#23906: "ValueError: Found wrong number (0) of constraints" when migrating
backwards with from alter_unique_together migration.
-------------------------------------+-------------------------------------
Reporter: liavkoren | Owner: nobody
Type: Bug | Status: closed
Component: Migrations | Version: 1.7
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage:
alter_unique_together migrations | Unreviewed
constraints valueError |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by ramiro):
FYI,
TL;DR: ''Review your schema as is exists before you start using Django
migrations. Chances are that the order of fields in the unique_together
constraint there doesn't match what the models.py says, and hasn't for a
long time.''
We found the same problem with a similar setup (Django 1.6 + South
migrations being ported to Django 1.7) and scenario.
What happened in our case is that the history of the model involved is
like this:
1) Model got created without unique_together constraint
2) A `unique_together = ('char_field', 'fk_field')` got added at some
point. South migtation syntax:
{{{
db.create_unique(u'app_outgoingpaymentcategory', ['char_field',
'fk_field_id'])
}}}
What happened is that stating from that point it seems South recorded
fields of the unique_together in the wrong order because all the frozen
representations of the model in every migration file created since then
contained the following excerpt:
{{{
'Meta': {'unique_together': "(('fk_field', 'str_field'),)", 'object_name':
'ModelName'},
}}}
So, when it came time to create a initial migration using Django 1.7
native migration framework, it picked the original `('char_field',
'fk_field')` read from the models.py file.
But when:
* No DB schema is created by Django 1.7 because it already exists and we
let `migrate` fake the initial Django migrations (or we pass `--fake` to
it) when migrating forward.
* We try to migrate back to migration zero (i.e. the state where the
South->Djnago 1.7 handover happened), Django detects the order of the
constraint as it exists in the DB doesn't match what it should be (i.e.
what models.py says)
and then the error message reported by the OP is shown.
--
Ticket URL: <https://code.djangoproject.com/ticket/23906#comment:3>
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