Add management of dynamic fields: if a normal field cannot be retrieved,
        check if it can be a dynamic field and in this case, create a copy with
        the given name and associate it to the model.

def get_class_field(cls, field_name):
        """
        Add management of dynamic fields: if a normal field cannot be retrieved,
        check if it can be a dynamic field and in this case, create a copy with
        the given name and associate it to the model.
        """
        try:
            field = super(ModelWithDynamicFieldMixin, cls).get_class_field(field_name)
        except AttributeError:
            # the "has_field" returned True but getattr raised... we have a DynamicField
            dynamic_field = cls._get_dynamic_field_for(field_name)
            field = cls._add_dynamic_field_to_model(dynamic_field, field_name)

        return field