#18599: GenericForeignKey field can't be set on init of model
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Reporter: dpantele | Owner: nobody
Type: Bug | Status: new
Component: contrib.contenttypes | Version: 1.4
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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Changes (by aaugustin):
* stage: Unreviewed => Accepted
Comment:
The cause is very simple:
[https://github.com/django/django/blob/master/django/contrib/contenttypes/generic.py#L44
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/18599#comment:2>
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