[Django]-Django models: mutual references between two classes and impossibility to use forward declaration in python

You can find the solution in the docs:

If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:

class Car(models.Model):
    manufacturer = models.ForeignKey('Manufacturer')
    # ...

class Manufacturer(models.Model):
    # ...