full-stack-fastapi-template: [BUG] SQLAlchemy InvalidRequestError on relationships if one of the model is not yet imported

Describe the bug

The application crashes at start-up, when initializing data if :

  1. a relationship is defined …
  2. … with a model not already imported at the time of execution.
backend_1        | INFO:__main__:Starting call to '__main__.init', this is the 2nd time calling it.
backend_1        | INFO:__main__:Service finished initializing
backend_1        | INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
backend_1        | INFO  [alembic.runtime.migration] Will assume transactional DDL.
backend_1        | INFO  [alembic.runtime.migration] Running upgrade  -> d4867f3a4c0a, First revision
backend_1        | INFO  [alembic.runtime.migration] Running upgrade d4867f3a4c0a -> ea9cad5d9292, Added SubItem models
backend_1        | INFO:__main__:Creating initial data
backend_1        | Traceback (most recent call last):
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 294, in __call__
backend_1        |     x = eval(self.arg, globals(), self._dict)
backend_1        |   File "<string>", line 1, in <module>
backend_1        | NameError: name 'SubItem' is not defined
...
backend_1        | sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Item->item, expression 'SubItem' failed to locate a name ("name 'SubItem' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.db_models.item.Item'> class after both dependent classes have been defined.
base-project_backend_1 exited with code 1

To Reproduce

create a new db_models/subitems.py (could be copied from item.py)

It is important that this new model has a relationship to another one, e.g Item

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.orm import relationship

from app.db.base_class import Base


class SubItem(Base):
    id = Column(Integer, primary_key=True, index=True)
    title = Column(String, index=True)
    description = Column(String, index=True)
    item_id = Column(Integer, ForeignKey("item.id"))
    item = relationship("Item", back_populates="subitems")

Adapt db_models/item.py with the new relationship

...

class Item(Base):
    ...
    subitems = relationship("SubItem", back_populates="item")

Declare the new SubItem in db/base.py as per the documentation

# Import all the models, so that Base has them before being
# imported by Alembic
from app.db.base_class import Base  # noqa
from app.db_models.user import User  # noqa
from app.db_models.item import Item  # noqa
from app.db_models.subitem import SubItem  # noqa

Re-build and start the application. The full traceback follows

backend_1        | INFO:__main__:Creating initial data
backend_1        | Traceback (most recent call last):
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 294, in __call__
backend_1        |     x = eval(self.arg, globals(), self._dict)
backend_1        |   File "<string>", line 1, in <module>
backend_1        | NameError: name 'SubItem' is not defined
backend_1        |
backend_1        | During handling of the above exception, another exception occurred:
backend_1        |
backend_1        | Traceback (most recent call last):
backend_1        |   File "/app/app/initial_data.py", line 21, in <module>
backend_1        |     main()
backend_1        |   File "/app/app/initial_data.py", line 16, in main
backend_1        |     init()
backend_1        |   File "/app/app/initial_data.py", line 11, in init
backend_1        |     init_db(db_session)
backend_1        |   File "/app/app/db/init_db.py", line 12, in init_db
backend_1        |     user = crud.user.get_by_email(db_session, email=config.FIRST_SUPERUSER)
backend_1        |   File "/app/app/crud/user.py", line 16, in get_by_email
backend_1        |     return db_session.query(User).filter(User.email == email).first()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/scoping.py", line 162, in do
backend_1        |     return getattr(self.registry(), name)(*args, **kwargs)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1543, in query
backend_1        |     return self._query_cls(entities, self, **kwargs)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 168, in __init__
backend_1        |     self._set_entities(entities)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 200, in _set_entities
backend_1        |     self._set_entity_selectables(self._entities)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 231, in _set_entity_selectables
backend_1        |     ent.setup_entity(*d[entity])
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 4077, in setup_entity
backend_1        |     self._with_polymorphic = ext_info.with_polymorphic_mappers
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 855, in __get__
backend_1        |     obj.__dict__[self.__name__] = result = self.fget(obj)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/mapper.py", line 2135, in _with_polymorphic_mappers
backend_1        |     configure_mappers()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/mapper.py", line 3229, in configure_mappers
backend_1        |     mapper._post_configure_properties()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/mapper.py", line 1947, in _post_configure_properties
backend_1        |     prop.init()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/interfaces.py", line 196, in init
backend_1        |     self.do_init()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/relationships.py", line 1860, in do_init
backend_1        |     self._process_dependent_arguments()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/relationships.py", line 1922, in _process_dependent_arguments
backend_1        |     self.target = self.entity.persist_selectable
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 855, in __get__
backend_1        |     obj.__dict__[self.__name__] = result = self.fget(obj)
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/relationships.py", line 1827, in entity
backend_1        |     argument = self.argument()
backend_1        |   File "/usr/local/lib/python3.7/site-packages/sqlalchemy/ext/declarative/clsregistry.py", line 306, in __call__
backend_1        |     % (self.prop.parent, self.arg, n.args[0], self.cls)
backend_1        | sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Item->item, expression 'SubItem' failed to locate a name ("name 'SubItem' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.db_models.item.Item'> class after both dependent classes have been defined.
base-project_backend_1 exited with code 1

Expected behavior The solution should have started normally

Additionnal context In (most of) real use-cases, the application would have defined some CRUD operation on the new defined model, and consequently imported it here and there.

Thus making it available at the time when creating initial data.

Nevertheless, the error is so annoying and obscure (when it happens) that it deserves a safeguard (see my PR for a suggestion)

About this issue

  • Original URL
  • State: closed
  • Created 5 years ago
  • Comments: 15 (6 by maintainers)

Commits related to this issue

Most upvoted comments

you are more than welcome ! It’s great to help on fastapi and its siblings 🐝 🐝 🐝

+8
ebreton on May 22, 2019

I found this issue on google because I had a similar error. I found a solution for my problem but even if it is not really related to the initial issue, I still want to write my solution here in case someone else had the same problem as me: I just had the same error message as discussed above, but it wasn’t a bug. I just miss-typed the string assigned to the back_populate argument. So just have a look at your back_populate argument referencing the column in your other table, it could be as simple as that 😊

+7
gruvw on Feb 4, 2021

@dmontagu In my code, I have implemented 2 models named Venue and User like this:

class User(Base):
    id = Column(Integer, primary_key=True, index=True)
    username = Column(String(64), index=True, unique=True)
    email = Column(String(120), index=True, unique=True)
    state = Column(String(30))
    venues = relationship("Venue")
    role = Column(String(10), default="customer")

class Venue(Base):
    id = Column(Integer, primary_key=True, index=True)
    name = Column(String(64), index=True)
    owner_id = Column(Integer, ForeignKey("user.id"))

And, this is the error which I am getting:

sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class User->user, expression 'Venue' failed to locate a name ("name 'Venue' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.db_models.user.User'> class after both dependent classes have been defined.
+6
Haider8 on Oct 30, 2019

@Haider8 This is documented in the sqlalchemy docs at the very end of the “Configuring Relationships” section here: https://docs.sqlalchemy.org/en/13/orm/extensions/declarative/relationships.html (just above “Configuring Many-to-Many Relationships”).

+4
dmontagu on Nov 16, 2019

I’m still having this issue, and this is 100% copy from the SQLAlchemy 2 docs.

# parent.py
class Parent(Base):
    __tablename__ = "parents"

    id: Mapped[int] = mapped_column(primary_key=True)
    ...
    children: Mapped[List["Child"]] = relationship(back_populates="parent")

# child.py
# Also tried from app.models.parent import Parent in this file, no effect.
# Also tried not using Mapped, and just parent = relationship("Parent", back_populates="parent"), also not working
class Child(Base):
    __tablename__ = "childs"  # 'classname + s'  in my case

    id: Mapped[int] = mapped_column(primary_key=True)
    ...
    parent: Mapped["Parent"] = relationship(back_populates="parent")
+1
dejoma on Feb 9, 2024

No, I couldn’t solve this. I was expecting some kind of reply from the maintainers.

@Haider8 I might know what is happening, just import “Venue” in your user file, that way the application will be aware of the Venue class…later you can change that import and put it some where else, in a path that the application executes at startup

+1
sebas-500 on Nov 16, 2019
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