#35877: Documentation on "Changing a ManyToManyField to use a through model"
does
not respect database index
-----------------------+-----------------------------------------
Reporter: robwa | Type: Bug
Status: new | Component: Documentation
Version: | Severity: Normal
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-----------------------+-----------------------------------------
When creating a `manytomany` field with an implicit `through` table, a
unique index is created on the database side for the `through` table for
the two foreign keys. This fact is not respected in the documentation on
[https://docs.djangoproject.com/en/dev/howto/writing-
migrations/#changing-a-manytomanyfield-to-use-a-through-model Changing a
ManyToManyField to use a through model]. When using the proposed code,
Django loses track of the index.
The addition of a simple options statement in the migration may be
sufficient:
{{{
// ...
operations = [
migrations.CreateModel(
name="AuthorBook",
// ...
options={
"unique_together": {("author", "book")},
},
),
]
}}}
I have not been able to check comprehensively how different database
systems behave when interacting with Django and in the context of renaming
the table. SQLite retains the index, but it is not renamed. In the case of
PostgreSQL, there were later bugs in an implementation that were difficult
to find.
--
Ticket URL: <https://code.djangoproject.com/ticket/35877>
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