[Django] #35508: Add an --ignore-deps flag to squashmigrations

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------
Reporter: Shai | Owner: nobody
Berger |
Type: New | Status: new
feature |
Component: | Version: dev
Migrations | Keywords: squash
Severity: Normal | squashmigrations
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 1 | Needs tests: 1
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
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 [https://forum.djangoproject.com/t/idea-make-squash-
migrations-no-deps/23986 discussion on the forum].
--
Ticket URL: <https://code.djangoproject.com/ticket/35508>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------

Reporter: Shai Berger | Owner: Clinton
| Christian
Type: New feature | Status: assigned
Component: Migrations | Version: dev
Severity: Normal | Resolution:
Keywords: squash | Triage Stage:
squashmigrations | Unreviewed
Has patch: 0 | Needs documentation: 1

Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Clinton Christian):

* owner: nobody => Clinton Christian
* status: new => assigned

--
Ticket URL: <https://code.djangoproject.com/ticket/35508#comment:1>

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------
Reporter: Shai Berger | Owner: Clinton
| Christian
Type: New feature | Status: assigned
Component: Migrations | Version: dev
Severity: Normal | Resolution:
Keywords: squash | Triage Stage:
squashmigrations | Unreviewed
Has patch: 0 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Clinton Christian):

I'm working on this and will finalize a PR shortly.
--
Ticket URL: <https://code.djangoproject.com/ticket/35508#comment:2>

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------
Reporter: Shai Berger | Owner: Clinton
| Christian
Type: New feature | Status: assigned
Component: Migrations | Version: dev
Severity: Normal | Resolution:

Keywords: squash | Triage Stage: Accepted
squashmigrations |

Has patch: 0 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Sarah Boyce):

* cc: Mariusz Felisiak, David Wobrock, David Sanders, Andrew Godwin
(added)
* stage: Unreviewed => Accepted

Comment:

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 👍
--
Ticket URL: <https://code.djangoproject.com/ticket/35508#comment:3>

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------
Reporter: Shai Berger | Owner: Clinton
| Christian
Type: New feature | Status: assigned
Component: Migrations | Version: dev
Severity: Normal | Resolution:
Keywords: squash | Triage Stage: Accepted
squashmigrations |

Has patch: 1 | Needs documentation: 1

Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Clinton Christian):

* has_patch: 0 => 1

--
Ticket URL: <https://code.djangoproject.com/ticket/35508#comment:4>

[Django]

#35508: Add an --ignore-deps flag to squashmigrations
-------------------------------------+-------------------------------------
Reporter: Shai Berger | Owner: Clinton
| Christian
Type: New feature | Status: assigned
Component: Migrations | Version: dev
Severity: Normal | Resolution:
Keywords: squash | Triage Stage: Accepted
squashmigrations |
Has patch: 1 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Clinton Christian):

Filtering out multi-app `FieldOperation`s is pretty straight forward, but
should we mutate a `ModelOperation` to exclude multi-app foreign key
references, or exclude the `ModelOperation` entirely?
--
Ticket URL: <https://code.djangoproject.com/ticket/35508#comment:5>