ValueError: Can't do subqueries with queries on different DBs.

[erwinfeser]

It happens when I try to save an object related to other objects, even if other objects were fetched from the same database that I am using to write.

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/query.pyc in create(self, **kwargs)
    320         obj = self.model(**kwargs)
    321         self._for_write = True
--> 322         obj.save(force_insert=True, using=self.db)
    323         return obj
    324 

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/base.pyc in save(self, force_insert, force_update, using, update_fields)
    543 
    544         self.save_base(using=using, force_insert=force_insert,
--> 545                        force_update=force_update, update_fields=update_fields)
    546     save.alters_data = True
    547 

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/base.pyc in save_base(self, raw, force_insert, force_update, using, update_fields)
    580         if not meta.auto_created:
    581             signals.post_save.send(sender=origin, instance=self, created=(not updated),
--> 582                                    update_fields=update_fields, raw=raw, using=using)
    583 
    584     save_base.alters_data = True

/app/.heroku/python/lib/python2.7/site-packages/django/dispatch/dispatcher.pyc in send(self, sender, **named)
    183 
    184         for receiver in self._live_receivers(sender):
--> 185             response = receiver(signal=self, sender=sender, **named)
    186             responses.append((receiver, response))
    187         return responses

/app/apps/grains/signals.py in update_answered_questions_num(sender, **kwargs)
    241     instance = kwargs.get('instance')
    242     if instance:
--> 243         instance.profile.answered_questions_num = instance.profile.get_answered_questions().count()
    244         instance.profile.save(update_fields=['answered_questions_num'])
    245 

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/query.pyc in count(self)
    292             return len(self._result_cache)
    293 
--> 294         return self.query.get_count(using=self.db)
    295 
    296     def get(self, *args, **kwargs):

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/query.pyc in get_count(self, using)
    388 
    389         obj.add_count_column()
--> 390         number = obj.get_aggregation(using=using)[None]
    391 
    392         # Apply offset and limit constraints manually, since using LIMIT/OFFSET

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/query.pyc in get_aggregation(self, using)
    354         query.related_select_cols = []
    355 
--> 356         result = query.get_compiler(using).execute_sql(SINGLE)
    357         if result is None:
    358             result = [None for q in query.aggregate_select.items()]

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/compiler.py in execute_sql(self, result_type)
    774         
    775         try:
--> 776             sql, params = self.as_sql()
    777             if not sql:
    778                 raise EmptyResultSet

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/compiler.py in as_sql(self, with_limits, with_col_aliases)
     92         qn = self.quote_name_unless_alias
     93 
---> 94         where, w_params = self.query.where.as_sql(qn=qn, connection=self.connection)
     95         having, h_params = self.query.having.as_sql(qn=qn, connection=self.connection)
     96         having_group_by = self.query.having.get_cols()

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/where.pyc in as_sql(self, qn, connection)
    101             try:
    102                 if hasattr(child, 'as_sql'):
--> 103                     sql, params = child.as_sql(qn=qn, connection=connection)
    104                 else:
    105                     # A leaf node in the tree.

/app/.heroku/python/lib/python2.7/site-packages/django/db/models/sql/where.pyc in as_sql(self, qn, connection)
    399         if hasattr(query, 'values'):
    400             if query._db and connection.alias != query._db:
--> 401                 raise ValueError("Can't do subqueries with queries on different DBs.")
    402             # Do not override already existing values.
    403             if not hasattr(query, 'field_names'):

ValueError: Can't do subqueries with queries on different DBs.

[jsocol]

Can you show us an example of the code that threw the error? The traceback will be a lot more helpful with some context.