Cannot pass id instead of related object to DjangoModelFactory

The problem

When I create an instance of a Django model, I can pass a related object directly, or its id.

article = Article.objects.create(author=author)
article = Article.objects.create(author_id=author.pk)

One would expect a Django model factory to work the same way, but it doesn’t. Everything breaks and there is no documentation regarding this use case.

Proposed solution

Ideally allow both strategies, but if not at least document this and explain how to do it.

[rbarrois]

Basically, factory_boy doesn’t care whether it’s working with IDs, instances, or anything else. It takes your declarations, computes the values (following subfactories, etc.), and sends the resulting payload as arguments and keyword arguments to the class you’ve provided for a model.

If you want a factory that will use a passed-in author_id if provided, but uses a SubFactory to create an Author if needed, you could use the following:

class ArticleFactory(factory.django.DjangoModelFactory):
    class Meta:
        model = Article

    author = factory.Maybe("author_id", no_declaration=factory.SubFactory(AuthorFactory))

This way:

• If you simply call the factory, since no author_id has been provided, the Maybe declaration will use its “else” branch and call AuthorFactory to get an instance;
• If you pass in an author_id (e.g ArticleFactory(author_id=42)), the Maybe declaration will be skipped and only the author_id will be passed to Article.objects.create().
• If you pass both author_id and author, the result will depend on Django’s implementation.

The _adjust_kwargs function is useful in more complex cases, but is only called once the declarations have been resolved; it can’t prevent calling an AuthorFactory earlier on, which might be an issue for your code.

[rbarrois]

@MRigal Indeed. Feel free to open a pull request with that prose in there 😉