Fix selecting fields on belongs to many relations

[rasmuscnielsen]

When using field selection on belongs-to-many relationships, it currently throws an "id in field list is ambiguous" error:

Integrity constraint violation: 1052 Column 'id' in field list is ambiguous (Connection: mysql, SQL: select id, name, pivot_models.test_model_id as pivot_test_model_id, pivot_models.related_through_pivot_model_id as pivot_related_through_pivot_model_id from related_through_pivot_models inner join pivot_models on related_through_pivot_models.id = pivot_models.related_through_pivot_model_id where pivot_models.test_model_id in (1))

This PR fixes that by qualifying relationship fields upon select.

[freekmurze]

Thanks!