Django custom field gives an error when editing in admin

Question

I'm trying to implement a custom Django field that would keep in DB a name of one of my model classes (e. g. "Cat", "Dog", "Restaurant", "SuperPrivateCommercialDataPiece", etc.) and return the class object when requested:

class Cat(models.Model):
    ...

class SomeDataPiece(models.Model):
    relatedTo = MyGloriousFieldType(null=True)

...

newPiece = SomeDataPiece()
newPiece.relatedTo = Cat
print newPiece.relatedTo # should print <class 'myproj.myapp.models.Cat'>

And I've really made this. I've subclassed models.Field, set __metaclass__, etc.:

class MyGloriousFieldType(models.Field):

    description = "No description, just gloriosity."

    __metaclass__ = models.SubfieldBase

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 40
        super(BlockTypeField, self).__init__(*args, **kwargs)

    def db_type(self, connection):
        return 'char(40)'

    def to_python(self, value):
        if not value:
            return None
        elif inspect.isclass(value) and issubclass(value, models.Model):
            return value
        elif hasattr(myproj.myapp.models, value):
            return getattr(myproj.myapp.models, value)
        else:
            raise ValidationError("Invalid model name string")

    def get_db_prep_value(self, value, **kwargs):
        if inspect.isclass(value) and issubclass(value, models.Model):
            return value.__name__
        else:
            # it is actually never raised
            raise ValidationError("Invalid value, Model subclass expected")

    def value_to_string(self, instance):
        value = self._get_val_from_obj(obj)
        return self.get_prep_value(value)

And in the code above it works just as expected. I've also registered some of my models containing such fields in admin, and now I can even create such objects; text input is created for MyGloriousFieldType fields.

The crisis begins when I try to edit already existing object with MyGloriousFieldType field. First, it fills the text field with "Cat object" instead of just "Cat". Second, when I change it back to "Cat" and click "save changes" it gives an error:

TypeError at /admin/myapp/somedatapiece/3/
  unbound method prepare_database_save() must be
  called with Cat instance as first argument (got 
  MyGloriousFieldType instance instead)

So, what am I doing wrong?

Answer 1

Well, I've figured that out. The problem is inside the very Django core. Fields just can't return Python objects containing some models.Field attributes like prepare_database_save, otherwise an error occures. My hack to solve that was to return tuple (MyClassObject, ) instead of just MyClassObject in to_python(). Odd, but what can I do... Maybe I'll try to post an issue somewhere on Django's Github page.