feat: Support Interfaces

README.md

@@ -44,6 +44,17 @@ class BaseObject < GraphQL::Schema::Object
 end
 ```
 
+Include the `ApolloFederation::Interface` module in your base interface module:
+
+```ruby
+module BaseInterface
+  include GraphQL::Schema::Interface
+  include ApolloFederation::Interface
+
+  field_class BaseField
+end
+```
+
 Finally, include the `ApolloFederation::Schema` module in your schema:
 
 ```ruby

lib/apollo-federation.rb

@@ -3,6 +3,7 @@
 require 'apollo-federation/version'
 require 'apollo-federation/schema'
 require 'apollo-federation/object'
+require 'apollo-federation/interface'
 require 'apollo-federation/field'
 require 'apollo-federation/tracing/proto'
 require 'apollo-federation/tracing/node_map'

lib/apollo-federation/federated_document_from_schema_definition.rb

@@ -26,6 +26,11 @@ def build_object_type_node(object_type)
       merge_directives(object_node, object_type.metadata[:federation_directives])
     end
 
+    def build_interface_type_node(interface_type)
+      field_node = super
+      merge_directives(field_node, interface_type.metadata[:federation_directives])
+    end
+
     def build_field_node(field_type)
       field_node = super
       merge_directives(field_node, field_type.metadata[:federation_directives])

lib/apollo-federation/interface.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'apollo-federation/has_directives'
+
+module ApolloFederation
+  module Interface
+    def self.included(klass)
+      klass.definition_methods do
+        include DefinitionMethods
+      end
+    end
+
+    module DefinitionMethods
+      include HasDirectives
+
+      def extend_type
+        add_directive(name: 'extends')
+      end
+
+      def key(fields:)
+        add_directive(
+          name: 'key',
+          arguments: [
+            name: 'fields',
+            values: fields,
+          ],
+        )
+      end
+    end
+  end
+end

lib/apollo-federation/object.rb

@@ -11,7 +11,6 @@ def self.included(klass)
     module ClassMethods
       include HasDirectives
 
-      # TODO: We should support extending interfaces at some point
       def extend_type
         add_directive(name: 'extends')
       end

lib/apollo-federation/schema.rb

@@ -29,7 +29,7 @@ def to_graphql
         end
 
         possible_entities = orig_defn.types.values.select do |type|
-          !type.introspection? && !type.default_scalar? &&
+          !type.introspection? && !type.default_scalar? && type.is_a?(GraphQL::ObjectType) &&
             type.metadata[:federation_directives]&.any? { |directive| directive[:name] == 'key' }
         end
 

spec/apollo-federation/service_field_spec.rb

@@ -5,6 +5,7 @@
 require 'apollo-federation/schema'
 require 'apollo-federation/field'
 require 'apollo-federation/object'
+require 'apollo-federation/interface'
 
 RSpec.describe ApolloFederation::ServiceField do
   let(:base_schema) do
@@ -144,6 +145,64 @@ def execute_sdl(schema)
     )
   end
 
+  it 'returns valid SDL for interface types' do
+    base_interface = Module.new do
+      include GraphQL::Schema::Interface
+      include ApolloFederation::Interface
+
+      graphql_name 'Interface'
+    end
+
+    product = Module.new do
+      include base_interface
+
+      graphql_name 'Product'
+
+      key fields: :upc
+      field :upc, String, null: false
+    end
+
+    book = Class.new(base_object) do
+      implements product
+
+      graphql_name 'Book'
+
+      extend_type
+
+      key fields: :upc
+      field :upc, String, null: false, external: true
+    end
+
+    pen = Class.new(base_object) do
+      implements product
+
+      graphql_name 'Pen'
+
+      key fields: :upc
+      field :upc, String, null: false
+    end
+
+    schema = Class.new(base_schema) do
+      orphan_types book, pen
+    end
+
+    expect(execute_sdl(schema)).to match_sdl(
+      <<~GRAPHQL,
+        type Book implements Product @extends @key(fields: "upc") {
+          upc: String! @external
+        }
+
+        type Pen implements Product @key(fields: "upc") {
+          upc: String!
+        }
+
+        interface Product @key(fields: "upc") {
+          upc: String!
+        }
+      GRAPHQL
+    )
+  end
+
   context 'when a Query object is provided' do
     it 'returns valid SDL for @key directives' do
       product = Class.new(base_object) do