Python SqlAlchemy - AttributeError: mapper

Question

based on my model:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship

Base = declarative_base()

class Session(Base):
    __tablename__ = 'sessions'

    id = Column(Integer, primary_key=True)
    token = Column(String(200))
    user_id = Column(Integer, ForeignKey('app_users.id'))
    user = relationship('model.user.User', back_populates='sessions')

I want to instantiate a new session through:

session = Session(token='test-token-123')

But i get:

AttributeError: mapper

The full stacktrace:

Traceback (most recent call last):
  File "/home/ubuntu/.local/lib/python3.5/site-packages/falcon/api.py", line 227, in __call__
    responder(req, resp, **params)
  File "./app_user/register.py", line 13, in on_post
    session = Session(token='test-token-123')
  File "<string>", line 2, in __init__
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/instrumentation.py", line 347, in _new_state_if_none
    state = self._state_constructor(instance, self)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 764, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/instrumentation.py", line 177, in _state_constructor
    self.dispatch.first_init(self, self.class_)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/event/attr.py", line 256, in __call__
    fn(*args, **kw)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 2976, in _event_on_first_init
    configure_mappers()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 2872, in configure_mappers
    mapper._post_configure_properties()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/mapper.py", line 1765, in _post_configure_properties
    prop.init()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/interfaces.py", line 184, in init
    self.do_init()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1653, in do_init
    self._process_dependent_arguments()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1710, in _process_dependent_arguments
    self.target = self.mapper.mapped_table
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 850, in __getattr__
    return self._fallback_getattr(key)
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 828, in _fallback_getattr
    raise AttributeError(key)

I have no idea where this error is coming from and i can not really debug it.. anybody could help me with this issue?

Thanks and Greetings!

Answer 1

Looking at the traceback you can see these lines:

  ...
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1653, in do_init
    self._process_dependent_arguments()
  File "/home/ubuntu/.local/lib/python3.5/site-packages/sqlalchemy/orm/relationships.py", line 1710, in _process_dependent_arguments
    self.target = self.mapper.mapped_table
  ...

which narrow your problem down quite a bit. The relationship

    user = relationship('model.user.User', back_populates='sessions')

uses a Python evaluable string as the argument, the use of which is further explained in "Configuring Relationships":

Relationships to other classes are done in the usual way, with the added feature that the class specified to relationship() may be a string name. The “class registry” associated with Base is used at mapper compilation time to resolve the name into the actual class object, which is expected to have been defined once the mapper configuration is used

If you've not imported models.user module anywhere before you try to instantiate a Session object for the first time, then the name resolving fails because the class User has not been created yet and does not exist in the registry. In other words for the name resolving to work, all classes must have been defined, which means that their bodies must have been executed.

And if you actually have imported the models.user module, check your other models and that their related model classes have been defined. Using your models for the first time triggers mapper compilation/configuration, so the source of the error could be other models as well.