Attempt to provide backwards compatible YAML deserialization

I should have done this in the first place. We are now serializing an
explicit version so we can make more careful changes in the future. This
will load Active Record objects which were serialized in Rails 4.1.

There will be bugs, as YAML serialization was at least partially broken
back then. There will also be edge cases that we might not be able to
handle, especially if the type of a column has changed.

In addition, we're passing this as `from_database`, since that is
required for serialized columns at minimum. All other types were
serializing the cast value. At a glance, there should be no types for
which this is a problem.

Finally, dirty checking information will be lost on records serialized
in 4.1, so no columns will be marked as changed.

activerecord/lib/active_record.rb

@@ -43,6 +43,7 @@ module ActiveRecord
   autoload :Explain
   autoload :Inheritance
   autoload :Integration
+  autoload :LegacyYamlAdapter
   autoload :Migration
   autoload :Migrator, 'active_record/migration'
   autoload :ModelSchema

activerecord/lib/active_record/core.rb

@@ -300,6 +300,7 @@ module ActiveRecord
     #   post.init_with('attributes' => { 'title' => 'hello world' })
     #   post.title # => 'hello world'
     def init_with(coder)
+      coder = LegacyYamlAdapter.convert(self.class, coder)
       @attributes = coder['attributes']
 
       init_internals
@@ -370,6 +371,7 @@ module ActiveRecord
       coder['raw_attributes'] = attributes_before_type_cast
       coder['attributes'] = @attributes
       coder['new_record'] = new_record?
+      coder['active_record_yaml_version'] = 1
     end
 
     # Returns true if +comparison_object+ is the same exact object, or +comparison_object+

activerecord/lib/active_record/legacy_yaml_adapter.rb

@@ -0,0 +1,30 @@
+module ActiveRecord
+  module LegacyYamlAdapter
+    def self.convert(klass, coder)
+      return coder unless coder.is_a?(Psych::Coder)
+
+      case coder["active_record_yaml_version"]
+      when 1 then coder
+      else
+        if coder["attributes"].is_a?(AttributeSet)
+          coder
+        else
+          Rails41.convert(klass, coder)
+        end
+      end
+    end
+
+    module Rails41
+      def self.convert(klass, coder)
+        attributes = klass.attributes_builder
+          .build_from_database(coder["attributes"])
+        new_record = coder["attributes"][klass.primary_key].blank?
+
+        {
+          "attributes" => attributes,
+          "new_record" => new_record,
+        }
+      end
+    end
+  end
+end

activerecord/test/cases/yaml_serialization_test.rb

@@ -83,4 +83,47 @@ class YamlSerializationTest < ActiveRecord::TestCase
     assert_equal 5, author.posts_count
     assert_equal 5, dumped.posts_count
   end
+
+  def test_a_yaml_version_is_provided_for_future_backwards_compat
+    coder = {}
+    Topic.first.encode_with(coder)
+
+    assert coder['active_record_yaml_version']
+  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)
+
+    assert topic.new_record?
+    assert_equal nil, topic.id
+    assert_equal "The First Topic", topic.title
+    assert_equal({ omg: :lol }, topic.content)
+  end
+
+  def test_deserializing_rails_42_yaml
+  end
 end