ManyToManyField with through on an abstract model

Question

Got an interesting one here.. I've shortened the models to make it easier to comprehend..

class Participant(Person):
    passport_number = models.IntegerField(verbose_name=_('Passport Number'), db_column=u'PassportNumber')

    class Meta:
        db_table = u'Participant'

class Journey(BaseModel):
    participants = models.ManyToManyField(Participant, related_name='%(app_label)s_%(class)s_participants', through=u'ParticipantJourney')

    class Meta:
        abstract = True

class PlaneJourney(Journey):
    flight_number = models.CharField(max_length=16, verbose_name=_('Flight Number'), db_column=u'FlightNumber')

    class Meta:
        db_table = u'PlaneJourney'

class ParticipantJourney(BaseModel):
    participant = models.ForeignKey(Participant, verbose_name=_('Participant'), db_column=u'ParticipantId')

    journey_content_type = models.ForeignKey(ContentType, related_name='journey_content_type')
    journey_object_id = models.PositiveIntegerField()
    journey = generic.GenericForeignKey('journey_content_type', 'journey_object_id') # models.ForeignKey(Journey, verbose_name=_('Journey'), db_column=u'JourneyId')

    payment_content_type = models.ForeignKey(ContentType, related_name='payment_content_type')
    payment_object_id = models.PositiveIntegerField()
    payment = generic.GenericForeignKey('payment_content_type', 'payment_object_id') # models.ForeignKey(Payment, verbose_name=_('Payment'), db_column=u'PaymentId')

    class Meta:
        db_table = u'ParticipantJourney'

The ParticipantJourney model links a participant to a journey, now a journey is abstract because it can be made by any number of different methods of transport each of which will have their own respective fields. I think this setup is correct but I'm getting the following error message:

Error: One or more models did not validate: kandersteg.planejourney: 'participants' is a manually-defined m2m relation through model ParticipantJourney, which does not have foreign keys to Participant and PlaneJourney

I need to keep the manual definition of the link table so I can also link a payment to said journey so I don't really know where to go next with this, if anyone could shed some light I would be really greatful!

Cheers, Alex

Answer 1

Django does not consider generic foreign keys as the required foreign keys in through model, and currently there is no way to define a ManyToMany relationship with through in an abstract base class.

However, there is a feature request in django (ticket #11760), that enables you to also use %(class)s in through field, e.g. by using through='Pariticipant_%(class)s' in Journey model, and then you must create the throught table manually for every child of Journey. But as I said, its just an open feature request yet.