Inheriting OneToOneField results in TypeError: __init__() got an unexpected keyword argument 'related_name'

Question

I'm using Django 2.2, Python 3.7 and my attempt was to set up some common kwargs (say, on_delete and related_name ) to the OneToOneField , by a sub class like the following

class MyOneToOneField(models.OneToOneField):
    def __init__(self, to):
        super().__init__(to, on_delete=models.CASCADE, related_name='extra_data')

And the model class is like

class UserExtraData(models.Model):
    entity = MyOneToOneField(USER_MODEL)

However, when running makemigrations , it results in:

TypeError: Couldn't reconstruct field entity on UserExtraData: init () got an unexpected keyword argument 'related_name'

(I tried removing all other fields, so I am pretty sure this is the field that caused the issue).

How can I fix this?

Answer 1

To fix this type of problem, you can change your project settings file. In the settings file, after INSTALLEDD_APPS :

AUTH_USER_MODEL= '<your app name>.<class name>'

Answer 2

if you tend to specify the related_name in MyOneToOneField not to use a arg passed to it. I prefer

class UserExtraData(models.Model):
    entity = models.OneToOneField(USER_MODEL, on_delete=models.CASCADE, related_name='extra_data')

or you could try:

class MyOneToOneField(models.OneToOneField):
    def __init__(self, to, *args, **kwargs):
        super().__init__(to, *args, **kwargs)
        # do others

class UserExtraData(models.Model):
    entity = MyOneToOneField(USER_MODEL, on_delete=models.CASCADE, related_name='extra_data')

Answer 3

Shouldn't it be like this according to doc 1

class MyOneToOneField(models.OneToOneField):
    def __init__(self, to, *args, **kwargs):
        kwargs['related_name'] = 'extra_data'
        kwargs['on_delete'] = models.CASCADE
        super().__init__(to, *args, **kwargs)

Answer 4

Found out it was because Django was going to clone the field for some reason, after digging a little to Django's suggested by the exception traceback:

  File ".../django/db/migrations/state.py", line 414, in from_model
    fields.append((name, field.clone()))
  File ".../django/db/models/fields/__init__.py", line 493, in clone
    return self.__class__(*args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'related_name'

So take a look at Field.clone , whose source is

    def clone(self):
        """
        Uses deconstruct() to clone a new copy of this Field.
        Will not preserve any class attachments/attribute names.
        """
        name, path, args, kwargs = self.deconstruct()
        return self.__class__(*args, **kwargs)

apparently the TypeError is due to kwargs contains related_name and on_delete , which is returned from deconstruct .

Then, the docstring of deconstruct says it is to "Return enough information to recreate the field as a 4-tuple:", so I guess what I missed is that I should have overridden it to provide a kwargs that does not contain on_delete and related_name , but only to .

So it finally works like this:

class MyOneToOneField(models.OneToOneField):
    def __init__(self, to):
        super().__init__(to, on_delete=models.CASCADE, related_name='extra_data')

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        return name, path, args, {'to': kwargs['to']}