Opened 3 years ago

Last modified 23 months ago

#18599 new Bug

GenericForeignKey field can't be set on init of model

Reported by: dpantele Owned by: nobody
Component: contrib.contenttypes Version: 1.4
Severity: Normal Keywords:
Cc: dpantele Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

If we are trying to set GenericForeignKey to non-saved object during model initialization, it is not set.

For example:

class A(models.model):
    pass

class B(models.model):
    object_id    = models.PositiveIntegerField()
    content_type = models.ForeignKey(ContentType)
    obj          = generic.GenericForeignKey()

...

a = A()
b = B(obj=a)

b.obj # None
b.obj.save() # 'NoneType' object has no attribute 'save'

Attachments (1)

18599-test.patch (614 bytes) - added by bouke 23 months ago.

Download all attachments as: .zip

Change History (6)

comment:1 Changed 3 years ago by dpantele

  • Needs documentation unset
  • Needs tests unset
  • Patch needs improvement unset
  • Summary changed from GenericForeignKey field can't be set on init of moel to GenericForeignKey field can't be set on init of model

comment:2 Changed 3 years ago by aaugustin

  • Triage Stage changed from Unreviewed to Accepted

The cause is very simple: GenericForeignKey.instance_pre_init extracts the content-type and the id of the related object and then throws the object itself away. If an unsaved object is passed in the constructor, its id is None, so the pointer to the object is lost at this point.

Here's what happens with a GFK:

>>> a = A()
>>> b = B(obj=a)
>>> b.obj
>>> b.content_type
<ContentType: a>
>>> b.object_id
>>> b.save()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/base.py", line 479, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/base.py", line 574, in save_base
    result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/manager.py", line 203, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/query.py", line 1589, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/sql/compiler.py", line 914, in execute_sql
    cursor.execute(sql, params)
  File "/Users/myk/Documents/dev/django-trunk/django/db/backends/util.py", line 42, in execute
    return self.cursor.execute(sql, params)
  File "/Users/myk/Documents/dev/django-trunk/django/db/backends/sqlite3/base.py", line 340, in execute
    return Database.Cursor.execute(self, query, params)
IntegrityError: test_app_b.object_id may not be NULL

In comparison here's what happens with a regular FK:

>>> a = A()
>>> b = B(fk=a)
>>> b.fk
<A: A object>
>>> b.save()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/base.py", line 479, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/base.py", line 574, in save_base
    result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/manager.py", line 203, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/query.py", line 1589, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/myk/Documents/dev/django-trunk/django/db/models/sql/compiler.py", line 914, in execute_sql
    cursor.execute(sql, params)
  File "/Users/myk/Documents/dev/django-trunk/django/db/backends/util.py", line 42, in execute
    return self.cursor.execute(sql, params)
  File "/Users/myk/Documents/dev/django-trunk/django/db/backends/sqlite3/base.py", line 340, in execute
    return Database.Cursor.execute(self, query, params)
IntegrityError: test_app_b.fk_id may not be NULL

In both cases, saving the B object will fail because the A object doesn't have an id yet, so there isn't much to gain in "fixing" this.

We should probably just raise an exception in this situation.

comment:3 Changed 3 years ago by dpantele

  • Cc dpantele added

It is very strange that any object which is passed to the constructor is thrown away. In FK situation I can do like that:

>>> a = A()
>>> b = B(fk=a)
>>> b.fk.save()
>>> b.fk_id = b.fk.id
>>> b.save()

In the GFK situation it is not possible for me to save b object if i don't pass a. So, I should everywhere pass two references or use an explicit notation:

>>> a = A()
>>> b = B()
>>> b.gfk = a
>>> b.gfk.save()
>>> b.gfk = b.gfk
>>> b.save()

Of course, it should be possible to keep reference to non-saved object. For example, I need to validate object 'b', and if it is not valid, I should not save 'a' object at all.

I see very simple way of solving this problem: we should keep cached object not in object instance, but in the field instance.

comment:4 Changed 3 years ago by ramiro

See also #7551.

Changed 23 months ago by bouke

comment:5 Changed 23 months ago by bouke

See also #16508, as this is a common problem of virtual fields (implementation of GenericForeignKey).

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