Understanding Django Model Inheritance

I'm having trouble understanding why some properties exist in some models, so I hope someone can explain them here.

Here's a basic example:

class Alpha(models.Model):
    one = models.PositiveIntegerField()

class Bravo(Alpha):
    two = models.PositiveIntegerField()

class Charlie(Alpha):
    three = models.PositiveIntegerField()

"bravo" in dir(Charlie())


How is it that an instance Charlie

has a property bravo

? Is there a reason for this? Did I miss something?

More importantly, why is it stopping me from adding a property bravo

to the class Charlie


class Charlie(Alpha):
    three = models.PositiveIntegerField()
    bravo = models.TextField()

ValueError: Cannot assign "''": "Charlie.bravo" must be a "Bravo" instance.



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2 answers

The inheritance interaction introduces links between the child model and each of its parents (via the automatically generated OneToOneField).

In your case, this means that Bravo Alpha is named "bravo" by definition.

See. Inheriting multiple tables and Inheritance and reverse relations documentation.



As far as I can tell, this is a consequence of the fact that the parent class has an implicit foreign key for all of its descendants, which in turn is inherited by the children. This probably shouldn't actually happen, but there can be no way to prevent it.

One possibility might be to explicitly set the parent link to Bravo to be related_name

something other than "bravo" so that you can reuse that name in Charlie.

class Bravo(Alpha):
    two = models.PositiveIntegerField()
    alpha = models.OneToOneField(Alpha, parent_link=True, related_name='not_bravo')




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