Django update_or_create (get part) using related object as kwarg

Question

I'm doing an update_or_create queryset operation, which internally uses the get queryset operation. I've read through the documentation on the update_or_create queryset method but I'm having some difficulty in understanding the parts relating to foreign keys and using objects as kwargs.

Consider an example where I have models Book and Chapter and I do something like this:

book = Book.objects.get(...)

kwargs = {'book': book, 'name': 'Chapter 3'}
defaults = {'text': '...'}
Chapter.objects.update_or_create(defaults=defaults, **kwargs)

Now the kwargs will be used in the 'get' method to check whether that chapter exists. But, since book is an object, how does Django know whether there is a chapter with a "matching" book? Does it check whether all fields of the book object match? Does it check only the unique field? Only the primary key of the book? If I have two books with the same fields but different primary key, could they match? Is it good practice to use an object in the kwargs in this way, or do I risk getting integrity error problems? Thank you.

Answer 1

In the context, I am assuming that Chapter has a FK to Book.

In that chase, Django will use the field ID or, if you changed the default setup, the primary key from the book instance to match. It is the same as doing:

Chapter.objects.update_or_create(defaults=defaults, book=book.id)

So you have have hundreds of books with the exact same fields except the primary key, django will know how to handle it.