3 files changed, 22 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d9f8ee7097..f57158f38e 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Fix bug in `becomes!` when changing from the base model to a STI sub-class.
+
+    Fixes #13272.
+
+    *the-web-dev*, *Yves Senn*
+
 *   Currently Active Record can be configured via the environment variable
     `DATABASE_URL` or by manually injecting a hash of values which is what Rails does,
     reading in `database.yml` and setting Active Record appropriately. Active Record
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index 4669c072a1..460fbdb3f8 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -196,7 +196,11 @@ module ActiveRecord
     # share the same set of attributes.
     def becomes!(klass)
       became = becomes(klass)
-      became.public_send("#{klass.inheritance_column}=", klass.sti_name) unless self.class.descends_from_active_record?
+      sti_type = nil
+      if !klass.descends_from_active_record?
+        sti_type = klass.sti_name
+      end
+      became.public_send("#{klass.inheritance_column}=", sti_type)
       became
     end
 
diff --git a/activerecord/test/cases/inheritance_test.rb b/activerecord/test/cases/inheritance_test.rb
index cb0d374ef7..7fd7d42354 100644
--- a/activerecord/test/cases/inheritance_test.rb
+++ b/activerecord/test/cases/inheritance_test.rb
@@ -128,6 +128,17 @@ class InheritanceTest < ActiveRecord::TestCase
     assert_kind_of Cabbage, cabbage
   end
 
+  def test_alt_becomes_bang_resets_inheritance_type_column
+    vegetable = Vegetable.create!(name: "Red Pepper")
+    assert_nil vegetable.custom_type
+
+    cabbage = vegetable.becomes!(Cabbage)
+    assert_equal "Cabbage", cabbage.custom_type
+
+    vegetable = cabbage.becomes!(Vegetable)
+    assert_nil cabbage.custom_type
+  end
+
   def test_inheritance_find_all
     companies = Company.all.merge!(:order => 'id').to_a
     assert_kind_of Firm, companies[0], "37signals should be a firm"