Django model with 2 foreign keys from the same table

Question

I wanted a Django model with 2 foreign keys from the same table. It's an event table which has 2 columns for employees: the 'actor' and the 'receiver'. But I get this error: Error: One or more models did not validate: tasks.task: Intermediary model TaskEvent has more than one foreign key to Employee, which is ambiguous and is not permitted. Is there a better way to model this? Thanks

I think I'm going to add a TaskEvent_to_Employee table. There will be two records in it, one for each of the two employees related to each TaskEvent. Anyone know an easier workaround?

Answer 1

I haven't done this yet, but I used inspectdb to generate the models.py file from an existing DB that does exactly that - this is what inspectdb threw back, so it should work:

creator = models.ForeignKey(Users, null=True, related_name='creator')
assignee = models.ForeignKey(Users, null=True, related_name='assignee')

Hope that works for you - if it doesn't I am going to have a problem too.

Answer 2

The fact that two columns are part of one table implies that the two fields are related, therefor to reference them individually is not ideal. The ForeignKey of your model should be the primary key of the table you are referencing:

event = models.ForeignKey('event')

You would then reference the columns as such:

foo.event.actor
foo.event.receiver

If you wish you could also change the way your class/model references the foreign attributes with properties. In your class you would do the following:

@property
def actor(self):
  return self.event.actor
@property
def receiver(self):
  return self.event.receiver

This would allow you to then call foo.actor and foo.receiver but I believe the longer, foo.event.actor would be more pythonic

Answer 3

I think what you're looking for is the related_name property on ForeignKeyFields. This will allow you to reference the same table, but give django special names for the relationship.

More Info:

• http://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ForeignKey.related_name
• http://docs.djangoproject.com/en/dev/topics/db/queries/#backwards-related-objects
• http://www.djangoproject.com/documentation/models/m2m_and_m2o/

Answer 4

From the error message, it sounds like you're trying to put two foreign keys to the same object on an intermediary table used via the through argument to ManyToManyField, the documentation for which states:

When you set up the intermediary model, you explicitly specify foreign keys to the models that are involved in the ManyToMany relation. This explicit declaration defines how the two models are related.

There are a few restrictions on the intermediate model:

• Your intermediate model must contain one - and only one - foreign key to the target model (this would be Person in our example). If you have more than one foreign key, a validation error will be raised.
• Your intermediate model must contain one - and only one - foreign key to the source model (this would be Group in our example). If you have more than one foreign key, a validation error will be raised.