#18864: Django models __eq__ and __hash__ treat all unsaved model instances as
identical
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Reporter: fengb | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.4
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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When a model instance is still new and unsaved, it has no primary key.
Both __eq__ and __hash__ rely solely on _get_pk_val(), which means all
unsaved model instances are treated as though they are identical.
>>> b1 = models.Base()
>>> b2 = models.Base()
>>> b1 == b2
True
>>> hash(b1) == hash(b2)
True
For integrity, we should add a check for Python object identity since if
both instances are saved, they will end up with different PKs.
For hashing, we can use the default Python hashing to prevent large
collisions based on hash(None). We would also need to cache the hash so
that hash(instance) will be the same both before save and after save such
that:
>>> b = models.Base()
>>> s = set()
>>> s.add(b)
>>> b.save()
>>> b in set
True
This last case actually has an edge case that I'm not sure is solvable.
>>> b = models.Base.object.get(id=b.id)
>>> b in set
False
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Ticket URL: <https://code.djangoproject.com/ticket/18864>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
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