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I’m building a project using FastAPI with SQLModel, implementing a relational data structure between multiple models. In my case, I’m defining a relationship between a User and a Signatory, where a Signatory has a foreign key referencing a User. Despite following standard SQLModel practices, I encounter an error when running the project.

Here is the simplified version of my User and Signatory models, followed by the error message and stack trace. The issue appears to relate to relationship initialization and class registry, with SQLAlchemy throwing an InvalidRequestError due to a failure to resolve the Signatory class.

I would like assistance in identifying the source of this error and resolving it. Below are the relevant code snippets and the complete error message for further context.
The user_model.py:

from datetime import datetime
from typing import List, Optional

from sqlmodel import Field, SQLModel, Relationship
from app.models.signatory_model import Signatory
from app.models.document_model import Document


class UserBase(SQLModel):
    email: str = Field(index=True, unique=True)
    is_active: bool = True
    is_superuser: bool = False


class User(UserBase, table=True):
    id: int | None = Field(default=None, primary_key=True)
    first_name: str
    last_name: str
    created_at: datetime = Field(default_factory=datetime.now)
    updated_at: datetime = Field(default_factory=datetime.now)

    user_as_signatory: Optional["Signatory"] = Relationship(back_populates="user")
    signatories: List["Signatory"] = Relationship(back_populates="creator")
    documents: List["Document"] = Relationship(back_populates="owner")

and this signatory_model.py:

from datetime import datetime
from typing import TYPE_CHECKING, Optional, List

from sqlalchemy import Enum
from sqlmodel import Field, Relationship, SQLModel

from app.models.document_model import Document, DocumentSignatoryLink

if TYPE_CHECKING:
    from .user_model import User
    

class SignatoryBase(SQLModel, table=True):
    id: int = Field(default=None, primary_key=True)
    first_name: str
    last_name: str
    email: str
    created_at: datetime = Field(default_factory=datetime.now)
    updated_at: datetime = Field(default_factory=datetime.now)


class Signatory(SignatoryBase):
    user_id: Optional[int] = Field(default=None, foreign_key="user.id")
    creator_id: int = Field(default=None, foreign_key="user.id")

    documents: List["Document"] = Relationship(
        back_populates="signatories",
        link_model=DocumentSignatoryLink,
    )
    user: Optional["User"] = Relationship(back_populates="user_as_signatory")
    creator: "User" = Relationship(back_populates="signatories")

when I try to run the project I get this error:


backend_1   | Traceback (most recent call last):
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/clsregistry.py", line 515, in _resolve_name
backend_1   |     rval = d[token]
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/util/_collections.py", line 344, in __missing__
backend_1   |     self[key] = val = self.creator(key)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/clsregistry.py", line 483, in _access_cls
backend_1   |     return self.fallback[key]
backend_1   | KeyError: 'Signatory'
backend_1   | 
backend_1   | The above exception was the direct cause of the following exception:
backend_1   | 
backend_1   | Traceback (most recent call last):
backend_1   |   File "/app/app/initial_data.py", line 23, in <module>
backend_1   |     main()
backend_1   |   File "/app/app/initial_data.py", line 18, in main
backend_1   |     init()
backend_1   |   File "/app/app/initial_data.py", line 13, in init
backend_1   |     init_db(session)
backend_1   |   File "/app/app/core/db.py", line 31, in init_db
backend_1   |     user = session.exec(user_query).first()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlmodel/orm/session.py", line 68, in exec
backend_1   |     results = super().execute(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/session.py", line 2306, in execute
backend_1   |     return self._execute_internal(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/session.py", line 2191, in _execute_internal
backend_1   |     result: Result[Any] = compile_state_cls.orm_execute_statement(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/context.py", line 293, in orm_execute_statement
backend_1   |     result = conn.execute(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/engine/base.py", line 1421, in execute
backend_1   |     return meth(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/elements.py", line 514, in _execute_on_connection
backend_1   |     return connection._execute_clauseelement(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/engine/base.py", line 1635, in _execute_clauseelement
backend_1   |     compiled_sql, extracted_params, cache_hit = elem._compile_w_cache(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/elements.py", line 702, in _compile_w_cache
backend_1   |     compiled_sql = self._compiler(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/elements.py", line 315, in _compiler
backend_1   |     return dialect.statement_compiler(dialect, self, **kw)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/compiler.py", line 1424, in __init__
backend_1   |     Compiled.__init__(self, dialect, statement, **kwargs)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/compiler.py", line 865, in __init__
backend_1   |     self.string = self.process(self.statement, **compile_kwargs)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/compiler.py", line 910, in process
backend_1   |     return obj._compiler_dispatch(self, **kwargs)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/visitors.py", line 141, in _compiler_dispatch
backend_1   |     return meth(self, **kw)  # type: ignore  # noqa: E501
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/compiler.py", line 4683, in visit_select
backend_1   |     compile_state = select_stmt._compile_state_factory(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/base.py", line 684, in create_for_statement
backend_1   |     return klass.create_for_statement(statement, compiler, **kw)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/context.py", line 1091, in create_for_statement
backend_1   |     _QueryEntity.to_compile_state(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/context.py", line 2546, in to_compile_state
backend_1   |     _MapperEntity(
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/context.py", line 2626, in __init__
backend_1   |     entity._post_inspect
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/util/langhelpers.py", line 1252, in __get__
backend_1   |     obj.__dict__[self.__name__] = result = self.fget(obj)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/mapper.py", line 2711, in _post_inspect
backend_1   |     self._check_configure()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/mapper.py", line 2388, in _check_configure
backend_1   |     _configure_registries({self.registry}, cascade=True)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/mapper.py", line 4203, in _configure_registries
backend_1   |     _do_configure_registries(registries, cascade)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/mapper.py", line 4244, in _do_configure_registries
backend_1   |     mapper._post_configure_properties()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/mapper.py", line 2405, in _post_configure_properties
backend_1   |     prop.init()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/interfaces.py", line 579, in init
backend_1   |     self.do_init()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/relationships.py", line 1642, in do_init
backend_1   |     self._setup_entity()
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/relationships.py", line 1856, in _setup_entity
backend_1   |     self._clsregistry_resolve_name(argument)(),
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/clsregistry.py", line 519, in _resolve_name
backend_1   |     self._raise_for_name(name, err)
backend_1   |   File "/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/clsregistry.py", line 500, in _raise_for_name
backend_1   |     raise exc.InvalidRequestError(
backend_1   | sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper[User(user)], expression 'Signatory' failed to locate a name ('Signatory'). If this is a class name, consider adding this relationship() to the <class 'app.models.user_model.User'> class after both dependent classes have been defined.
esign_backend_1 exited with code 1

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raise exc.InvalidRequestError( sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper[User(user)]