Can't get relationships to work. Unexpected type ForwardRef?

[dentatar]

python 3.11
sqlalchemy 2.0.23
strawberry-graphql 0.216.0
strawberry-sqlalchemy-mapper 0.4.0

Trying to use this mapper and was playing around with some basics but can't get relationships to work. It doesn't seem like i do something wrong, this is literally the example from readme:

models.py

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

Base = declarative_base()


class Employee(Base):
    __tablename__ = "employee"
    id = Column(UUID, primary_key=True)
    name = Column(String, nullable=False)
    password_hash = Column(String, nullable=False)
    department_id = Column(UUID, ForeignKey("department.id"))
    department = relationship("Department", back_populates="employees")


class Department(Base):
    __tablename__ = "department"
    id = Column(UUID, primary_key=True)
    name = Column(String, nullable=False)
    employees = relationship("Employee", back_populates="department")

app.py

from fastapi import FastAPI
from strawberry.fastapi import GraphQLRouter
import strawberry, models
from strawberry_sqlalchemy_mapper import StrawberrySQLAlchemyMapper

strawberry_sqlalchemy_mapper = StrawberrySQLAlchemyMapper()


@strawberry_sqlalchemy_mapper.type(models.Employee)
class Employee:
    __exclude__ = ["password_hash"]


@strawberry_sqlalchemy_mapper.type(models.Department)
class Department:
    pass


@strawberry.type
class Query:
    @strawberry.field
    def departments(self) -> Department:
        return Department(name="Test", employees=[Employee(name="1"), Employee(name="2")])


strawberry_sqlalchemy_mapper.finalize()
schema = strawberry.Schema(
    query=Query,
)
app = FastAPI()
graphql_app = GraphQLRouter(schema)
app.include_router(graphql_app, prefix='/graphql')

main.py

import uvicorn
from app import app

if __name__ == "__main__":
    uvicorn.run(app, port=5000, log_level="info")

Got this error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/graphql/type/definition.py", line 808, in fields
    fields = resolve_thunk(self._fields)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/graphql/type/definition.py", line 300, in resolve_thunk
    return thunk() if callable(thunk) else thunk
           ^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 514, in <lambda>
    fields=lambda: self.get_graphql_fields(object_type),
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 373, in get_graphql_fields
    return _get_thunk_mapping(
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 133, in _get_thunk_mapping
    thunk_mapping[name_converter(field)] = field_converter(
                                           ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 314, in from_field
    self.from_maybe_optional(
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 768, in from_maybe_optional
    return GraphQLNonNull(self.from_type(type_))
                          ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema_converter.py", line 801, in from_type
    raise TypeError(f"Unexpected type '{type_}'")
TypeError: Unexpected type 'ForwardRef('Employee')'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/app/app.py", line 28, in <module>
    schema = strawberry.Schema(
             ^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/strawberry/schema/schema.py", line 141, in __init__
    self._schema = GraphQLSchema(
                   ^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/graphql/type/schema.py", line 224, in __init__
    collect_referenced_types(query)
  File "/usr/local/lib/python3.11/site-packages/graphql/type/schema.py", line 433, in collect_referenced_types
    collect_referenced_types(field.type)
  File "/usr/local/lib/python3.11/site-packages/graphql/type/schema.py", line 433, in collect_referenced_types
    collect_referenced_types(field.type)
  File "/usr/local/lib/python3.11/site-packages/graphql/type/schema.py", line 433, in collect_referenced_types
    collect_referenced_types(field.type)
  File "/usr/local/lib/python3.11/site-packages/graphql/type/schema.py", line 432, in collect_referenced_types
    for field in named_type.fields.values():
                 ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/functools.py", line 1001, in __get__
    val = self.func(instance)
          ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/graphql/type/definition.py", line 811, in fields
    raise cls(f"{self.name} fields cannot be resolved. {error}") from error
TypeError: EmployeeEdge fields cannot be resolved. Unexpected type 'ForwardRef('Employee')'

[Gianfranco753]

+1

Have the same exception

[0x587]

+1

Have the same exception

[johansigfrids]

I have the same issue. Downgrading to 0.3.1 fixed it, so I guess something in 0.4.0 broke it.

[erikwrede]

Thanks for the report! I'll try to look into it this weekend. Has anyone gathered some more detail of what could have caused the issue? That would really help me dive straight into the fixing 🙂

[erikwrede]

I've investigated further. The issue is this change, specifically L289. The node type names are now passed as a ForwardRef to the generic. This ref is not resolved correctly.

strawberry-sqlalchemy/src/strawberry_sqlalchemy_mapper/mapper.py

Lines 278 to 293 in 4fa950f

	def _edge_type_for(self, type_name: str) -> Type[Any]:
	"""
	Get or create a corresponding Edge model for the given type
	(to support future pagination)
	"""
	edge_name = f"{type_name}Edge"
	if edge_name not in self.edge_types:
	self.edge_types[edge_name] = edge_type = strawberry.type(
	dataclasses.make_dataclass(
	edge_name,
	[],
	bases=(relay.Edge[type_name],), # type: ignore[valid-type]
	)
	)
	setattr(edge_type, _GENERATED_FIELD_KEYS_KEY, ["node"])
	return self.edge_types[edge_name]

Since relay.Edge is generic, we save the value of the TypeVar for the node type inside of StrawberryField.specialized_type_var_map to later replace it in StrawberryField.resolve_type:

https://github.com/strawberry-graphql/strawberry/blob/aad305b0036de8bfd4d08ef9571aee68f04eb0d1/strawberry/field.py#L360-L375

The problem in this case: the mapped type is not a StrawberryType, but a ForwardRef equaling the type name we expect it to be. This cannot be resolved, because the generic resolver does not convert the ForwardRef into a StrawberryAnnotation to call evaluate on it.

I did this as a test, but this yielded another error: the type Employee is not defined in the scope. This is because we have no strawberry.lazy info about the actual location of the type Employee because we just expect that it will exist under this name later.

I'm still looking for a quick and easy fix, but long-term I see this being fixed through one of these options:

• Have a type registry for all mapped types and a custom lazy type that utilizes this registry
• Adopt strawberry-django's behavior /cc @bellini666 how are you implementing lazy refs in relationships?

For me personally, a central registry is the preferred approach since it gives full control over the way we access types. This is a mid-term effort and requires heavy refactoring, so having something that works short-term to fix the occurring issues would be preferable.

/cc @mattalbr

[bellini666]

Adopt strawberry-django's behavior /cc @bellini666 how are you implementing lazy refs in relationships?

Actually in strawberry django all relations and the types to those relations should be created explicitly: https://strawberry-graphql.github.io/strawberry-graphql-django/guide/fields/#relationships , and thus, those can be done using Lazy types

If we try to map a relationship using auto or passing to the fields list, we map them to a type that only has a pk attribute (https://github.com/strawberry-graphql/strawberry-graphql-django/blob/main/strawberry_django/fields/types.py#L250) or to the Node interface when using relay. But we usually suggest users to map relations explicitly

Having said that, based on how this code works, a type registry is probably the way to go for this.

TBH, I'm not totally familiar with this code but doesn't something like that already exists when creating the Edge/Connection? i.e. relay.Edge[self.mapped_types[type_name]]

Also, it seems that I'm the one who caused this issue so I can try to fix it as well :)

[bellini666]

This should fix the issue: #106

[erikwrede]

I've merged #106 to ensure a quick fix for the affected projects. We can keep this issue open until we are certain about the definitive future solution for this. The use of lazy types is exactly what I've envisioned, but I'm looking forward to hear matt's opinion on the matter.

@dentatar @Gianfranco753 @0x587 @johansigfrids please try out the new release 0.4.1 and let us know if you encounter any additional issues. Thanks for reporting the bug! 🙂