expand_join_condition

  __PACKAGE__->has_many(minions => 'Blah::Person' => sub {
    my ($args) = @_;
    $args->{self_resultsource}
         ->schema->storage->sql_maker
         ->expand_join_condition(
             $args
           );
  });

on

  __PACKAGE__->has_many(minions => 'Blah::Person' => on {
    { 'self.group_id' => 'foreign.group_id',
      'self.rank' => { '>', 'foreign.rank' } }
  });

Or with ParameterizedJoinHack,

  __PACKAGE__->parameterized_has_many(
      priority_tasks => 'MySchema::Result::Task',
      [['min_priority'] => sub {
          my $args = shift;
          return +{
              "$args->{foreign_alias}.owner_id" => {
                  -ident => "$args->{self_alias}.id",
              },
              "$args->{foreign_alias}.priority" => {
                  '>=' => $_{min_priority},
              },
          };
      }],
  );

becomes

  __PACKAGE__->parameterized_has_many(
      priority_tasks => 'MySchema::Result::Task',
      [['min_priority'] => on {
        { 'foreign.owner_id' => 'self.id',
          'foreign.priority' => { '>=', { -value => $_{min_priority} } } }
      }]
  );

Note that foreign/self can appear in such a condition on either side, BUT if you want DBIx::Class to be able to use a join-less version you must ensure that the LHS is all foreign columns, i.e.

  on {
    +{
      'foreign.x' => 'self.x',
      'self.y' => { -between => [ 'foreign.y1', 'foreign.y2' ] }
    }
  }

is completely valid but DBIC will insist on doing a JOIN even if you have a fully populated row object to call "search_related" on - to avoid the spurious JOIN, you must specify it with explicit LHS foreign cols as:

  on {
    +{
      'foreign.x' => 'self.x',
      'foreign.y1' => { '<=', 'self.y' },
      'foreign.y2' => { '>=', 'self.y' },
    }
  }