Deprecate the behavior of AR::Dirty inside of after_(create|update|save) callbacks

We pretty frequently get bug reports that "dirty is broken inside of
after callbacks". Intuitively they are correct. You'd expect
`Model.after_save { puts changed? }; model.save` to do the same thing as
`model.save; puts model.changed?`, but it does not.

However, changing this goes much farther than just making the behavior
more intuitive. There are a _ton_ of places inside of AR that can be
drastically simplified with this change. Specifically, autosave
associations, timestamps, touch, counter cache, and just about anything
else in AR that works with callbacks have code to try to avoid "double
save" bugs which we will be able to flat out remove with this change.

We introduce two new sets of methods, both with names that are meant to
be more explicit than dirty. The first set maintains the old behavior,
and their names are meant to center that they are about changes that
occurred during the save that just happened. They are equivalent to
`previous_changes` when called outside of after callbacks, or once the
deprecation cycle moves.

The second set is the new behavior. Their names imply that they are
talking about changes from the database representation. The fact that
this is what we really care about became clear when looking at
`BelongsTo.touch_record` when tests were failing. I'm unsure that this
set of methods should be in the public API. Outside of after callbacks,
they are equivalent to the existing methods on dirty.

Dirty itself is not deprecated, nor are the methods inside of it. They
will only emit the warning when called inside of after callbacks. The
scope of this breakage is pretty large, but the migration path is
simple. Given how much this can improve our codebase, and considering
that it makes our API more intuitive, I think it's worth doing.

1 files changed, 83 insertions, 0 deletions

diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 3bd8475bb0..0e58e65a07 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -726,6 +726,89 @@ class DirtyTest < ActiveRecord::TestCase
     assert person.changed?
   end
 
+  test "saved_change_to_attribute? returns whether a change occurred in the last save" do
+    person = Person.create!(first_name: "Sean")
+
+    assert person.saved_change_to_first_name?
+    refute person.saved_change_to_gender?
+    assert person.saved_change_to_first_name?(from: nil, to: "Sean")
+    assert person.saved_change_to_first_name?(from: nil)
+    assert person.saved_change_to_first_name?(to: "Sean")
+    refute person.saved_change_to_first_name?(from: "Jim", to: "Sean")
+    refute person.saved_change_to_first_name?(from: "Jim")
+    refute person.saved_change_to_first_name?(to: "Jim")
+  end
+
+  test "saved_change_to_attribute returns the change that occurred in the last save" do
+    person = Person.create!(first_name: "Sean", gender: "M")
+
+    assert_equal [nil, "Sean"], person.saved_change_to_first_name
+    assert_equal [nil, "M"], person.saved_change_to_gender
+
+    person.update(first_name: "Jim")
+
+    assert_equal ["Sean", "Jim"], person.saved_change_to_first_name
+    assert_nil person.saved_change_to_gender
+  end
+
+  test "attribute_before_last_save returns the original value before saving" do
+    person = Person.create!(first_name: "Sean", gender: "M")
+
+    assert_nil person.first_name_before_last_save
+    assert_nil person.gender_before_last_save
+
+    person.first_name = "Jim"
+
+    assert_nil person.first_name_before_last_save
+    assert_nil person.gender_before_last_save
+
+    person.save
+
+    assert_equal "Sean", person.first_name_before_last_save
+    assert_equal "M", person.gender_before_last_save
+  end
+
+  test "saved_changes? returns whether the last call to save changed anything" do
+    person = Person.create!(first_name: "Sean")
+
+    assert person.saved_changes?
+
+    person.save
+
+    refute person.saved_changes?
+  end
+
+  test "saved_changes returns a hash of all the changes that occurred" do
+    person = Person.create!(first_name: "Sean", gender: "M")
+
+    assert_equal [nil, "Sean"], person.saved_changes[:first_name]
+    assert_equal [nil, "M"], person.saved_changes[:gender]
+    assert_equal %w(id first_name gender created_at updated_at).sort, person.saved_changes.keys.sort
+
+    travel(1.second) do
+      person.update(first_name: "Jim")
+    end
+
+    assert_equal ["Sean", "Jim"], person.saved_changes[:first_name]
+    assert_equal %w(first_name lock_version updated_at).sort, person.saved_changes.keys.sort
+  end
+
+  test "changed? in after callbacks returns true but is deprecated" do
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "people"
+
+      after_save do
+        ActiveSupport::Deprecation.silence do
+          raise "changed? should be true" unless changed?
+        end
+        raise "has_changes_to_save? should be false" if has_changes_to_save?
+      end
+    end
+
+    person = klass.create!(first_name: "Sean")
+    refute person.changed?
+  end
+
   private
     def with_partial_writes(klass, on = true)
       old = klass.partial_writes?