move ActiveRecord::Persistance#becomes logic into initialize block

activerecord/lib/active_record/persistence.rb

@@ -569,12 +569,15 @@ module ActiveRecord
     # If you want to change the sti column as well, use #becomes! instead.
     def becomes(klass)
       became = klass.allocate
-      became.send(:initialize)
-      became.instance_variable_set(:@attributes, @attributes)
-      became.instance_variable_set(:@mutations_from_database, @mutations_from_database ||= nil)
-      became.instance_variable_set(:@new_record, new_record?)
-      became.instance_variable_set(:@destroyed, destroyed?)
-      became.errors.copy!(errors)
+
+      became.send(:initialize) do |becoming|
+        becoming.instance_variable_set(:@attributes, @attributes)
+        becoming.instance_variable_set(:@mutations_from_database, @mutations_from_database ||= nil)
+        becoming.instance_variable_set(:@new_record, new_record?)
+        becoming.instance_variable_set(:@destroyed, destroyed?)
+        becoming.errors.copy!(errors)
+      end
+
       became
     end
 

activerecord/test/cases/inheritance_test.rb

@@ -227,6 +227,16 @@ class InheritanceTest < ActiveRecord::TestCase
     assert_kind_of Cabbage, cabbage
   end
 
+  def test_becomes_sets_variables_before_initialization_callbacks
+    vegetable = Vegetable.create!(name: "yelling carrot")
+    assert_kind_of Vegetable, vegetable
+    assert_equal "yelling carrot", vegetable.name
+
+    yelling_veggie = vegetable.becomes(YellingVegetable)
+    assert_equal "YELLING CARROT", yelling_veggie.name, "YellingVegetable name should be YELLING CARROT"
+    assert_equal "YELLING CARROT", vegetable.name, "Vegetable name should be YELLING CARROT after becoming a YellingVegetable"
+  end
+
   def test_becomes_and_change_tracking_for_inheritance_columns
     cucumber = Vegetable.find(1)
     cabbage = cucumber.becomes!(Cabbage)

activerecord/test/models/vegetables.rb

@@ -23,3 +23,11 @@ end
 class RedCabbage < Cabbage
   belongs_to :seller, class_name: "Company"
 end
+
+class YellingVegetable < Vegetable
+  after_initialize :format_name
+
+  def format_name
+    self.name = name&.upcase
+  end
+end