list_id | user_idAnd so on. The ORM will deal with everything once you have your models defined, so you can do things like
1 | 1
1 | 2
2 | 1
2 | 3
// assign user 123 to existing list 1:
$list = Model_List::find(1, 'related' => array('users'));
$list->users[] = Model_User::find(123);
$list->save();
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