Invalid Type for Union Error

Description

This error is thrown when trying to create an union with one or more type that’s are allowed in unions, for example the following code will throw this error:

import strawberry

from typing import Union, Annotated


@strawberry.type
class Example:
    name: str


ExampleUnion = Annotated[Union[Example, int], strawberry.union("ExampleUnion")]


@strawberry.type
class Query:
    field: ExampleUnion


schema = strawberry.Schema(query=Query)

This happens because GraphQL doesn’t support scalars as union members.

How to fix this error

Since GraphQL doesn’t allow scalars as union members, a workaround is to create a wrapper type that contains the scalar value and use that instead. For example the following code will create a union type between Example and IntWrapper which is a wrapper on top of the int scalar:

import strawberry

from typing import Union, Annotated


@strawberry.type
class Example:
    name: str


@strawberry.type
class IntWrapper:
    value: int


ExampleUnion = Annotated[Union[Example, IntWrapper], strawberry.union("ExampleUnion")]


@strawberry.type
class Query:
    field: ExampleUnion


schema = strawberry.Schema(query=Query)