Recovers error_messages for ActiveRecordInstanceTag. [#4078 state:resolved]

Signed-off-by: Nwycats <wycats@gmail.com>

actionpack/lib/action_view/helpers/active_model_helper.rb

@@ -295,6 +295,10 @@ def error_wrapping(html_tag)
         end
       end
 
+      def error_message
+        object.errors[@method_name]
+      end
+
       def column_type
         object.send(:column_for_attribute, @method_name).type
       end

actionpack/test/template/active_model_helper_test.rb

@@ -147,6 +147,20 @@ def test_text_field_with_errors
     )
   end
 
+  def test_field_error_proc
+    old_proc = ActionView::Base.field_error_proc
+    ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
+      %(<div class=\"fieldWithErrors\">#{html_tag} <span class="error">#{[instance.error_message].join(', ')}</span></div>).html_safe
+    end
+
+    assert_dom_equal(
+      %(<div class="fieldWithErrors"><input id="post_author_name" name="post[author_name]" size="30" type="text" value="" /> <span class="error">can't be empty</span></div>),
+      text_field("post", "author_name")
+    )
+  ensure
+    ActionView::Base.field_error_proc = old_proc if old_proc
+  end
+
   def test_form_with_string
     assert_dom_equal(
       %(<form action="create" method="post"><p><label for="post_title">Title</label><br /><input id="post_title" name="post[title]" size="30" type="text" value="Hello World" /></p>\n<p><label for="post_body">Body</label><br /><div class="fieldWithErrors"><textarea cols="40" id="post_body" name="post[body]" rows="20">Back to the hill and over it again!</textarea></div></p><input name="commit" type="submit" value="Create" /></form>),