#29085: Possible data loss on .save() with unsaved related model
-------------------------------------+-------------------------------------
Reporter: Jonas Haag | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | 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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Comment (by Ian Foote):
I've been looking into this a bit and I don't see how we can do the right
thing here. The problem I see is that the {{{Child}}} model has two
sources of truth about the {{{Parent}}} instance and we don't know which
we can trust. These sources are {{{Child.parent_id}}} and
{{{Child.parent.id}}}. When we save the {{{Parent}}} instance,
{{{Child.parent.id}}} changes, but {{{Child.parent_id}}} does not and I
don't see a way to push the new {{{pk}}} to {{{Child.parent_id}}} behind
the scenes. One can reassign {{{Child.parent}}} explicitly (as noted
above), which ensures {{{Child.parent_id}}} is correct.
I looked into adjusting {{{ForeignKey.pre_save}}} to read from
{{{Child.parent}}} and {{{Model.save}}} to avoid clearing
{{{Child._state.fields_cache}}}, but this either didn't work or broke many
other Django tests.
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Ticket URL: <https://code.djangoproject.com/ticket/29085#comment:7>
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