Mock instance isn't using property code

I have some Django models I need some unit test coverage on and in doing so I mock out some instances of them. Here is an example class I want coverage of

class MyMixin(object):
    @property
    def sum(self):
        return field_one + field_two + field_three

class MyModel(Model, MyMixin):

    field_one = IntegerField()
    field_two = IntegerField()
    field_three = IntegerField()

So I can mock out an instance of it like so:

mock_inst = mock.Mock(spec=MyModel, field_one=1, field_two=2, field_3=3)

However when I go to execute mock_inst.sum, it doesn't execute the code properly, it gives me something from the mock class. Shouldn't it execute the code given the spec in the instance?