Support both conventions for translations for namespaced models.

3.0.0 - 3.0.1 required 'namespace/model'
3.0.2 - 3.0.5 required 'namespace.model' (nested). It has the advantage of
keeping the i18n file DRY when multiple models are in the same namespace,
but can lead to translation key conflicts if models are nested within
models.

[rails#6448, rails#5572]

activemodel/lib/active_model/naming.rb

@@ -14,7 +14,7 @@ def initialize(klass)
       @human = ActiveSupport::Inflector.humanize(@element).freeze
       @collection = ActiveSupport::Inflector.tableize(self).freeze
       @partial_path = "#{@collection}/#{@element}".freeze
-      @i18n_key = self.underscore.to_sym
+      @i18n_key = ActiveSupport::Inflector.underscore(self).tr('/', '.').to_sym
     end
 
     # Transform the model name into a more humane format, using I18n. By default,
@@ -28,8 +28,9 @@ def human(options={})
                            @klass.respond_to?(:i18n_scope)
 
       defaults = @klass.lookup_ancestors.map do |klass|
-        klass.model_name.i18n_key
-      end
+        [klass.model_name.i18n_key,
+         klass.model_name.i18n_key.to_s.tr('.', '/').to_sym]
+      end.flatten
 
       defaults << options.delete(:default) if options[:default]
       defaults << @human

activemodel/lib/active_model/translation.rb

@@ -44,8 +44,9 @@ def lookup_ancestors
     # Specify +options+ with additional translating options.
     def human_attribute_name(attribute, options = {})
       defaults = lookup_ancestors.map do |klass|
-        :"#{self.i18n_scope}.attributes.#{klass.model_name.i18n_key}.#{attribute}"
-      end
+        [:"#{self.i18n_scope}.attributes.#{klass.model_name.i18n_key}.#{attribute}",
+         :"#{self.i18n_scope}.attributes.#{klass.model_name.i18n_key.to_s.tr('.', '/')}.#{attribute}"]
+      end.flatten
 
       defaults << :"attributes.#{attribute}"
       defaults << options.delete(:default) if options[:default]

activemodel/test/cases/translation_test.rb

@@ -53,5 +53,15 @@ def test_translated_model_names_with_ancestors_fallback
     I18n.backend.store_translations 'en', :activemodel => {:models => {:person => 'person model'} }
     assert_equal 'person model', Child.model_name.human
   end
+
+  def test_alternate_namespaced_model_attribute_translation
+    I18n.backend.store_translations 'en', :activemodel => {:attributes => {:person => {:gender => {:attribute => 'person gender attribute'}}}}
+    assert_equal 'person gender attribute', Person::Gender.human_attribute_name('attribute')
+  end
+
+  def test_alternate_namespaced_model_translation
+    I18n.backend.store_translations 'en', :activemodel => {:models => {:person => {:gender => 'person gender model'}}}
+    assert_equal 'person gender model', Person::Gender.model_name.human
+  end
 end