override_model_field.py

from django.db.models.fields.related import ReverseSingleRelatedObjectDescriptor

def override_model_field(model, name, column, field):
    """Force override a field in a Django Model.
    Usage: override_model_field(
        MyModel, models.ForeignKey(OtherModel), 'other', 'other_id')
    :type model: django.db.models.base.ModelBase
    :type name: basestring
    :type column: basestring
    :type field: django.db.models.fields.Field
    """
    class FieldOverrideProxy(type(field)):
        # noinspection PyMissingConstructor
        def __init__(self, original_field):
            self.original_field = original_field
            self.new_field = field
            self.name = self.new_field.name = name
            self.attname = self.new_field.attname = column
            self.default = original_field.default
            self.new_field.verbose_name = (field.verbose_name or
                                           original_field.verbose_name)

        def __getattr__(self, item):
            try:
                return getattr(self.new_field, item)
            except AttributeError:
                return getattr(self.original_field, item)

    for i, f in enumerate(model._meta.fields):
        if f.name == name:
            field_proxy = FieldOverrideProxy(f)
            model._meta.fields[i] = field_proxy
            break
    else:
        raise TypeError('Model {!r} does not have a field {!r}.'
                        .format(model, name))
    model.add_to_class(name, ReverseSingleRelatedObjectDescriptor(field_proxy))

# Example usage:

from django.db import models

class ChildModel(models.Model):
    pass

class TheModel(models.Model):
    child = models.ForeignKey(ChildModel)

class ChildModelProxy(ChildModel):
    class Meta:
        proxy = True

class TheModelProxy(TheModel):
    class Meta:
        proxy = True

override_model_field(TheModelProxy, 'child', 'child_id',
                     models.ForeignKey(ChildModelProxy))