feat: automatically include implementers of interfaces in schema

strawberry/schema/schema.py

@@ -187,6 +187,30 @@ class Query:
                 },
             )
 
+            # due to 7dc715c9a65e6e0b6ea0ea3e903cf20284e3316b (#1684, #1586),
+            # fields are evaluated lazily. This means, that we only know about all
+            # interfaces after the schema is created.
+            # We need to find a way to add the extra implementations to the schema after creating it.
+            # This is not officially supported by GraphQL core and would be somewhat hacky.
+
+            # TODO: prevent duplicates - no error, but duplicate processing is inefficient
+            for (
+                extra_interface_type
+            ) in self.schema_converter.extra_interface_child_map.values():
+                graphql_type = self.schema_converter.from_object(extra_interface_type)
+                graphql_types.append(graphql_type)
+
+            self._schema = GraphQLSchema(
+                query=query_type,
+                mutation=mutation_type,
+                subscription=subscription_type if subscription else None,
+                directives=specified_directives + tuple(graphql_directives),
+                types=graphql_types,
+                extensions={
+                    GraphQLCoreConverter.DEFINITION_BACKREF: self,
+                },
+            )
+
         except TypeError as error:
             # GraphQL core throws a TypeError if there's any exception raised
             # during the schema creation, so we check if the cause was a

strawberry/schema/schema_converter.py

@@ -92,7 +92,6 @@
     from strawberry.types.info import Info
     from strawberry.types.scalar import ScalarDefinition
 
-
 FieldType = TypeVar(
     "FieldType", bound=Union[GraphQLField, GraphQLInputField], covariant=True
 )
@@ -245,6 +244,7 @@ def __init__(
         scalar_registry: Dict[object, Union[ScalarWrapper, ScalarDefinition]],
         get_fields: Callable[[StrawberryObjectDefinition], List[StrawberryField]],
     ) -> None:
+        self.extra_interface_child_map: Dict[str, StrawberryObjectDefinition] = {}
         self.type_map: Dict[str, ConcreteType] = {}
         self.config = config
         self.scalar_registry = scalar_registry
@@ -583,6 +583,20 @@ def resolve_type(
             definition=interface, implementation=graphql_interface
         )
 
+        # get all subclasses of the interface
+        subclasses = interface.origin.__subclasses__()
+        for subclass in subclasses:
+            # check if subclass is strawberry type
+
+            subclass_object_definition = get_object_definition(subclass, strict=False)
+            object_type_name = self.config.name_converter.from_type(
+                subclass_object_definition
+            )
+
+            if object_type_name not in self.type_map:
+                self.extra_interface_child_map[object_type_name] = (
+                    subclass_object_definition
+                )
         return graphql_interface
 
     def from_list(self, type_: StrawberryList) -> GraphQLList:

tests/schema/test_interface.py

@@ -8,6 +8,94 @@
 from strawberry.types.base import StrawberryObjectDefinition
 
 
+def test_query_interface_without_extra_types_duplicate_reference():
+    @strawberry.interface
+    class Cheese:
+        name: str
+
+    @strawberry.type
+    class Swiss(Cheese):
+        canton: str
+
+    @strawberry.type
+    class Italian(Cheese):
+        province: str
+
+    @strawberry.type
+    class Root:
+        @strawberry.field
+        def assortment(self) -> List[Cheese]:
+            return [
+                Italian(name="Asiago", province="Friuli"),
+                Swiss(name="Tomme", canton="Vaud"),
+            ]
+
+        @strawberry.field
+        def italians(self) -> List[Italian]:
+            return [Italian(name="Asiago", province="Friuli")]
+
+    schema = strawberry.Schema(query=Root)
+
+    query = """{
+        assortment {
+            name
+            ... on Italian { province }
+            ... on Swiss { canton }
+        }
+    }"""
+
+    result = schema.execute_sync(query)
+
+    assert not result.errors
+    assert result.data is not None
+    assert result.data["assortment"] == [
+        {"name": "Asiago", "province": "Friuli"},
+        {"canton": "Vaud", "name": "Tomme"},
+    ]
+
+
+def test_query_interface_without_extra_types():
+    @strawberry.interface
+    class Cheese:
+        name: str
+
+    @strawberry.type
+    class Swiss(Cheese):
+        canton: str
+
+    @strawberry.type
+    class Italian(Cheese):
+        province: str
+
+    @strawberry.type
+    class Root:
+        @strawberry.field
+        def assortment(self) -> List[Cheese]:
+            return [
+                Italian(name="Asiago", province="Friuli"),
+                Swiss(name="Tomme", canton="Vaud"),
+            ]
+
+    schema = strawberry.Schema(query=Root)
+
+    query = """{
+        assortment {
+            name
+            ... on Italian { province }
+            ... on Swiss { canton }
+        }
+    }"""
+
+    result = schema.execute_sync(query)
+
+    assert not result.errors
+    assert result.data is not None
+    assert result.data["assortment"] == [
+        {"name": "Asiago", "province": "Friuli"},
+        {"canton": "Vaud", "name": "Tomme"},
+    ]
+
+
 def test_query_interface():
     @strawberry.interface
     class Cheese: