Round usec when writing timestamp attribute.

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -59,11 +59,14 @@ def #{attr_name}=(original_time)
                 unless time.acts_like?(:time)
                   time = time.is_a?(String) ? Time.zone.parse(time) : time.to_time rescue time
                 end
-                time = time.in_time_zone rescue nil if time
-                changed = read_attribute(:#{attr_name}) != time
-                write_attribute(:#{attr_name}, original_time)
-                #{attr_name}_will_change! if changed
-                @attributes_cache["#{attr_name}"] = time
+                zoned_time   = time && time.in_time_zone rescue nil
+                rounded_time = rounded_current_value(zoned_time)
+                attr = rounded_current_value(read_attribute("#{attr_name}"))
+                if (attr != rounded_time) || (!attr && original_time)
+                  write_attribute("#{attr_name}", original_time)
+                  #{attr_name}_will_change!
+                  @attributes_cache["#{attr_name}"] = zoned_time
+                end
               end
             EOV
             generated_attribute_methods.module_eval(method_body, __FILE__, line)
@@ -79,6 +82,12 @@ def create_time_zone_conversion_attribute?(name, column)
             [:datetime, :timestamp].include?(column.type)
         end
       end
+
+      private
+      def rounded_current_value(value)
+        return unless value
+        value.change(:usec => 0)
+      end
     end
   end
 end

activerecord/test/cases/dirty_test.rb

@@ -525,6 +525,21 @@ def test_field_named_field
     end
   end
 
+  def test_setting_time_attributes_with_time_zone_field_to_same_time_should_not_be_marked_as_a_change
+    in_time_zone 'Paris' do
+      target = Class.new(ActiveRecord::Base)
+      target.table_name = 'pirates'
+
+      created_on = Time.now
+
+      pirate = target.create(:created_on => created_on)
+      pirate.reload # Here mysql truncate the usec value to 0
+
+      pirate.created_on = created_on
+      assert !pirate.created_on_changed?
+    end
+  end
+
   private
     def with_partial_updates(klass, on = true)
       old = klass.partial_updates?