=head1 NAME

Tangram::Set - maps Set::Object using a link table

=head1 SYNOPSIS

   use Tangram;

   # or
   use Tangram::Core;
   use Tangram::Set;

   $schema = Tangram::Schema->new(

      classes => { Company => { fields => {

      set =>
      {
         # long form
         employee =>
         {
            class => 'Person',
            table => 'Company_employees',
            coll => 'company',
            item => 'employee',
         },

         # short form
         assets => 'Asset',
      }

=head1 DESCRIPTION

Maps a reference to a Set::Object. The persistent fields are grouped in
a hash under the C<set> key in the field hash.

The set may contain only objects of persistent classes. These classes
must have a common persistent base class.

Tangram uses a link table to save the state of the collection. The
table has two columns: one contains the id of the container objects;
the other contains the ids of the elements.

The field names are passed in a hash that associates a field name with
a field descriptor. The field descriptor may be either a hash or a
string. The hash uses the following fields:

=over 4

=item * class

=item * aggreg

=item * table

=item * coll

=item * item

=item * deep_update

=back

Mandatory field C<class> specifies the class of the elements.

Optional field C<aggreg> specifies that the elements of the collection
must be removed (erased) from persistent storage along with the
containing object. The default is not to aggregate.

Optional field C<table> sets the name of the link table. This
defaults to 'C_F', where C is the class of the containing object and F
is the field name.

Optional field C<coll> sets the name the column containing the ids of
the containing objects. This defaults to 'coll'.

Optional field C<item> sets the name the column containing the ids of
the elements. This defaults to 'item'.

Optional field C<deep_update> specificies that all elements have to be
updated automatically when C<update> is called on the collection
object. Automatic update ensures consisitency between the Perl
representation and the DBMS state, but degrades update performance so
use it with caution. The default is not to do automatic updates.

If the descriptor is a string, it is interpreted as the name of the
element's class. This is equivalent to specifying only the C<class>
field in the hash variant.