#32563: Cannot override database used with RelatedManager
-------------------------------------+-------------------------------------
Reporter: Lucas | Owner: nobody
Gruber |
Type: Bug | Status: new
Component: Database | Version: dev
layer (models, ORM) | Keywords: RelatedManager
Severity: Normal | mullti databases
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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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
--
Ticket URL: <https://code.djangoproject.com/ticket/32563>
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