Easy to use natural keys from a tuple on meta

[Brian Faherty]

Having to implement two methods on two different objects in order to use natural keys is very cumbersome. I would think we could implement something on a models meta to take a tuple of field names for use in both the natural_key and get_by_natural_key methods. I implemented this in a way that would be overridable to keep compatible with the current methods but would allow for easy use in the future. Working code with this is here ​https://github.com/scrummyin/django/commit/7e562127c3a9610ae3ad9fab6855a005f4d4706f. Includes 6 more tests around a two new methods, with same name but on the model and manager. The old tests are still working with minimal changes. The minimal change is to use a new method to check for a natural_key instead of hasattr. The pull request branch has all

[Brian Faherty]

Sorry, I just realized I broke some other tests. After I broke them and took another look at them, I think I didn't break them, but there were already messed up. I have adjusted those tests for what I think is the proper use case. I believe in their original form the serializer output the models in the wrong order. I reorder the data in the test and got it to work. When I looked at the new order I realized that the new order reflected the decencies correctly. A membership had fk's to person and group and it should have come after those two objects. This is correctly reflected in the revised tests. With this git commit ​https://github.com/scrummyin/django/commit/ce6d437c72de478c681a934509a186cb242f84f7.