Don't call will_change! for datetime nil->"".

Setting a nil datetime attribute to a blank string should not cause the
attribute to be dirty.

Fix #8310

1 files changed, 14 insertions, 0 deletions

diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index d4fc5f204b..6fd9a9d11d 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -203,6 +203,20 @@ class DirtyTest < ActiveRecord::TestCase
     end
   end
 
+  def test_nullable_datetime_not_marked_as_changed_if_new_value_is_blank
+    in_time_zone 'Edinburgh' do
+      target = Class.new(ActiveRecord::Base)
+      target.table_name = 'topics'
+
+      topic = target.create
+      assert_equal nil, topic.written_on
+
+      topic.written_on = ""
+      assert_equal nil, topic.written_on
+      assert !topic.written_on_changed?
+    end
+  end
+
   def test_integer_zero_to_string_zero_not_marked_as_changed
     pirate = Pirate.new
     pirate.parrot_id = 0