Address `test_assign_ids_for_through_a_belongs_to` failure
Address `test_assign_ids_for_through_a_belongs_to` failure

If `:readers` fixture is loaded before the test, it will be failed.
Use `firm.developer_ids` instead because we don't have `:contracts`
fixture for now.

https://travis-ci.org/rails/rails/jobs/268976230#L729

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