#17579: Delete cascade fails for nullable field
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Reporter: amichair | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.3
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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In an app with models A, B and C, with foreign key fields from B to A and
from C to B, the fields are defined with the default on_delete CASCADE,
and the field of C (pointing to B) may be null; if A is deleted, B is
deleted as well, but a C pointing to the deleted B is not deleted as
expected - instead, a constraint violation exception is raised.
I traced this back to django/db/models/deletion.py, in the definition of
Collector.add (line 103 in django 1.3). There is a comment explaining why
dependencies do not need to be processed for nullable fields, however this
is exactly what causes the bug - the dependency order is wrong and so the
constraint is violated. If the 'and not nullable' condition is removed, it
seems to work ok. Perhaps the code fails to consider the possibility that
a field can be both nullable and with cascading deletes, for different
purposes (i.e. a field is optional, but if it is set, must be a proper
delete-cascading foreign key). Also the bug might be with whomever passes
the nullable parameter to this method (i.e. CASCADE) - I don't know enough
about the django design to tell.
The expected behavior would be for all foreign keys defined with on_delete
CASCADE (the default behavior) to have the models deleted in a cascading
manner, regardless of whether the fields are defined as being nullable or
not.
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Ticket URL: <https://code.djangoproject.com/ticket/17579>
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