-If you need to express really complex joins, you
-can supply literal SQL to C<from> via a scalar reference. In this case
-the contents of the scalar will replace the table name associated with the
-resultsource.
-
-WARNING: This technique might very well not work as expected on chained
-searches - you have been warned.
-
- # Assuming the Event resultsource is defined as:
-
- MySchema::Event->add_columns (
- sequence => {
- data_type => 'INT',
- is_auto_increment => 1,
- },
- location => {
- data_type => 'INT',
- },
- type => {
- data_type => 'INT',
- },
- );
- MySchema::Event->set_primary_key ('sequence');
-
- # This will get back the latest event for every location. The column
- # selector is still provided by DBIC, all we do is add a JOIN/WHERE
- # combo to limit the resultset
-
- $rs = $schema->resultset('Event');
- $table = $rs->result_source->name;
- $latest = $rs->search (
- undef,
- { from => \ "
- (SELECT e1.* FROM $table e1
- JOIN $table e2
- ON e1.location = e2.location
- AND e1.sequence < e2.sequence
- WHERE e2.sequence is NULL
- ) me",
- },
- );
-
- # Equivalent SQL (with the DBIC chunks added):
-
- SELECT me.sequence, me.location, me.type FROM
- (SELECT e1.* FROM events e1
- JOIN events e2
- ON e1.location = e2.location
- AND e1.sequence < e2.sequence
- WHERE e2.sequence is NULL
- ) me;
-
=head2 for
=over 4
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