Fix flush with reverse m2m changed signal

[k4rl85]

Fix issue #377

[clintonb]

Can you explain why this is necessary, please? Is there anyway to do this without a database call?

[k4rl85]

Hi Clinton, first of all thx for the review on this patch.

The problem here is m2m_changed handling from Django.

If i execute the following code

from my_app.models import MyUserModel
from waffle.models import Flag

flag = Flag.objects.first()
user = MyUserModel.objects.first()

flag.users.add(user) # all fine, cache is flushed without errors

user.flag_set.add(flag) # crash

the last line will crash because the instance received by flag_membership_changed is a MyUserModel instance instead of a Flag instance.

The only solution that i can see when instance is of type user/group is to retrieve all the flag involved and flush them like i do here

The reason why i query the db is that is not possible to guarantee that the flag involved are still accessible from instance when flag_membership_changed is executed.

Let me know if you want some of this explanation in the code.

I have added 2 tests to ensure that the m2m_changed signal is tested when the change are performed from the User/Group instance

[k4rl85]

I have changed the code to use reverse instead of try/except to be more explicit of what is happening here

[clintonb]

Thanks for the explanation. Would you mind adding a comment with a version of this explanation, too?

Also, instead of checking for a kwarg, what if you checked: if isinstance(instance, get_waffle_flag_model())? That seems a bit safer than relying on the kwarg.

[k4rl85]

I have applied the required changes.

[clintonb]

Thanks for the explanation. Would you mind adding a comment with a version of this explanation, too?

Also, instead of checking for a kwarg, what if you checked: if isinstance(instance, get_waffle_flag_model())? That seems a bit safer than relying on the kwarg.

[k4rl85]

Thx @clintonb for the merge.

What are the policy for this repository to have a new release?

Unfortunately this bug impact my production code and i would avoid if possible to install django-waffle from github.

[clintonb]

@k4rl85 new release is out.