Serialized attribute should be able to be defined in abstract classes

Ref: rails#47191

This feature recently regressed when rails#47191
was merged to handle defaults.

The `attribute` definition callback is set on the parent class and
is triggered when the child class schema is loaded.

I don't know if this is fixeable.

Author: byroot

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -219,7 +219,7 @@ def serialize(attr_name, class_name_or_coder = nil, coder: nil, type: Object, ya
             end
 
             cast_type = cast_type.subtype if Type::Serialized === cast_type
-            Type::Serialized.new(cast_type, column_serializer, default: columns_hash[attr_name.to_s]&.default)
+            Type::Serialized.new(cast_type, column_serializer)
           end
         end
 

activerecord/lib/active_record/type/serialized.rb

@@ -9,10 +9,9 @@ class Serialized < DelegateClass(ActiveModel::Type::Value) # :nodoc:
 
       attr_reader :subtype, :coder
 
-      def initialize(subtype, coder, default: nil)
+      def initialize(subtype, coder)
         @subtype = subtype
         @coder = coder
-        @default = default
         super(subtype)
       end
 
@@ -26,7 +25,7 @@ def deserialize(value)
 
       def serialize(value)
         return if value.nil?
-        unless default_value?(value) && @default.nil?
+        unless default_value?(value)
           super coder.dump(value)
         end
       end

activerecord/test/cases/serialized_attribute_test.rb

@@ -403,51 +403,18 @@ def test_values_cast_from_nil_are_persisted_as_nil
     assert_equal [topic, topic2], Topic.where(content: nil).sort_by(&:id)
   end
 
-  # MySQL doesn't support default values for text columns, so we need to skip this test for MySQL
-  if !current_adapter?(:Mysql2Adapter)
-    def test_serialized_attribute_with_default_can_update_to_default
-      @verbose_was = ActiveRecord::Migration.verbose
-      ActiveRecord::Migration.verbose = false
-
-      ActiveRecord::Schema.define do
-        create_table :tmp_posts, force: true do |t|
-          t.text     :content, null: false, default: "{}"
-        end
-      end
-      klass = Class.new(ActiveRecord::Base) do
-        self.table_name = "tmp_posts"
-        serialize(:content, type: Hash)
-      end
-
-      t = klass.create!(content: { "other_key" => "new_value" })
-      assert_equal({ "other_key" => "new_value" }, t.content)
-
-      t.update!(content: {})
-      assert_equal({}, t.content)
-    ensure
-      ActiveRecord::Migration.verbose = @verbose_was
+  def test_serialized_attribute_can_be_defined_in_abstract_classes
+    klass = Class.new(ActiveRecord::Base) do
+      self.abstract_class = true
+      self.table_name = nil
+      serialize(:content, type: Hash)
     end
 
-    def test_nil_is_always_persisted_as_default
-      @verbose_was = ActiveRecord::Migration.verbose
-      ActiveRecord::Migration.verbose = false
-
-      ActiveRecord::Schema.define do
-        create_table :tmp_posts, force: true do |t|
-          t.text     :content, null: false, default: "{}"
-        end
-      end
-      klass = Class.new(ActiveRecord::Base) do
-        self.table_name = "tmp_posts"
-        serialize(:content, type: Hash)
-      end
-
-      t = klass.create!(content: { foo: "bar" })
-      t.update_attribute :content, nil
-      assert_equal({}, t.content)
-    ensure
-      ActiveRecord::Migration.verbose = @verbose_was
+    subclass = Class.new(klass) do
+      self.table_name = "posts"
     end
+
+    subclass.define_attribute_methods
   end
 
   def test_nil_is_always_persisted_as_null