#32563: Cannot override database used with RelatedManager
-------------------------------------+-------------------------------------
Reporter: Lucas Gruber | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: RelatedManager | Triage Stage:
mullti databases | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Lucas Gruber:
Old description:
> I suppose an app with models:
> {{{
> ############################
> class Blog(models.Model):
> title = models.CharField(max_length=100)
>
> class Person(models.Model):
> name = models.CharField(max_length=100)
> subscribed_blogs = models.ManyToManyField(Blog,
> related_name="subscribers", through="Subscription")
>
> class Subscription(models.Model):
> person = models.ForeignKey(Person, related_name="subscriptions")
> blog = models.ForeignKey(Blog, related_name="subscriptions")
> ############################
> }}}
>
> In thes case we are using multiple databases and we need to create custom
> migration, we have to write something like
> {{{
> ############################
> def custom_migrations(apps, schema_editor):
> db_alias = schema_editor.connection.alias
>
> person = Person.objects.using(db_alias).get(pk=1)
> blog = Blog.objects.using(db_alias).get(pk=1)
> blog.subscribers.set([person])
>
> blog.save(using=db_alias)
>
> ############################
> }}}
>
> The line blog.subscribers.set(...) does not permit to add parameter for
> overriding database to use.
> The source code for this function is in
> django.db.models.fields.related_descriptors when we can see:
> {{{
> [...]
> def set(self, objs, *, bulk=True, clear=False):
> # Force evaluation of `objs` in case it's a queryset whose
> value
> # could be affected by `manager.clear()`. Refs #19816.
> objs = tuple(objs)
>
> if self.field.null:
> db = router.db_for_write(self.model,
> instance=self.instance)
> with transaction.atomic(using=db, savepoint=False):
> if clear:
> self.clear(bulk=bulk)
> self.add(*objs, bulk=bulk)
> else:
> old_objs = set(self.using(db).all())
> new_objs = []
> for obj in objs:
> if obj in old_objs:
> old_objs.remove(obj)
> else:
> new_objs.append(obj)
>
> self.remove(*old_objs, bulk=bulk)
> self.add(*new_objs, bulk=bulk)
> else:
> self.add(*objs, bulk=bulk)
> set.alters_data = True
> [...]
> }}}
>
> Code always calls database router, but in migration process, the router
> can not find the appropriate database because we just use without request
> :
> {{{
> python manage.py migrate --database db2
> }}}
>
> I noticed that all the methods of ''RelatedManager'' directly call the
> router object to find the database while the ''Manager'' objects always
> exploits the possibility of overriding the database with the call to
> ''using()'' on the QuerySet or to pass parameter ''using=db'' for save
> model method for example.
>
> Thank you in advance for your answer
New description:
I suppose an app with models:
{{{
############################
class Blog(models.Model):
title = models.CharField(max_length=100)
class Person(models.Model):
name = models.CharField(max_length=100)
subscribed_blogs = models.ManyToManyField(Blog,
related_name="subscribers", through="Subscription")
class Subscription(models.Model):
person = models.ForeignKey(Person, related_name="subscriptions")
blog = models.ForeignKey(Blog, related_name="subscriptions")
############################
}}}
In thes case we are using multiple databases and we need to create custom
migration, we have to write something like
{{{
############################
def custom_migrations(apps, schema_editor):
db_alias = schema_editor.connection.alias
person = Person.objects.using(db_alias).get(pk=1)
blog = Blog.objects.using(db_alias).get(pk=1)
blog.subscribers.set([person])
blog.save(using=db_alias)
############################
}}}
The line blog.subscribers.set(...) does not permit to add parameter for
overriding database to use.
The source code for this function is in
django.db.models.fields.related_descriptors when we can see:
{{{
[...]
def set(self, objs, *, clear=False, through_defaults=None):
# Force evaluation of `objs` in case it's a queryset whose
value
# could be affected by `manager.clear()`. Refs #19816.
objs = tuple(objs)
db = router.db_for_write(self.through, instance=self.instance)
with transaction.atomic(using=db, savepoint=False):
if clear:
self.clear()
self.add(*objs, through_defaults=through_defaults)
else:
old_ids =
set(self.using(db).values_list(self.target_field.target_field.attname,
flat=True))
new_objs = []
for obj in objs:
fk_val = (
self.target_field.get_foreign_related_value(obj)[0]
if isinstance(obj, self.model) else obj
)
if fk_val in old_ids:
old_ids.remove(fk_val)
else:
new_objs.append(obj)
self.remove(*old_ids)
self.add(*new_objs, through_defaults=through_defaults)
set.alters_data = True
[...]
}}}
Code always calls database router, but in migration process, the router
can not find the appropriate database because we just use without request
:
{{{
python manage.py migrate --database db2
}}}
I noticed that all the methods of ''RelatedManager'' directly call the
router object to find the database while the ''Manager'' objects always
exploits the possibility of overriding the database with the call to
''using()'' on the QuerySet or to pass parameter ''using=db'' for save
model method for example.
Thank you in advance for your answer
--
--
Ticket URL: <https://code.djangoproject.com/ticket/32563#comment:1>
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