#702: document that ManyToMany fields can't be in unique_together
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Reporter: hugo | Owner: nobody
Type: New feature | Status: new
Milestone: | Component: Documentation
Version: SVN | Severity: Normal
Resolution: | Keywords: model-validation
Triage Stage: Accepted | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Changes (by carljm):
* component: Database layer (models, ORM) => Documentation
Comment:
I don't think we should allow m2m fields to be listed in unique_together.
In practical terms, this is cross-model validation between two models (the
main model and the m2m through model, which may be implicit). This is
quite challenging and different from a normal unique_together, because the
validation code for the main model runs before the m2m models have even
been saved, so implementing this requires either listening to an
m2m_changed signal, or putting the validation code onto the m2m through
model. In a regular unique_together, there is an actual UNIQUE constraint
created in the database covering the unique_together fields; with an m2m
unique-together it could only be enforced by the model-validation code.
IOW, the underlying implementation would be quite different from the
implementation of a normal non-m2m unique_together, and those differences
would be bound to leak up into application code.
Also, the meaning of listing an m2m field in a unique_together is not
self-evident, as demonstrated by the IRC conversation in the description
on this ticket. If "charfield" is unique_together with "m2mfield", does
that mean charfield must be unique with respect to each individual value
of m2mfield, or with respect to the combination of m2mfield values?
Clearly in the `FlatPage` case the former is desired, but I can certainly
imagine cases in which the latter would be desired.
Lastly, there's already a sort of precedent here - `ManyToManyFields`
cannot be set as "unique" (presumably because it would result in similar
semantic questions and implementation challenges). It seems pretty
reasonable that a field type which can't be set as unique also can't be
included in unique_together.
In light of all this complexity, I think validation of uniqueness related
to m2m fields should be left up to the application developer, via signals
or an explicit through model, rather than built in to the ORM. I'm
changing the component here to documentation, and the summary to reflect
that we need to document that m2m fields can't be included in
unique_together.
--
Ticket URL: <https://code.djangoproject.com/ticket/702#comment:13>
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