class ModelSerializationMixin(object):
    def as_dict(self, fields=None, exclude=None):
        """
        Return this model as a dictionary, replacing ForeignKey values with
        their pks.

        fields - The fields to return. If omitted, returns all fields (minus
                 excluded).
        exclude - The fields to return. If omitted, returns the fields from
                  `fields`.
        """
        data = {}
        for field in self._meta.fields:
            if exclude and field.name in exclude:
                continue

            if fields is not None and field.name not in fields:
                continue

            if hasattr(field, "get_foreign_related_value"):
                # If the field is a FK or such, get the foreign ID.
                value = field.get_foreign_related_value(self)[0]
            else:
                value = getattr(self, field.name)
            data[field.name] = value
        return data

class MyModel(models.Model, ModelSerializationMixin):
    my_field = models.CharField(max_length=100)
    my_fk = models.ForeignKey(OtherModel)

>>> mymodel_instance.as_dict()
{"my_field": "test", "my_fk": 4}