#33191: Avoid unnecessary clear of cached reference
-------------------------------------+-------------------------------------
Reporter: | Owner: nobody
callerbear |
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: 3.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Consider this case of ORM models "Parent" and "Child", where Child has a
foreign key reference to Parent (and the database can return generated IDs
following insert operations):
{{{
parent = Parent(name='parent_object')
child = child(parent=parent)
parent.save()
child.save()
print(child.parent.name)
}}}
The print statement will cause an unnecessary lazy read of the parent
object.
In the application where this behavior was first observed, the application
was creating thousands of parent and child objects using bulk_create().
The subsequent lazy reads occurred when creating log entries to record the
action, and added thousands of unwanted SELECT queries.
Closed ticket 29497 solved a problem with potential data loss in this
situation by essentially executing {{{child.parent_id = child.parent.pk}}}
while preparing the child object to be saved. However, when the child's
ForeignKeyDeferredAttrbute "parent_id" changes value from None to the
parent's ID, the child's internal cache containing the reference to
"parent" is cleared. The subsequent reference to child.parent then must
do a lazy read and reload parent from the database.
A workaround to avoid this lazy read is to explicitly update both the
"parent_id" and "parent" cache entry by adding this non-intuitive
statement:
{{{child.parent = child.parent}}}
after executing parent.save()
But it appears that a simple change could avoid clearing the cache in this
narrow case.
Within Model._prepare_related_fields_for_save(), replace
{{{setattr(self, field.attname, obj.pk)}}}
with
{{{self.__dict__[field.attname] = obj.pk}}}
This suggested code has -not- been tested.
This change would set the associated "parent_id" attribute to the desired
value without affecting the cache. In this spot of the code, "obj" is
currently set to the cached parent object that we want to preserve, and
we're just reconciling the associated copy of the parent's primary key.
--
Ticket URL: <https://code.djangoproject.com/ticket/33191>
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