Assert that class_manager(cls) is present on InstanceState deserialize

In a Django application using Elixir to map models to DB, very few often I get this exception on the logs:

mod_wsgi (pid=13845): Exception occurred processing WSGI script '/var/www/webtv/entry.wsgi'.
Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/django/core/handlers/wsgi.py", line 241, in __call__
    response = self.get_response(request)
  File "/usr/lib/python2.6/site-packages/django/core/handlers/base.py", line 73, in get_response
    response = middleware_method(request)
  File "/var/www/webtv/master/handler.py", line 38, in process_request
    return handler.processRequest(request)
  File "/var/www/webtv/master/gateway/handler.py", line 24, in processRequest
    return client.select(clientId)
  File "/var/www/webtv/master/client.py", line 61, in select
    databaseCredentials = DatabaseCredentials.get_by(client=client, type=u'main')
  File "/usr/lib/python2.6/site-packages/elixir/entity.py", line 1000, in get_by
    return cls.query.filter_by(*args, **kwargs).first()
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/query.py", line 1231, in first
    ret = list(self[0:1](0:1))
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/query.py", line 1152, in __getitem__
    return list(res)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/query.py", line 1292, in __iter__
    return self._execute_and_instances(context)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/query.py", line 1295, in _execute_and_instances
    result = self.session.execute(querycontext.statement, params=self._params, mapper=self._mapper_zero_or_none())
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/session.py", line 755, in execute
    clause, params or {})
  File "/usr/lib/python2.6/site-packages/sqlalchemy/engine/base.py", line 824, in execute
    return Connection.executors[c](c)(self, object, multiparams, params)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/engine/base.py", line 872, in _execute_clauseelement
    parameters=params
  File "/usr/lib/python2.6/site-packages/sqlalchemy/engine/base.py", line 938, in __create_execution_context
    return dialect.execution_ctx_cls(dialect, connection=self, **kwargs)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/engine/default.py", line 167, in __init__
    self.compiled_parameters = [for m in parameters](compiled.construct_params(m))
  File "/usr/lib/python2.6/site-packages/sqlalchemy/sql/compiler.py", line 243, in construct_params
    pd[self.bind_names[bindparam](self.bind_names[bindparam)] = bindparam.value()
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/strategies.py", line 398, in <lambda>
    bindparam.value = lambda: mapper._get_committed_attr_by_column(o, bind_to_col[bindparam.key](bindparam.key))
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/mapper.py", line 1089, in _get_committed_attr_by_column
    return self._get_committed_state_attr_by_column(state, column)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/mapper.py", line 1092, in _get_committed_state_attr_by_column
    return self._get_col_to_prop(column).getcommitted(state, column, passive=passive)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/properties.py", line 102, in getcommitted
    return state.get_impl(self.key).get_committed_value(state, state.dict, passive=passive)
  File "/usr/lib/python2.6/site-packages/sqlalchemy/orm/state.py", line 92, in get_impl
    return self.manager.get_impl(key)
AttributeError: 'NoneType' object has no attribute 'get_impl

Notice that subsequent requests are handled correctly, leaving me with no clue on the problem.

I start the connection at the module level in a custom middleware I set in Django, the code there is:

engine = sqlalchemy.create_engine(connectionString,    
    pool_recycle=7200,                                 
#    echo=masterSettings.DEBUG,                        
#    echo_pool=masterSettings.DEBUG,                   
    )

elixir.metadata.bind = engine                          
elixir.options_defaults['autosetup']('autosetup') = True

class DatabaseSession(object):                         
    """                                                
    SQLalchemy/Elixir master database session controler
    """                                                
    def process_response(self, request, response):     
        """                                            
        Close database session                         
        """                                            
        try:                                           
            elixir.session.close()                     
        except:                                        
           pass                                        
        return response

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