#36344: Accessing a related field after using defer() or only() unexpectedly
overwrites unsaved state on model instances
-------------------------------------+-------------------------------------
Reporter: Erick | Type:
| Uncategorized
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
When working with a model instance that was originally loaded with
`only()` or `defer()`, accessing any related object not cached on the
instance reverts unrelated fields to the value currently present in the
database.
This toy example reproduces the behavior:
{{{
class SomethingElse(Model):
...
class Something(Model):
foo = CharField(max_length=2)
bar = CharField(max_length=2)
other = ForeignKey(SomethingElse)
Something.objects.create(
foo="ab",
bar="cd",
other=SomethingElse.objects.create()
)
something = Something.objects.only("bar").first()
something.foo = "ef"
assert something.foo == "ef"
something.other # Accessing this deferred related field triggers the
behavior
assert something.foo == "ef" # AssertionError
}}}
This is surprising behavior, because application code may interact with
model instances without knowing how they were loaded from the database.
--
Ticket URL: <https://code.djangoproject.com/ticket/36344>
Django <https://code.djangoproject.com/>
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