diff --git a/activerecord/lib/active_record/legacy_yaml_adapter.rb b/activerecord/lib/active_record/legacy_yaml_adapter.rb
index 0d018d4352b..89dee584236 100644
--- a/activerecord/lib/active_record/legacy_yaml_adapter.rb
+++ b/activerecord/lib/active_record/legacy_yaml_adapter.rb
@@ -7,13 +7,29 @@ module ActiveRecord
       when 1 then coder
       else
         if coder["attributes"].is_a?(AttributeSet)
-          coder
+          Rails420.convert(klass, coder)
         else
           Rails41.convert(klass, coder)
         end
       end
     end
 
+    module Rails420
+      def self.convert(klass, coder)
+        attribute_set = coder["attributes"]
+
+        klass.attribute_names.each do |attr_name|
+          attribute = attribute_set[attr_name]
+          if attribute.type.is_a?(Delegator)
+            type_from_klass = klass.type_for_attribute(attr_name)
+            attribute_set[attr_name] = attribute.with_type(type_from_klass)
+          end
+        end
+
+        coder
+      end
+    end
+
     module Rails41
       def self.convert(klass, coder)
         attributes = klass.attributes_builder
diff --git a/activerecord/test/cases/yaml_serialization_test.rb b/activerecord/test/cases/yaml_serialization_test.rb
index 65cd54e3807..56909a86302 100644
--- a/activerecord/test/cases/yaml_serialization_test.rb
+++ b/activerecord/test/cases/yaml_serialization_test.rb
@@ -92,31 +92,7 @@ class YamlSerializationTest < ActiveRecord::TestCase
   end
 
   def test_deserializing_rails_41_yaml
-    yaml = <<-YAML.strip_heredoc
-      --- !ruby/object:Topic
-        attributes:
-          id: 
-          title: The First Topic
-          author_name: David
-          author_email_address: david@loudthinking.com
-          written_on: 2003-07-16 14:28:11.223300000 Z
-          bonus_time: 2000-01-01 14:28:00.000000000 Z
-          last_read: 2004-04-15
-          content: |
-            ---
-            :omg: :lol
-          important: 
-          approved: false
-          replies_count: 1
-          unique_replies_count: 0
-          parent_id: 
-          parent_title: 
-          type: 
-          group: 
-          created_at: 2015-03-10 17:05:42.000000000 Z
-          updated_at: 2015-03-10 17:05:42.000000000 Z
-    YAML
-    topic = YAML.load(yaml)
+    topic = YAML.load(yaml_fixture("rails_4_1"))
 
     assert topic.new_record?
     assert_equal nil, topic.id
@@ -124,6 +100,22 @@ class YamlSerializationTest < ActiveRecord::TestCase
     assert_equal({ omg: :lol }, topic.content)
   end
 
-  def test_deserializing_rails_42_yaml
+  def test_deserializing_rails_4_2_0_yaml
+    topic = YAML.load(yaml_fixture("rails_4_2_0"))
+
+    assert_not topic.new_record?
+    assert_equal 1, topic.id
+    assert_equal "The First Topic", topic.title
+    assert_equal("Have a nice day", topic.content)
+  end
+
+  private
+
+  def yaml_fixture(file_name)
+    path = File.expand_path(
+      "../../support/yaml_compatibility_fixtures/#{file_name}.yml",
+      __FILE__
+    )
+    File.read(path)
   end
 end
diff --git a/activerecord/test/support/yaml_compatibility_fixtures/rails_4_1.yml b/activerecord/test/support/yaml_compatibility_fixtures/rails_4_1.yml
new file mode 100644
index 00000000000..20b128db9ca
--- /dev/null
+++ b/activerecord/test/support/yaml_compatibility_fixtures/rails_4_1.yml
@@ -0,0 +1,22 @@
+--- !ruby/object:Topic
+  attributes:
+    id: 
+    title: The First Topic
+    author_name: David
+    author_email_address: david@loudthinking.com
+    written_on: 2003-07-16 14:28:11.223300000 Z
+    bonus_time: 2000-01-01 14:28:00.000000000 Z
+    last_read: 2004-04-15
+    content: |
+      ---
+      :omg: :lol
+    important: 
+    approved: false
+    replies_count: 1
+    unique_replies_count: 0
+    parent_id: 
+    parent_title: 
+    type: 
+    group: 
+    created_at: 2015-03-10 17:05:42.000000000 Z
+    updated_at: 2015-03-10 17:05:42.000000000 Z
diff --git a/activerecord/test/support/yaml_compatibility_fixtures/rails_4_2_0.yml b/activerecord/test/support/yaml_compatibility_fixtures/rails_4_2_0.yml
new file mode 100644
index 00000000000..b3d3b331414
--- /dev/null
+++ b/activerecord/test/support/yaml_compatibility_fixtures/rails_4_2_0.yml
@@ -0,0 +1,182 @@
+--- !ruby/object:Topic
+raw_attributes:
+  id: 1
+  title: The First Topic
+  author_name: David
+  author_email_address: david@loudthinking.com
+  written_on: '2003-07-16 14:28:11.223300'
+  bonus_time: '2005-01-30 14:28:00.000000'
+  last_read: '2004-04-15'
+  content: |
+    --- Have a nice day
+    ...
+  important:
+  approved: f
+  replies_count: 1
+  unique_replies_count: 0
+  parent_id:
+  parent_title:
+  type:
+  group:
+  created_at: '2015-03-10 17:44:41'
+  updated_at: '2015-03-10 17:44:41'
+attributes: !ruby/object:ActiveRecord::AttributeSet
+  attributes: !ruby/object:ActiveRecord::LazyAttributeHash
+    types:
+      id: &5 !ruby/object:ActiveRecord::Type::Integer
+        precision:
+        scale:
+        limit:
+        range: !ruby/range
+          begin: -2147483648
+          end: 2147483648
+          excl: true
+      title: &6 !ruby/object:ActiveRecord::Type::String
+        precision:
+        scale:
+        limit: 250
+      author_name: &1 !ruby/object:ActiveRecord::Type::String
+        precision:
+        scale:
+        limit:
+      author_email_address: *1
+      written_on: &4 !ruby/object:ActiveRecord::Type::DateTime
+        precision:
+        scale:
+        limit:
+      bonus_time: &7 !ruby/object:ActiveRecord::Type::Time
+        precision:
+        scale:
+        limit:
+      last_read: &8 !ruby/object:ActiveRecord::Type::Date
+        precision:
+        scale:
+        limit:
+      content: !ruby/object:ActiveRecord::Type::Serialized
+        coder: &9 !ruby/object:ActiveRecord::Coders::YAMLColumn
+          object_class: !ruby/class 'Object'
+        subtype: &2 !ruby/object:ActiveRecord::Type::Text
+          precision:
+          scale:
+          limit:
+      important: *2
+      approved: &10 !ruby/object:ActiveRecord::Type::Boolean
+        precision:
+        scale:
+        limit:
+      replies_count: &3 !ruby/object:ActiveRecord::Type::Integer
+        precision:
+        scale:
+        limit:
+        range: !ruby/range
+          begin: -2147483648
+          end: 2147483648
+          excl: true
+      unique_replies_count: *3
+      parent_id: *3
+      parent_title: *1
+      type: *1
+      group: *1
+      created_at: *4
+      updated_at: *4
+    values:
+      id: 1
+      title: The First Topic
+      author_name: David
+      author_email_address: david@loudthinking.com
+      written_on: '2003-07-16 14:28:11.223300'
+      bonus_time: '2005-01-30 14:28:00.000000'
+      last_read: '2004-04-15'
+      content: |
+        --- Have a nice day
+        ...
+      important:
+      approved: f
+      replies_count: 1
+      unique_replies_count: 0
+      parent_id:
+      parent_title:
+      type:
+      group:
+      created_at: '2015-03-10 17:44:41'
+      updated_at: '2015-03-10 17:44:41'
+    additional_types: {}
+    materialized: true
+    delegate_hash:
+      id: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: id
+        value_before_type_cast: 1
+        type: *5
+      title: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: title
+        value_before_type_cast: The First Topic
+        type: *6
+      author_name: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: author_name
+        value_before_type_cast: David
+        type: *1
+      author_email_address: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: author_email_address
+        value_before_type_cast: david@loudthinking.com
+        type: *1
+      written_on: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: written_on
+        value_before_type_cast: '2003-07-16 14:28:11.223300'
+        type: *4
+      bonus_time: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: bonus_time
+        value_before_type_cast: '2005-01-30 14:28:00.000000'
+        type: *7
+      last_read: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: last_read
+        value_before_type_cast: '2004-04-15'
+        type: *8
+      content: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: content
+        value_before_type_cast: |
+          --- Have a nice day
+          ...
+        type: !ruby/object:ActiveRecord::Type::Serialized
+          coder: *9
+          subtype: *2
+      important: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: important
+        value_before_type_cast:
+        type: *2
+      approved: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: approved
+        value_before_type_cast: f
+        type: *10
+      replies_count: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: replies_count
+        value_before_type_cast: 1
+        type: *3
+      unique_replies_count: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: unique_replies_count
+        value_before_type_cast: 0
+        type: *3
+      parent_id: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: parent_id
+        value_before_type_cast:
+        type: *3
+      parent_title: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: parent_title
+        value_before_type_cast:
+        type: *1
+      type: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: type
+        value_before_type_cast:
+        type: *1
+      group: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: group
+        value_before_type_cast:
+        type: *1
+      created_at: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: created_at
+        value_before_type_cast: '2015-03-10 17:44:41'
+        type: *4
+      updated_at: !ruby/object:ActiveRecord::Attribute::FromDatabase
+        name: updated_at
+        value_before_type_cast: '2015-03-10 17:44:41'
+        type: *4
+new_record: false