#29129: Child model updates parent model with empty fields making an extra
query in
multi-inheritance use case
-------------------------------------+-------------------------------------
Reporter: user0007 | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 2.0
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 |
-------------------------------------+-------------------------------------
While creating a new model object (using multi-inheritance model =>
`Child(Parent)`), Django does an extra update query setting parent model
fields to empty values. This situation occurs *only* if we define a custom
primary key in a parent model (eg. as an UUID field).
An example *without* custom primary key (correct behavior):
{{{
class Parent(models.Model):
title = models.TextField()
class Child(Parent):
body = models.TextField()
>> Child.objects.create()
1. INSERT INTO "app_parent" ("title") VALUES ('') RETURNING
"app_parent"."id"
2. INSERT INTO "app_child" ("parent_ptr_id", "body") VALUES (1, '')
}}}
An example *with* custom primary key (incorrect behavior):
{{{
class Parent(models.Model):
id = models.UUIDField(
primary_key=True,
default=uuid.uuid4,
editable=False
)
title = models.TextField()
class Child(Parent):
body = models.TextField()
>> Child.objects.create()
1. UPDATE "app_parent" SET "title" = '' WHERE "app_parent"."id" =
'd750cfdd-ae7b-48a6-a2e0-d49e70e28686'::uuid
2. INSERT INTO "app_parent" ("id", "title") VALUES ('d750cfdd-ae7b-
48a6-a2e0-d49e70e28686'::uuid, '')
3. INSERT INTO "app_child" ("parent_ptr_id", "body") VALUES ('d750cfdd-
ae7b-48a6-a2e0-d49e70e28686'::uuid, '')
}}}
Python 3.6, PostgreSQL 9.6
--
Ticket URL: <https://code.djangoproject.com/ticket/29129>
Django <https://code.djangoproject.com/>
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