Timestamp values should be present on callbacks

This reverts commit dd3ea17191 and add a
regression test.

Fixes #15418

activerecord/CHANGELOG.md

@@ -532,10 +532,6 @@
 
     *Luke Steensen*
 
-*   Make possible to change `record_timestamps` inside Callbacks.
-
-    *Tieg Zaharia*
-
 *   Fixed error where .persisted? throws SystemStackError for an unsaved model with a
     custom primary key that didn't save due to validation error.
 

activerecord/lib/active_record/base.rb

@@ -311,8 +311,8 @@ module ActiveRecord #:nodoc:
     include Locking::Optimistic
     include Locking::Pessimistic
     include AttributeMethods
-    include Timestamp
     include Callbacks
+    include Timestamp
     include Associations
     include ActiveModel::SecurePassword
     include AutosaveAssociation

activerecord/test/cases/timestamp_test.rb

@@ -71,24 +71,6 @@ class TimestampTest < ActiveRecord::TestCase
     assert_equal @previously_updated_at, @developer.updated_at
   end
 
-  def test_saving_when_callback_sets_record_timestamps_to_false_doesnt_update_its_timestamp
-    klass = Class.new(Developer) do
-      before_update :cancel_record_timestamps
-      def cancel_record_timestamps
-        self.record_timestamps = false
-        return true
-      end
-    end
-
-    developer = klass.first
-    previously_updated_at = developer.updated_at
-
-    developer.name = "New Name"
-    developer.save!
-
-    assert_equal previously_updated_at, developer.updated_at
-  end
-
   def test_touching_an_attribute_updates_timestamp
     previously_created_at = @developer.created_at
     @developer.touch(:created_at)
@@ -399,6 +381,19 @@ class TimestampTest < ActiveRecord::TestCase
     assert_not_equal time, pet.updated_at
   end
 
+  def test_timestamp_column_values_are_present_in_the_callbacks
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = 'people'
+
+      before_create do
+        self.born_at = self.created_at
+      end
+    end
+
+    person = klass.create first_name: 'David'
+    assert_not_equal person.born_at, nil
+  end
+
   def test_timestamp_attributes_for_create
     toy = Toy.first
     assert_equal [:created_at, :created_on], toy.send(:timestamp_attributes_for_create)

activerecord/test/schema/schema.rb

@@ -529,6 +529,7 @@ ActiveRecord::Schema.define do
     t.references :best_friend
     t.references :best_friend_of
     t.integer    :insures, null: false, default: 0
+    t.timestamp :born_at
     t.timestamps
   end