Allow migrations.Operation to control their output.

[loic84]

Currently MigrationWriter controls the output of migrations and to do so it renders the various operations with a generic format.

The problem is operations such as CreateModel end up writing all the fields on a single line, which makes the migration file difficult to review.

I propose that Operations should control their own output.

I had a go at this in ​https://github.com/django/django/pull/1681.

[Andrew Godwin]

Me and loic84 discussed this on IRC a while back; I'm not a fan of how this currently looks (it makes the operations very hard to read and is confusing logic with presentation in a way) but it's a nice idea in principle.

[loic84]

I've cleaned up the code a bit by introducing a MigrationFormatter class, but ultimately there isn't so much we can do; presentational logic in Python is quite verbose. Also the way the serializer is implemented requires formatting as we go, fixing that would require a significant refactor.

If we move to a better template system with white spaces handling (Jinja2), CreateModel.serialize() could be shrunk significantly.

IMO being able to review the output of CreateModel is sufficiently critical to justify an imperfect CreateModel.serialize() method.

[loic84]

I've reworked the code in a way that doesn't affect the Operation classes anymore, the changes are now contained in migrations/writer.py.

[Loic Bistuer <loic.bistuer@…>]

In 374faa472150e2e1fb70224a764b82d3d51994f7:

Fixed #21323 -- Improved readability of serialized Operation.

[Andrew Godwin <andrew@…>]

In e8d4aed3b97493a050f69dca1f6562991d5c511b:

Merge pull request #1681 from loic/migrations.format

Fixed #21323 -- Improved readability of serialized Operation.