Revert "Round usec when writing timestamp attribute."

This reverts commit e9d2ad395ec2ef929d74752f3d71c80674044fbe.

Closes #8460

Conflicts:
	activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
	activerecord/test/cases/dirty_test.rb

2 files changed, 5 insertions, 28 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
index 47a8b576c0..7701001da9 100644
--- a/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
+++ b/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb
@@ -39,14 +39,11 @@ module ActiveRecord
                 unless time.acts_like?(:time)
                   time = time.is_a?(String) ? Time.zone.parse(time) : time.to_time rescue time
                 end
-                zoned_time   = time && time.in_time_zone rescue nil
-                rounded_time = round_usec(zoned_time)
-                rounded_value = round_usec(read_attribute("#{attr_name}"))
-                if (rounded_value != rounded_time) || (!rounded_value && original_time)
-                  write_attribute("#{attr_name}", original_time)
-                  #{attr_name}_will_change!
-                  @attributes_cache["#{attr_name}"] = zoned_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
               end
             EOV
             generated_attribute_methods.module_eval(method_body, __FILE__, line)
@@ -62,11 +59,6 @@ module ActiveRecord
             [:datetime, :timestamp].include?(column.type)
         end
       end
-
-      private
-      def round_usec(value)
-        value.change(usec: 0) if value
-      end
     end
   end
 end
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index c8796a94f3..4ac82f6880 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -551,21 +551,6 @@ class DirtyTest < ActiveRecord::TestCase
     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
-
   def test_datetime_attribute_can_be_updated_with_fractional_seconds
     in_time_zone 'Paris' do
       target = Class.new(ActiveRecord::Base)