Just noticed this happen and I found it strange. I have two models that should be comprised entirely of abstract ancestors. Something like this:

class CoreEntry(models.Model):
    title = models.CharField(...)
    slug = models.SlugField(...)

    class Meta:
        abstract = True

class ContentEntry(models.Model):
    content = models.TextField(...)

    class Meta:
        abstract = True

class ExcerptEntry(models.Model):
    excerpt = models.TextField(...)

    class Meta:
        abstract = True

# a few more abstract models here, then the actual models I'm using

class Post(CoreEntry, ContentEntry, ExcerptEntry, ...):
    pass

class Page(CoreEntry, ContentEntry, ExcerptEntry, ...):
    pass

Nothing weird so far. However, I just realized that I forgot to make ExcerptEntry an abstract base class. So Django created a separate table and a foreign key from Post and Page pointing to ExcerptEntry (which is to be expected).

The weird thing is that Django never created an ID field on Page and Post. Instead it used the FK to the ExcerptEntry as the Primary Key field.

There's probably a reason Django behaves this way but I'm not really sure I get it.