Keep the types of virtual columns after yaml serialization

On MySQL and PostgreSQL, the adapter does not type cast virtual columns
for us.

activerecord/lib/active_record/connection_adapters/column.rb

@@ -13,7 +13,7 @@ module Format
         ISO_DATETIME = /\A(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)(\.\d+)?\z/
       end
 
-      attr_reader :name, :cast_type, :null, :sql_type, :default_function
+      attr_reader :name, :cast_type, :sql_type, :default_function
 
       delegate :type, :precision, :scale, :limit, :klass, :accessor,
         :text?, :number?, :binary?, :serialized?, :changed?,
@@ -34,7 +34,7 @@ def initialize(name, default, cast_type, sql_type = nil, null = true)
         @name             = name
         @cast_type        = cast_type
         @sql_type         = sql_type
-        @null             = null
+        @nullable         = null
         @original_default = default
         @default_function = nil
       end
@@ -61,6 +61,10 @@ def with_type(type)
           clone.instance_variable_set('@cast_type', type)
         end
       end
+
+      def null
+        @nullable
+      end
     end
 
     class NullColumn < Column

activerecord/lib/active_record/core.rb

@@ -356,6 +356,7 @@ def initialize_dup(other) # :nodoc:
     def encode_with(coder)
       coder['raw_attributes'] = @raw_attributes
       coder['attributes'] = @attributes
+      coder['column_types'] = @column_types_override
       coder['new_record'] = new_record?
     end
 

activerecord/lib/active_record/type/serialized.rb

@@ -36,6 +36,17 @@ def accessor
         ActiveRecord::Store::IndifferentHashAccessor
       end
 
+      def init_with(coder)
+        @subtype = coder['subtype']
+        @coder = coder['coder']
+        __setobj__(@subtype)
+      end
+
+      def encode_with(coder)
+        coder['subtype'] = @subtype
+        coder['coder'] = @coder
+      end
+
       private
 
       def is_default_value?(value)

activerecord/test/cases/yaml_serialization_test.rb

 require 'cases/helper'
 require 'models/topic'
+require 'models/post'
+require 'models/author'
 
 class YamlSerializationTest < ActiveRecord::TestCase
-  fixtures :topics
+  fixtures :topics, :authors, :posts
 
   def test_to_yaml_with_time_with_zone_should_not_raise_exception
     with_timezone_config aware_attributes: true, zone: "Pacific Time (US & Canada)" do
@@ -69,4 +71,15 @@ def test_new_records_remain_new_after_round_trip
     assert_not topic.new_record?, "Loaded records without ID are not new"
     assert_not YAML.load(YAML.dump(topic)).new_record?, "Record should not be new after deserialization"
   end
+
+  def test_types_of_virtual_columns_are_not_changed_on_round_trip
+    author = Author.select('authors.*, count(posts.id) as posts_count')
+      .joins(:posts)
+      .group('authors.id')
+      .first
+    dumped = YAML.load(YAML.dump(author))
+
+    assert_equal 5, author.posts_count
+    assert_equal 5, dumped.posts_count
+  end
 end