#23906: "ValueError: Found wrong number (0) of constraints" when migrating
backwards with from alter_unique_together migration.
-------------------------------------+-------------------------------------
Reporter: liavkoren | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.7
Severity: Normal | Resolution:
Keywords: | Triage Stage:
alter_unique_together migrations | Unreviewed
constraints valueError | Needs documentation: 0
Has patch: 0 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
-------------------------------------+-------------------------------------
Changes (by carljm):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
I would guess that reproducing for purposes of writing a tests shouldn't
be too hard with some raw SQL; it seems this is simply a matter of the
order in which columns are listed in a constraint. The question is whether
the suggested fix is actually correct. Does it break any existing tests?
How does migrations currently handle it if you change the ordering of a
`unique_together` without otherwise changing the constituent fields?
I notice there is an `ORDER BY ordinal_position` in at least the
PostgreSQL implementation of `get_constraints`, suggesting that the order
of fields in a constraint is relevant.
I think that's the basic question here: either a constraint with the same
fields in a different order is semantically the same constraint (in which
case your suggested fix seems right), or its semantically a different
constraint (in which case the current behavior is correct, and you just
have a mismatch between the state of your database and what migrations
thinks is the state of your database, which is something you need to deal
with locally).
I don't actually know which of those is correct in terms of the underlying
database semantics, though the fact that at least PG clearly tracks field
ordering within a constraint suggests the latter.
--
Ticket URL: <https://code.djangoproject.com/ticket/23906#comment:1>
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