default_resolver is not called when type implements an interface

[marmor157]

Describe the Bug

There's an example in strawberry docs that shows how to return a dict from a resolver and get it automatically converted to a GraphQL type. It does not work, when returned type implements an interface.

System Information

• Operating system: MacOS
• Strawberry version (if applicable): 0.247.2

Additional Context

Reproduction:

def custom_resolver(obj, field):
    try:
        return obj[field]
    except (KeyError, TypeError):
        return getattr(obj, field)

@strawberry.interface
class UserInterface:
    name: str

@strawberry.type
class Client(UserInterface):
    companyName: str


@strawberry.type
class Query:
    def user(self, info) -> UserInterface:  # this won't type check, but will work at runtime
        return {"name": "Patrick", "companyName": "company", "__typename": "Client"}


schema = strawberry.Schema(
    query=Query, types=[Client], config=StrawberryConfig(default_resolver=custom_resolver)
)

It does not work when type hint on function return type is either UserInterface nor Client. Client needs to stop implementing interface for it to start working.

I've checked it with debugger and when it implements the interface it does not even go into the custom_resolver

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[patrick91]

Thanks for the bug report!

I'm not sure yet if and how we can properly allow this, but here's an example code that makes it work:

from typing import Any

import strawberry
from strawberry.schema.config import StrawberryConfig
from strawberry.types.base import get_object_definition


def test_can_change_default_resolver():
    def custom_resolver(obj, field):
        try:
            return obj[field]
        except (KeyError, TypeError):
            return getattr(obj, field)

    @strawberry.interface
    class UserInterface:
        name: str

        @classmethod
        def is_type_of(cls, obj: Any, _info: strawberry.Info) -> bool:
            strawberry_definition = get_object_definition(cls, strict=True)

            return obj.get("__typename") == strawberry_definition.name

    @strawberry.type
    class Client(UserInterface):
        companyName: str

    @strawberry.type
    class Query:
        @strawberry.field
        def user(self) -> UserInterface:
            return {"name": "Patrick", "companyName": "company", "__typename": "Client"}  # type: ignore

    schema = strawberry.Schema(
        query=Query,
        config=StrawberryConfig(default_resolver=custom_resolver),
        types=[Client],
    )

    query = "{ user { __typename, name, ... on Client { companyName } } }"

    result = schema.execute_sync(query)

    assert not result.errors
    assert result.data == {
        "user": {"__typename": "Client", "name": "Patrick", "companyName": "company"}
    }

[patrick91]

Maybe we can also allow to customise the default is_type_of 🤔