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 instance.

def get_instance_field(self, 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 instance.
        """
        try:
            field = super(ModelWithDynamicFieldMixin, self).get_instance_field(field_name)
        except AttributeError:
            # the "has_field" returned True but getattr raised... we have a DynamicField
            dynamic_field = self._get_dynamic_field_for(field_name)  # it's a model bound field
            dynamic_field = self.get_field(dynamic_field.name)  # we now have an instance bound field
            field = self._add_dynamic_field_to_instance(dynamic_field, field_name)

        return field