should, set this attribute to a true or false value to override the detection
of when to create constraints.
+=item is_deferrable
+
+Tells L<SQL::Translator> that the foreign key constraint it creates should be
+deferrable. In other words, the user may request that the constraint be ignored
+until the end of the transaction. Currently, only the PostgreSQL producer
+actually supports this.
+
=back
=head2 register_relationship