Fix subscription inheritance/override

lib/graphql/schema.rb

@@ -132,7 +132,13 @@ def deprecated_graphql_definition
       end
 
       # @return [GraphQL::Subscriptions]
-      attr_accessor :subscriptions
+      def subscriptions(inherited: true)
+        defined?(@subscriptions) ? @subscriptions : (inherited ? find_inherited_value(:subscriptions, nil) : nil)
+      end
+
+      def subscriptions=(new_implementation)
+        @subscriptions = new_implementation
+      end
 
       # Returns the JSON response of {Introspection::INTROSPECTION_QUERY}.
       # @see {#as_json}

lib/graphql/subscriptions.rb

@@ -26,7 +26,7 @@ class SubscriptionScopeMissingError < GraphQL::Error
     def self.use(defn, options = {})
       schema = defn.is_a?(Class) ? defn : defn.target
 
-      if schema.subscriptions
+      if schema.subscriptions(inherited: false)
         raise ArgumentError, "Can't reinstall subscriptions. #{schema} is using #{schema.subscriptions}, can't also add #{self}"
       end
 

spec/graphql/schema_spec.rb

@@ -23,6 +23,9 @@ class Subscription < GraphQL::Schema::Object
       field :some_field, String
     end
 
+    class CustomSubscriptions < GraphQL::Subscriptions::ActionCableSubscriptions
+    end
+
     let(:base_schema) do
       Class.new(GraphQL::Schema) do
         query Query
@@ -47,6 +50,7 @@ class Subscription < GraphQL::Schema::Object
         multiplex_analyzer Object.new
         rescue_from(StandardError) { }
         use GraphQL::Backtrace
+        use GraphQL::Subscriptions::ActionCableSubscriptions, action_cable: nil, action_cable_coder: JSON
       end
     end
 
@@ -73,11 +77,15 @@ class Subscription < GraphQL::Schema::Object
       assert_equal base_schema.query_analyzers, schema.query_analyzers
       assert_equal base_schema.multiplex_analyzers, schema.multiplex_analyzers
       assert_equal base_schema.disable_introspection_entry_points?, schema.disable_introspection_entry_points?
-      assert_equal [GraphQL::Backtrace], schema.plugins.map(&:first)
+      assert_equal [GraphQL::Backtrace, GraphQL::Subscriptions::ActionCableSubscriptions], schema.plugins.map(&:first)
+      assert_instance_of GraphQL::Subscriptions::ActionCableSubscriptions, schema.subscriptions
     end
 
     it "can override configuration from its superclass" do
-      schema = Class.new(base_schema)
+      schema = Class.new(base_schema) do
+        use CustomSubscriptions, action_cable: nil, action_cable_coder: JSON
+      end
+
       query = Class.new(GraphQL::Schema::Object) do
         graphql_name 'Query'
         field :some_field, String
@@ -141,9 +149,10 @@ class Subscription < GraphQL::Schema::Object
       assert_equal schema.directives, GraphQL::Schema.default_directives.merge(DummyFeature1.graphql_name => DummyFeature1, DummyFeature2.graphql_name => DummyFeature2)
       assert_equal base_schema.query_analyzers + [query_analyzer], schema.query_analyzers
       assert_equal base_schema.multiplex_analyzers + [multiplex_analyzer], schema.multiplex_analyzers
-      assert_equal [GraphQL::Backtrace], schema.plugins.map(&:first)
+      assert_equal [GraphQL::Backtrace, GraphQL::Subscriptions::ActionCableSubscriptions, CustomSubscriptions], schema.plugins.map(&:first)
       assert_equal [GraphQL::Tracing::DataDogTracing, GraphQL::Backtrace::Tracer], base_schema.tracers
       assert_equal [GraphQL::Tracing::DataDogTracing, GraphQL::Backtrace::Tracer, GraphQL::Tracing::NewRelicTracing], schema.tracers
+      assert_instance_of CustomSubscriptions, schema.subscriptions
     end
   end