array( self::COLUMNS => 'listing_id', self::REF_TABLE_CLASS => '\Ppb\Db\Table\Listings', self::REF_COLUMNS => 'id', ), 'Sale' => array( self::COLUMNS => 'sale_id', self::REF_TABLE_CLASS => '\Ppb\Db\Table\Sales', self::REF_COLUMNS => 'id', ), ); }