id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
20047	ORM objects store connection name, but django does not include it during memcache lookup	kush@…	nobody	"Hi,

We ran into a strange problem with the way ORM objects are being looked up in memcache, by django.
Our setup include two application servers that share the same memcache instance.
We also run a mysql-master and read-replicas.

Appserver1 uses the default mechanism of connecting to the read-replicas (using DATABASES dictionary in settings.py).
Appserver2 uses a DB-router to route writes to the master and reads to the replicas. Please see below for  appserver2 settings file:


We noticed that Appserver1 was trying to make a connection to the DB alias called ""'''read'''"", while it only had default settings to connect to the DB (no references to ""read"" instance are present on appserver-1).

On investigating how this happened we found the following:

1. Appserver-2 got a read request and used the ""read"" alias to fetch and store the corresponding ORM object in memcache.
2. The ORM object that is stored seems to maintain a reference to the DB connection/alias (""read"" in this case).
3. Appserver-1 does a memcache lookup and finds the same ORM object (that has a reference to the DB connection ""read"").
4. When Appserver-1 tries to use the ORM object, it throws an error saying that it does not know about the ""read"" connection.


The error message is shown below:
---------------- --------------
...
...
....
File ""/usr/local/lib/python2.6/dist-packages/django/db/models/manager.py"", line 120, in count
    return self.get_query_set().count()

  File ""/usr/local/lib/python2.6/dist-packages/django/db/models/query.py"", line 332, in count
    return self.query.get_count(using=self.db)

  File ""/usr/local/lib/python2.6/dist-packages/django/db/models/sql/query.py"", line 407, in get_count
    number = obj.get_aggregation(using=using)[None]

  File ""/usr/local/lib/python2.6/dist-packages/django/db/models/sql/query.py"", line 373, in get_aggregation
    result = query.get_compiler(using).execute_sql(SINGLE)

  File ""/usr/local/lib/python2.6/dist-packages/django/db/models/sql/query.py"", line 213, in get_compiler
    connection = connections[using]

  File ""/usr/local/lib/python2.6/dist-packages/django/db/utils.py"", line 90, in __getitem__
    self.ensure_defaults(alias)

  File ""/usr/local/lib/python2.6/dist-packages/django/db/utils.py"", line 72, in ensure_defaults
    raise ConnectionDoesNotExist(""The connection %s doesn't exist"" % alias)

ConnectionDoesNotExist: The connection read doesn't exist


---------------- 
Django Version: 1.2.7
---------------------


Appserver-2 settings:
--------------------

    DATABASES = {
        'read': {
            ......
            /**** Connects to the read-replicas ********/
        },
        'default': {
            ......
            /**** Connects to the write-master ********/
        },
"	Bug	closed	Database layer (models, ORM)	1.2	Normal	wontfix			Unreviewed	0	0	0	0	0	0