Merge pull request rails#54314 from byroot/serialize-json-default

Fix `serialize coder: JSON, type: Hash` to wrap the coder in `ColumnSerializer`

Author: byroot

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -220,10 +220,12 @@ def build_column_serializer(attr_name, coder, type, yaml = nil)
             # to ensure special objects (e.g. Active Record models) are dumped correctly
             # using the #as_json hook.
 
+            if coder == ::JSON || coder == Coders::JSON
+              coder = Coders::JSON.new
+            end
+
             if coder == ::YAML || coder == Coders::YAMLColumn
               Coders::YAMLColumn.new(attr_name, type, **(yaml || {}))
-            elsif coder == ::JSON || coder == Coders::JSON
-              Coders::JSON.new
             elsif coder.respond_to?(:new) && !coder.respond_to?(:load)
               coder.new(attr_name, type)
             elsif type && type != Object

activerecord/test/cases/serialized_attribute_test.rb

@@ -149,6 +149,12 @@ def test_json_read_db_null
     assert_nil t.content
   end
 
+  def test_json_type_hash_default_value
+    Topic.serialize :content, coder: JSON, type: Hash
+    t = Topic.new
+    assert_equal({}, t.content)
+  end
+
   def test_json_symbolize_names_returns_symbolized_names
     Topic.serialize :content, coder: ActiveRecord::Coders::JSON.new(symbolize_names: true)
     my_post = posts(:welcome)