should be able to delete target instances based on filters in HasManyThrough relation

[agnes512]

Steps to reproduce

a Doctor has many Patients through Appointment, the relation called patients.

Doctor :           [{id: 1, dec: 'doctor'}]
Appointment:       [{doctorId: 1, patientId: 1}, {doctorId: 1, patientId: 2}]
Patient:           [{id: 1, name: 'p1'}, {id: 2, name: 'p2'}]

Current Behavior

// delete all patients that belongs to doctor id 1 where the patient name is p2
doctorRepo.patients(1).delete({name: 'p2'});

The target p2 gets deleted, but patient p1 is no longer traversable as the through model Appointment{doctorId: 1, patientId: 1} gets deleted too.

Expected Behavior

Should delete the related through models instead of deleting all throughs that belong to the same source key.

Acceptance Criteria

• redesign the delete method in hasManyThrough repository
• add tests to verify the new behaviour