diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 1431228..4ee8ed2 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -289,8 +289,8 @@ module ActiveModel
       end
 
       defaults = @base.class.lookup_ancestors.map do |klass|
-        [ :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.underscore}.attributes.#{attribute}.#{type}",
-          :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.underscore}.#{type}" ]
+        [ :"#{@base.class.i18n_scope}.errors.models.#{klass.i18n_key}.attributes.#{attribute}.#{type}",
+          :"#{@base.class.i18n_scope}.errors.models.#{klass.i18n_key}.#{type}" ]
       end
 
       defaults << options.delete(:message)
diff --git a/activemodel/lib/active_model/naming.rb b/activemodel/lib/active_model/naming.rb
index d79635c..2fa0655 100644
--- a/activemodel/lib/active_model/naming.rb
+++ b/activemodel/lib/active_model/naming.rb
@@ -27,7 +27,7 @@ module ActiveModel
                            @klass.respond_to?(:i18n_scope)
 
       defaults = @klass.lookup_ancestors.map do |klass|
-        klass.model_name.underscore.to_sym
+        klass.i18n_key
       end
 
       defaults << options.delete(:default) if options[:default]
@@ -40,7 +40,7 @@ module ActiveModel
 
   # == Active Model Naming
   #
-  # Creates a +model_name+ method on your object.
+  # Creates +model_name+ and +i18n_key+ methods on your object.
   #
   # To implement, just extend ActiveModel::Naming in your object:
   #
@@ -51,6 +51,9 @@ module ActiveModel
   #   BookCover.model_name        # => "BookCover"
   #   BookCover.model_name.human  # => "Book cover"
   #
+  #   BookCover.i18n_key              # => "book_cover"
+  #   BookModule::BookCover.i18n_key  # => "book_module.book_cover"
+  #
   # Providing the functionality that ActiveModel::Naming provides in your object
   # is required to pass the Active Model Lint test.  So either extending the provided
   # method below, or rolling your own is required..
@@ -61,6 +64,11 @@ module ActiveModel
       @_model_name ||= ActiveModel::Name.new(self)
     end
 
+    # Returns an i18n key symbol for module.
+    def i18n_key
+      @_i18n_key ||= model_name.underscore.tr('/', '.').to_sym
+    end
+
     # Returns the plural class name of a record or class. Examples:
     #
     #   ActiveModel::Naming.plural(post)             # => "posts"
diff --git a/activemodel/lib/active_model/translation.rb b/activemodel/lib/active_model/translation.rb
index 6c1cecd..a03c6f6 100644
--- a/activemodel/lib/active_model/translation.rb
+++ b/activemodel/lib/active_model/translation.rb
@@ -44,7 +44,7 @@ module ActiveModel
     # 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.underscore}.#{attribute}"
+        :"#{self.i18n_scope}.attributes.#{klass.i18n_key}.#{attribute}"
       end
 
       defaults << :"attributes.#{attribute}"
diff --git a/activemodel/test/cases/validations/i18n_validation_test.rb b/activemodel/test/cases/validations/i18n_validation_test.rb
index e9f0e43..a193364 100644
--- a/activemodel/test/cases/validations/i18n_validation_test.rb
+++ b/activemodel/test/cases/validations/i18n_validation_test.rb
@@ -55,6 +55,14 @@ class I18nValidationTest < ActiveModel::TestCase
     assert_equal ["Person's name not found"], @person.errors.full_messages
   end
 
+  def test_errors_full_messages_translates_human_attribute_name_for_model_in_module_attributes
+    I18n.backend.store_translations('en', :activemodel => {:attributes => {:person_module => {:person => {:name => "Person in Module's name"}}}})
+    person = PersonModule::Person.new
+    person.errors.add(:name, 'not found')
+    PersonModule::Person.expects(:human_attribute_name).with(:name, :default => 'Name').returns("Person in Module's name")
+    assert_equal ["Person in Module's name not found"], person.errors.full_messages
+  end
+
   def test_errors_full_messages_uses_format
     I18n.backend.store_translations('en', :errors => {:format => "Field %{attribute} %{message}"})
     @person.errors.add('name', 'empty')
@@ -363,4 +371,17 @@ class I18nValidationTest < ActiveModel::TestCase
     assert_equal ["I am a custom error"], @person.errors[:title]
   end
 
+  # test class i18n scope with Module
+
+  def test_model_with_module_i18n_scope
+      I18n.backend.store_translations 'en', :activemodel => {:errors => {:models => {:person_module => {:person => {:blank => 'generic blank'}}}}}
+      PersonModule::Person.validates_presence_of :title
+      person = PersonModule::Person.new :title => nil
+      person.valid?
+      assert_equal ['generic blank'], person.errors[:title]
+
+      I18n.backend.store_translations 'en', :activemodel => {:errors => {:models => {:person_module => {:person => {:attributes => {:title => {:blank => 'title cannot be blank'}}}}}}}
+      person.valid?
+      assert_equal ['title cannot be blank'], person.errors[:title]
+  end
 end
diff --git a/activemodel/test/models/person.rb b/activemodel/test/models/person.rb
index cf16a38..eb84f7a 100644
--- a/activemodel/test/models/person.rb
+++ b/activemodel/test/models/person.rb
@@ -11,3 +11,8 @@ end
 
 class Child < Person
 end
+
+module PersonModule
+  class Person < ::Person
+  end
+end