Add support for generic unions

RELEASE.md

@@ -0,0 +1,3 @@
+Release type: patch
+
+Attempt to merge union types during schema conversion.

strawberry/schema/schema_converter.py

@@ -856,9 +856,14 @@ def from_union(self, union: StrawberryUnion) -> GraphQLUnionType:
 
             if isinstance(graphql_type, GraphQLInputObjectType):
                 raise InvalidTypeInputForUnion(graphql_type)
-            assert isinstance(graphql_type, GraphQLObjectType)
+            assert isinstance(graphql_type, (GraphQLObjectType, GraphQLUnionType))
 
-            graphql_types.append(graphql_type)
+            # If the graphql_type is a GraphQLUnionType, merge its child types
+            if isinstance(graphql_type, GraphQLUnionType):
+                # Add the child types of the GraphQLUnionType to the list of graphql_types
+                graphql_types.extend(graphql_type.types)
+            else:
+                graphql_types.append(graphql_type)
 
         graphql_union = GraphQLUnionType(
             name=union_name,

tests/schema/test_union.py

@@ -856,3 +856,117 @@
 
     assert not result.errors
     assert result.data["something"] == {"__typename": "A", "a": 5}
+
+
+@pytest.mark.xfail(reason="Not supported yet")
+def test_generic_union_with_annotated():
+    @strawberry.type
+    class SomeType:
+        id: strawberry.ID
+        name: str
+
+    @strawberry.type
+    class NotFoundError:
+        id: strawberry.ID
+        message: str
+
+    T = TypeVar("T")
+
+    @strawberry.type
+    class ObjectQueries(Generic[T]):
+        @strawberry.field
+        def by_id(
+            self, id: strawberry.ID
+        ) -> Annotated[Union[T, NotFoundError], strawberry.union("ByIdResult")]: ...
+
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def some_type_queries(self, id: strawberry.ID) -> ObjectQueries[SomeType]:
+            raise NotImplementedError()
+
+    schema = strawberry.Schema(Query)
+
+    # TODO: check the name
+    assert (
+        str(schema)
+        == textwrap.dedent(
+            """
+            type NotFoundError {
+              id: ID!
+              message: String!
+            }
+
+            type Query {
+              someTypeQueries(id: ID!): SomeTypeByIdResult!
+            }
+
+            type SomeType {
+              id: ID!
+              name: String!
+            }
+
+            union SomeTypeNotFoundError = SomeType | NotFoundError
+
+            type SomeTypeObjectQueries {
+              byId(id: ID!): SomeTypeByIdResult!
+            }
+            """
+        ).strip()
+    )
+
+
+def test_generic_union_with_annotated_inside():
+    @strawberry.type
+    class SomeType:
+        id: strawberry.ID
+        name: str
+
+    @strawberry.type
+    class NotFoundError:
+        id: strawberry.ID
+        message: str
+
+    T = TypeVar("T")
+
+    @strawberry.type
+    class ObjectQueries(Generic[T]):
+        @strawberry.field
+        def by_id(
+            self, id: strawberry.ID
+        ) -> Union[T, Annotated[NotFoundError, strawberry.union("ByIdResult")]]: ...
+
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def some_type_queries(self, id: strawberry.ID) -> ObjectQueries[SomeType]:
+            return ObjectQueries(SomeType)
+
+    schema = strawberry.Schema(Query)
+
+    assert (
+        str(schema)
+        == textwrap.dedent(
+            """
+            type NotFoundError {
+              id: ID!
+              message: String!
+            }
+
+            type Query {
+              someTypeQueries(id: ID!): SomeTypeObjectQueries!
+            }
+
+            type SomeType {
+              id: ID!
+              name: String!
+            }
+
+            union SomeTypeByIdResult = SomeType | NotFoundError
+
+            type SomeTypeObjectQueries {
+              byId(id: ID!): SomeTypeByIdResult!
+            }
+            """
+        ).strip()
+    )