#37034: Improve writing migrations how-to add through field on a ManyToManyField
-------------------------------------+-------------------------------------
Reporter: Clifford | Owner: Clifford Gama
Gama |
Type: Bug | Status: assigned
Component: | Version: dev
Documentation | Keywords: migrations,
Severity: Normal | ManyToManyField, through
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
1. Inaccurate description of how Django handles this change: The section
states that "the default migration will delete the existing table and
create a new one". This is not accurate. Django
[https://github.com/django/django/blob/d61f33f03b3177afdf1d76153014bad4107b1224/django/db/backends/base/schema.py#L894
refuses to apply a migration] when `through=` is added/changed on an
existing `ManyToManyField`.
2. The through model example does not accurately reflect the database: The
current example uses `on_delete=DO_NOTHING` and `models.UniqueConstraint`,
whereas Django's auto-generated through tables use `CASCADE` and
`unique_together`. In other words, future model states will not bear a
correct representation of what's in the db.
3. The example can be simplified by setting `Meta.db_table` on the new
through model to match the existing table name, eliminating the need for a
`RunSQL` rename operation.
The section also suggests using `sqlmigrate` or `dbshell` to find the
existing table name, which is indirect. The simplest approach is to
inspect `field.through._meta.db_table` before modifying the field.
Incidentally for the case where db_table remains the same, #36803 may
allow us to suggest a simpler alternative in the docs, one that does not
need to use `SeparateDatabaseAndState`.
--
Ticket URL: <https://code.djangoproject.com/ticket/37034>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion visit
https://groups.google.com/d/msgid/django-updates/0107019d8b9a3938-21ed26f9-4366-448a-b2a4-dcdb9cc95dce-000000%40eu-central-1.amazonses.com.
