Add an --ignore-deps flag to squashmigrations

[Shai Berger]

In large, long-going projects -- the ones whose need for migration squashing is the greatest -- apps have a tendency to develop dependencies upon each other. This entails problems with migrations -- dependencies could be circular, or an app could be removed from the project but the migrations that hold dependencies on it still remain, and squashmigrations faces constraints that makes it very hard to do correctly. Current implementation often generates migrations that make no sense, with dependencies on several migrations from the same app.

As a partial solution, let's add a --ignore-deps flag to squashmigrations. When this flag is applied, the generated squashed migration will

• Only depend on the dependencies of the first migration in the range squashed
• Not include the operations that create or modify relation fields, if these fields refer to models from other apps (deletion can be included)

The idea is that such a migration will be correct, but not complete. Assuming the range of migrations includes the last migrations of the app, it will then be possible to add the missing pieces by a regular makemigrations; but the squashing will still include all the non-ellidable RunSQL and RunPython operations -- the squashmigrations --ignore-deps + makemigrations dance is proposed as a safer alternative to "migration bankruptcy", the practice of just removing all the migrations and stating afresh

On a side note: It may be safer to add an operation which enforces that it is indeed called with the last migration of the app in range, and maybe even automatically invokes the following makemigrations, but I think we should trust developers with the lower-level building bricks, which may be useful in other situations as well.

This follows from a ​discussion on the forum.

[Clinton Christian]

I'm working on this and will finalize a PR shortly.

[Sarah Boyce]

Hi Shai, thank you for raising this ticket
cc-ing a few people here in case they have further insights or opinions on this
Tentatively accepting as the discussion seems positive and I can see the value of the feature 👍

[Clinton Christian]

Filtering out multi-app FieldOperations is pretty straight forward, but should we mutate a ModelOperation to exclude multi-app foreign key references, or exclude the ModelOperation entirely?