Django Model Inheritance And Foreign Keys

Question

Basically, I have a model where I've created a superclass that many other classes share, and then each of those classes has some unique features that differ from each other. Let's say class A is the superclass, and class B, C, and D are children of that class.

Both class B and class C can have multiples of class D, however I've seen that it's best to put the foreign key relationship in class D, which then refers to its parent class. Now in other languages, I could simply say it has a ForeignKey relationship to class A, and then the language recognizes the classes' true type. However, I don't think that's how it works with Python.

What's the best recommended way of pursuing this issue?

EDIT: Here is roughly what I mean...

class A(models.Model):
    field = models.TextField()

class B(A):
    other = <class specific functionality>

class C(A):
    other2 = <different functionality>

class D(A):
    #I would like class D to have a foreign key to either B or C, but not both.

Essentially, class B and class C both have multiple class D's. But a particular class D only belongs to one of them.

Answer 1

One way to do this is to add an intermediate class as follows:

class A(Model):
    class Meta(Model.Meta):
        abstract = True
    # common definitions here

class Target(A):
    # this is the target for links from D - you then need to access the 
    # subclass through ".b" or ".c"
    # (no fields here)

class B(Target):
    # additional fields here

class C(Target):
    # additional fields here        

class D(A):
    b_or_c = ForeignKey(Target)
    def resolve_target(self):
        # this does the work for you in testing for whether it is linked 
        # to a b or c instance
        try:
            return self.b_or_c.b
        except B.DoesNotExist:
            return self.b_or_c.c

Using an intermediate class (Target) guarantees that there will only be one link from D to either B or C. Does that make sense? See http://docs.djangoproject.com/en/1.2/topics/db/models/#model-inheritance for more information.

In your database there will be tables for Target, B, C and D, but not A, because that was marked as abstract (instead, columns related to attributes on A will be present in Target and D).

[Warning: I have not actually tried this code - any corrections welcome!]

Answer 2

I see a problem here:

class D(A):
    #D has foreign key to either B or C, but not both.

Can't do it. You'll have to add both because in SQL columns must be defined exactly.

Also even though inherited models like you have compile with syncdb - they don't seem to behave like you would expect - at least I could not make them work. I can't explain why.

This is how FK works in Django

class A(models.Model):
    a = models.CharField(max_length=5)

class B(models.Model):
    a = model.ForeignKey(A, related_name='A')
    b = models.CharField(max_length=5)

class D(models.Model):
    a = model.ForeignKey(A, related_name='A')
    parent = model.ForeignKey(B, related_name='D')

this way you can effectively have multiples of D in B.

Inheritance in models (e.g. class B(A)) doesn't work as I would expect it. Maybe someone else can explain it better.

Take a look at this doc page. It's about many-to-one relationship in django.

b = B()
b.D_set.create(...)

Answer 3

From the Django Docs:

For example, if you were building a database of "places", you would build pretty standard stuff such as address, phone number, etc. in the database. Then, if you wanted to build a database of restaurants on top of the places, instead of repeating yourself and replicating those fields in the Restaurant model, you could make Restaurant have a OneToOneField to Place (because a restaurant "is a" place; in fact, to handle this you'd typically use inheritance, which involves an implicit one-to-one relation).

Normally, you would just have Restaurant inherit from Place. Sadly, you need what I consider a hack: making a one-to-one reference from subclass to superclass (Restaurant to Place)

Answer 4

You could also do a generic relation http://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#id1 and check the types to constrain it to B or C when setting or saving. This is probably more work than figuring out the direct reference, but might feel cleaner.