Set_relation_n_n with attributes of set_relation

Set_relation_n_n with attributes of set_relation.

I have the table structure above. I want to select only the entries (crew_name) in the middle table (relation table) to display in a drop down (crew_id) in the first table. However, it must be displayed using “lastname” and “firstname” in the third table (selection table).

My set relation code is:
$crud->set_relation_n_n(‘crew_id’, ‘currency_fc’, ‘members’, ‘crew_name’, ‘crew_name’, ‘{lastname} {firstname}’,null);

My problem is that it is listing all the items in the selection table instead of only the items in the relation table. Am I doing something stupid here?