No related save for non inline

[philipn]

This fixes #397. Based on tests and work by @joshbohde.

Beyond simply improving performance, saving related fields when they don't need to be saved can cause undesirable things to happen (signal handlers, etc).

[buzzeante]

@philipn Love the patch. This same issue was bugging me. I've changed slightly using also obj._state_adding in save_related.

You can check it here https://github.com/TwoApart/django-tastypie/tree/no_related_save_for_non_inline

Cheers,
Javi

[georgedorn]

I've merged this manually (the code's drifted too much in the last 6 months to do it automatically) but I'm pushing to a new branch pending a test for a specific regression I'm worried about:

If you are posting a change to an explicit through model, does that change get saved? The relationship model already exists, so we're not adding it, but we are changing the extra data stored within that model. I tried to find a test that exercised this, but we don't appear to currently have such a test, thus my trepidation at pushing this to master.

The manual merge changeset is here: toastdriven@e4a43ed

[numan]

@georgedorn I added a test for the senario you described in d11d224.

The test is failing. I'm not sure about the best way to fix it.

[georgedorn]

One approach I can think of is to check to see whether the relationship model has additional data (probably by interrogating the relationship manager) and if so, save it. Otherwise, it's safe to skip it.

I'm not sure if it's the best approach, but if it makes the test pass and it works for the 80% of cases where it's just a normal ManyToManyField, I'm fine with that.