#20257: QuerySet that prefetches related object with a ManyToMany field cannot
be
pickled.
-------------------------------------+-------------------------------------
Reporter: bryced | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.5
(models, ORM) | Resolution:
Severity: Release blocker | Triage Stage: Accepted
Keywords: ORM, pickle, cache | Needs documentation: 0
Has patch: 0 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
-------------------------------------+-------------------------------------
Comment (by akaariai):
The problem is that fields for automatic models (for example the
intermediary m2m table) can't be pickled in 1.5. In master this is fixed
by introducing Field.reduce. In 1.4 the field wasn't used directly, only
the field.name was used so there was no problems. So, to fix this, one of
the following three things needs to be done:
- alter `QuerySet.__getstate__` and `__setstate__` to do something
similar that sql.query:Query's state methods do. That is, store the
field's name in getstate and restore back the real field instance in
setstate.
- remove direct storage of the field in QuerySet
- backpatch Field.reduce changes from master
I think the Field.reduce changes are too risky to backpatch, and I assume
there was a good reason to store the field directly in QuerySet instead of
field.name. So, changes to getstate and setstate seem like the best choice
to me.
--
Ticket URL: <https://code.djangoproject.com/ticket/20257#comment:2>
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