feat: adds support for Interface Entities

Context: In Federation v2.3 there was added support for interfaces to be
Entities. There are two ways to consume this new feature.

1. Extend an Interface Entity with the `@interfaceObject` directive,
   when the Entity is defined by a different subgraph.

2. Implement the Interface Entity with its implementing Type Entitites.

There was previos work into getting the first part covered in this gem,
but we were still lacking support for the 2nd part. This work address
that.

The main issue here is the fact that an `Union` in `GraphQL` can't be an
`Interface` according to the [spec](https://spec.graphql.org/October2021/#sec-Unions.Type-Validation), but at the same time, according to the Apollo Federation [spec](https://www.apollographql.com/docs/federation/federated-types/interfaces), an interface can be an Entity, and an Entity is an Union. Therefore, we have to extend the validation (`assert_valid_union_member`) for the Entity union to allow `Interfaces` (more exactly `Modules`) as possible types.

lib/apollo-federation/entities_field.rb

@@ -40,13 +40,12 @@ def _entities(representations:)
 
         # TODO: Use warden or schema?
         type = context.warden.get_type(typename)
-        if type.nil? || type.kind != GraphQL::TypeKinds::OBJECT
+        if type.nil? || (type.kind != GraphQL::TypeKinds::OBJECT && type.kind != GraphQL::TypeKinds::INTERFACE)
           # TODO: Raise a specific error class?
           raise "The _entities resolver tried to load an entity for type \"#{typename}\"," \
                 ' but no object type of that name was found in the schema'
         end
 
-        # TODO: What if the type is an interface?
         type_class = class_of_type(type)
 
         if type_class.underscore_reference_keys
@@ -89,7 +88,7 @@ def _entities(representations:)
     private
 
     def class_of_type(type)
-      if defined?(GraphQL::ObjectType) && type.is_a?(GraphQL::ObjectType)
+      if (defined?(GraphQL::ObjectType) && type.is_a?(GraphQL::ObjectType)) || (defined?(GraphQL::InterfaceType) && type.is_a?(GraphQL::InterfaceType))
         type.metadata[:type_class]
       else
         type

lib/apollo-federation/entity.rb

@@ -9,5 +9,25 @@ class Entity < GraphQL::Schema::Union
     def self.resolve_type(object, context)
       context[object]
     end
+
+    # The main issue here is the fact that an union in GraphQL can't be an interface
+    # according to the [spec](https://spec.graphql.org/October2021/#sec-Unions.Type-Validation),
+    # but at the same time, according to the Federation spec, an interface can be an Entity,
+    # and an Entity is an union. Therefore, we have to extend this validation to allow interfaces
+    # (more precisely Modules) as possible types.
+    def self.assert_valid_union_member(type_defn)
+      case type_defn
+      when Class
+        if !type_defn.kind.object?
+          raise ArgumentError, "Union possible_types can only be object types (not #{type_defn.kind.name}, #{type_defn.inspect})"
+        end
+      when Module
+        # It's an interface entity, defined as a module
+      when String, GraphQL::Schema::LateBoundType
+        # Ok - assume it will get checked later
+      else
+        raise ArgumentError, "Union possible_types can only be class-based GraphQL types (not #{type_defn.inspect} (#{type_defn.class.name}))."
+      end
+    end
   end
 end

lib/apollo-federation/schema.rb

@@ -82,8 +82,8 @@ def schema_entities
         # Walk through all of the types and determine which ones are entities (any type with a
         # "key" directive)
         types_schema.send(:non_introspection_types).values.flatten.select do |type|
-          # TODO: Interfaces can have a key...
-          type.include?(ApolloFederation::Object) &&
+          # TODO: Find Objects that implement interfaces that are entities. Make sure they are also entities.
+          (type.include?(ApolloFederation::Object) || type.include?(ApolloFederation::Interface)) &&
             type.federation_directives&.any? { |directive| directive[:name] == 'key' }
         end
       end