Function join_table_name 

pub fn join_table_name(class_name: &str, key: &str) -> String

The join collection backing one Relation field.

_Join:<key>:<className> (DatabaseController.js:319-321). Note the argument order: the key comes first and the owning class second, so _Role.users is _Join:users:_Role. Getting it backwards produces a collection parse-server will never read.