Refactor inheritance in attributes and backend plugins

- removes register methods which we don't need
- freezes attributes/backends after inheritance, to ensure they are not
  modified later

lib/mobility/plugins/attributes.rb

@@ -32,8 +32,15 @@ def inspect
       end
 
       included_hook do |klass|
-        klass.extend ClassMethods
-        @names.each { |name| klass.register_mobility_attribute(name) }
+        names = @names
+
+        klass.class_eval do
+          extend ClassMethods
+          names.each { |name| mobility_attributes << name.to_s }
+          mobility_attributes.uniq!
+        rescue FrozenError
+          raise FrozenAttributesError, "Attempting to translate these attributes on #{klass}, which has already been subclassed: #{names.join(', ')}."
+        end
       end
 
       module ClassMethods
@@ -44,23 +51,20 @@ def mobility_attribute?(name)
           mobility_attributes.include?(name.to_s)
         end
 
-        # Register a new attribute name. Public, but treat as internal.
-        # @param [String, Symbol] Attribute name
-        def register_mobility_attribute(name)
-          (self.mobility_attributes << name.to_s).uniq!
-        end
-
-        def inherited(klass)
-          super
-          mobility_attributes.each { |name| klass.register_mobility_attribute(name) }
-        end
-
         # Return translated attribute names on this model.
         # @return [Array<String>] Attribute names
         def mobility_attributes
           @mobility_attributes ||= []
         end
+
+        def inherited(klass)
+          super
+          attrs = mobility_attributes.freeze # ensure attributes are not modified after being inherited
+          klass.class_eval { @mobility_attributes = attrs.dup }
+        end
       end
+
+      class FrozenAttributesError < Error; end
     end
 
     register_plugin(:attributes, Attributes)

lib/mobility/plugins/backend.rb

@@ -56,8 +56,11 @@ def included(klass)
 
           backend_class.setup_model(klass, names)
 
-          @names.each do |name|
-            klass.register_mobility_backend_class(name, @backend_class)
+          names = @names
+          backend_class = @backend_class
+
+          klass.class_eval do
+            names.each { |name| mobility_backend_classes[name.to_sym] = backend_class }
           end
 
           backend_class
@@ -137,12 +140,9 @@ def mobility_backend_class(name)
           raise KeyError, "No backend for: #{name}"
         end
 
-        def register_mobility_backend_class(name, backend_class)
-          mobility_backend_classes[name.to_sym] = backend_class
-        end
-
         def inherited(klass)
-          klass.mobility_backend_classes.merge!(@mobility_backend_classes)
+          parent_classes = mobility_backend_classes.freeze # ensure backend classes are not modified after being inherited
+          klass.class_eval { @mobility_backend_classes = parent_classes.dup }
           super
         end
 

spec/mobility/plugins/attributes_spec.rb

@@ -22,6 +22,18 @@
     end
   end
 
+  describe ".mobility_attributes" do
+    it "returns attributes included in one of multiple pluggable modules" do
+      translations1 = translations_class.new("title", "content")
+      translations2 = translations_class.new("author")
+      klass = Class.new
+      klass.include translations1
+      klass.include translations2
+
+      expect(klass.mobility_attributes).to eq(%w[title content author])
+    end
+  end
+
   describe ".mobility_attribute?" do
     it "returns true if name is an attribute" do
       translations = translations_class.new("title", "content")
@@ -46,4 +58,39 @@
       expect(klass.mobility_attribute?("foo")).to eq(false)
     end
   end
+
+  describe "inheritance" do
+    it "inherits mobility attributes from parent" do
+      mod1 = translations_class.new("title", "content")
+      klass1 = Class.new
+      klass1.include mod1
+
+      klass2 = Class.new(klass1)
+
+      expect(klass1.mobility_attributes).to match_array(%w[title content])
+      expect(klass2.mobility_attributes).to match_array(%w[title content])
+
+      mod2 = translations_class.new("author")
+      klass2.include mod2
+
+      expect(klass1.mobility_attributes).to match_array(%w[title content])
+      expect(klass2.mobility_attributes).to match_array(%w[title content author])
+    end
+
+    it "freezes inherited attributes to ensure they are not changed after subclassing" do
+      mod1 = translations_class.new("title")
+      stub_const("Foo", Class.new)
+      klass1 = Foo
+      klass1.include mod1
+
+      Class.new(klass1)
+      expect(klass1.mobility_attributes).to be_frozen
+
+      mod2 = translations_class.new("content")
+      expect {
+        klass1.include mod2
+      }.to raise_error(Mobility::Plugins::Attributes::FrozenAttributesError,
+                       "Attempting to translate these attributes on Foo, which has already been subclassed: content.")
+    end
+  end
 end

spec/mobility/plugins/backend_spec.rb

@@ -100,6 +100,18 @@
         content_backend_class = model_subclass.mobility_backend_class("content")
         expect(content_backend_class).to be <(backend_class_2)
       end
+
+      it "freezes backends after subclassing" do
+        backend_class_1 = Class.new
+        backend_class_1.include Mobility::Backend
+
+        mod1 = translations_class.new("title", backend: backend_class_1)
+        model_class.include mod1
+
+        Class.new(model_class)
+
+        expect(model_class.send(:mobility_backend_classes)).to be_frozen
+      end
     end
 
     describe "#mobility_backends" do