fix: Adds support for defining Interface entities resolvers

README.md

@@ -282,6 +282,20 @@ class Product < BaseObject
 end
 ```
 
+### The `@interfaceObject` directive (Apollo Federation v2.3)
+
+[Apollo documentation](https://www.apollographql.com/docs/federation/federated-types/federated-directives/#interfaceobject)
+
+Call `interface_object` within your class definition:
+
+```ruby
+class Product < BaseObject
+  interface_object
+  key fields: :id
+  field :id, ID, null: false
+end
+```
+
 ### The `@tag` directive (Apollo Federation v2)
 
 [Apollo documentation](https://www.apollographql.com/docs/federation/federated-types/federated-directives/#tag)
@@ -327,7 +341,7 @@ Define a `resolve_reference` class method on your object. The method will be pas
 class User < BaseObject
   key fields: :user_id
   field :user_id, ID, null: false
-  
+
   def self.resolve_reference(reference, context)
     USERS.find { |user| user[:userId] == reference[:userId] }
   end
@@ -341,7 +355,7 @@ class User < BaseObject
   key fields: :user_id
   field :user_id, ID, null: false
   underscore_reference_keys true
-  
+
   def self.resolve_reference(reference, context)
     USERS.find { |user| user[:user_id] == reference[:user_id] }
   end
@@ -358,6 +372,61 @@ class BaseObject < GraphQL::Schema::Object
 end
 ```
 
+### Reference resolvers for Interfaces
+
+[Apollo documentation](https://www.apollographql.com/docs/federation/federated-types/interfaces/#required-resolvers)
+
+```ruby
+  module Product
+    include BaseInterface
+
+    key fields: :id
+    field :id, ID, null: false
+    field :title, String, null: true
+
+    definition_methods do
+      def resolve_type(obj, _ctx)
+        if obj.is_a?(Book)
+          BookType
+        elsif obj.is_a?(Movie)
+          MovieType
+      end
+
+      def resolve_reference(reference, _context)
+        PRODUCTS.find { |product| product[:id] == reference[:id] }
+      end
+    end
+  end
+
+  class BookType < BaseObject
+    implements Product
+    graphql_name 'Book'
+
+    key fields: :id
+    field :id, ID, null: false
+    field :title, String, null: true
+    field :pages, Integer, null: true
+
+    def self.resolve_reference(reference, _context)
+      BOOKS.find { |book| book[:id] == reference[:id] }
+    end
+  end
+
+  class MovieType < BaseObject
+    implements Product
+    graphql_name 'Movie'
+
+    key fields: :id
+    field :id, ID, null: false
+    field :title, String, null: true
+    field :minutes, Integer, null: true
+
+    def self.resolve_reference(reference, _context)
+      MOVIES.find { |movie| movie[:id] == reference[:id] }
+    end
+  end
+```
+
 ### Tracing
 
 To support [federated tracing](https://www.apollographql.com/docs/apollo-server/federation/metrics/):

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 +9,18 @@ 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 as possible types.
+    def self.assert_valid_union_member(type_defn)
+      if type_defn.is_a?(Module) &&
+          type_defn.included_modules.include?(ApolloFederation::Interface)
+        # It's an interface entity, defined as a module
+      else
+        super(type_defn)
+      end
+    end
   end
 end

lib/apollo-federation/interface.rb

@@ -35,6 +35,18 @@ def key(fields:, camelize: true)
           ],
         )
       end
+
+      def underscore_reference_keys(value = nil)
+        if value.nil?
+          if @underscore_reference_keys.nil?
+            find_inherited_value(:underscore_reference_keys, false)
+          else
+            @underscore_reference_keys
+          end
+        else
+          @underscore_reference_keys = value
+        end
+      end
     end
   end
 end

lib/apollo-federation/schema.rb

@@ -79,13 +79,53 @@ def schema_entities
         # infinite recursion
         types_schema.orphan_types(original_query)
 
-        # 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) &&
-            type.federation_directives&.any? { |directive| directive[:name] == 'key' }
+        # Walk through all of the types and interfaces and determine which ones are entities
+        # (any type with a "key" directive)
+        # However, for interface entities, don't add them straight away, but first check that
+        # all implementing types of the interfaces are also entities.
+        federation_interfaces = []
+        interface_types_map = {}
+
+        entities_collection = types_schema.send(:non_introspection_types).values.flatten.select do |type|
+          # keep track of the interfaces -> type relations.
+          if type.respond_to?(:implements)
+            type.implements.each do |interface|
+              interface_types_map[interface.abstract_type.graphql_name] ||= []
+              interface_types_map[interface.abstract_type.graphql_name] << type.graphql_name
+            end
+          end
+
+          # Only add Type entities to the collection
+          # Interface entities will be added later if all implementing types are entities
+          if type.include?(ApolloFederation::Object) && includes_key_directive?(type)
+            true
+          elsif type.include?(ApolloFederation::Interface) && includes_key_directive?(type)
+            federation_interfaces << type
+            false
+          else
+            false
+          end
         end
+
+        if federation_interfaces.any?
+          entity_names = entities_collection.map(&:graphql_name)
+
+          federation_interfaces.each do |interface|
+            members = interface_types_map.fetch(interface.graphql_name, [])
+            not_entity_members = members.reject { |member| entity_names.include?(member) }
+
+            # If all interface members are entities, it is valid so we add it to the collection
+            if not_entity_members.empty?
+              entities_collection << interface
+            else
+              raise "Interface #{interface.graphql_name} is not valid. " \
+                "Types `#{not_entity_members.join(', ')}` do not have a @key directive. " \
+                "All types that implement an interface with a @key directive must also have a @key directive."
+            end
+          end
+        end
+
+        entities_collection
       end
 
       def federation_query(query_obj)
@@ -110,6 +150,10 @@ def federation_query(query_obj)
         klass.define_service_field
         klass
       end
+
+      def includes_key_directive?(type)
+        type.federation_directives&.any? { |directive| directive[:name] == 'key' }
+      end
     end
   end
 end