Add Feature#disabled_groups

lib/flipper/feature.rb

@@ -228,9 +228,12 @@ def gate_values
     def groups
       groups_value.map { |name| Flipper.group(name) }.to_set
     end
-
     alias_method :enabled_groups, :groups
 
+    def disabled_groups
+      Flipper.groups - enabled_groups
+    end
+
     # Public: Returns the Set of group Symbol names enabled.
     def groups_value
       gate_values.groups

spec/flipper/feature_spec.rb

@@ -299,6 +299,33 @@
     end
   end
 
+  describe "#disabled_groups" do
+    context "when no groups enabled" do
+      it "returns empty set" do
+        subject.disabled_groups.should eq(Set.new)
+      end
+    end
+
+    context "when one or more groups enabled" do
+      before do
+        @staff = Flipper.register(:staff) { |thing| true }
+        @preview_features = Flipper.register(:preview_features) { |thing| true }
+        @not_enabled = Flipper.register(:not_enabled) { |thing| true }
+        @disabled = Flipper.register(:disabled) { |thing| true }
+        subject.enable @staff
+        subject.enable @preview_features
+        subject.disable @disabled
+      end
+
+      it "returns set of groups that are not enabled" do
+        subject.disabled_groups.should eq(Set[
+          @not_enabled,
+          @disabled,
+        ])
+      end
+    end
+  end
+
   describe "#groups_value" do
     context "when no groups enabled" do
       it "returns empty set" do