Fix assertion.
Fix hasOne relationship on array of foreign keys.

Scenario:

1. Parent has an array of `child_ids`.
2. Parent specifies a hasOne relationship on these child ids, using 'order' to get the desired child.

Note:

This is going to be inefficient if there are a lot of child records, since there is no way to do a `'limit' => 1` on the query (due to having to eager-load using a single query for mutiple parent records).
Revert change to foreach order and use an isset() call instead.
Merge pull request #16 from BlaineSch/feature/has-one-array-of-keys

Add ability to do a hasOne relationship using an array of keys on the parent.