Make ActiveModel::Errors backward compatible with 4.2

If a Error object was serialized in the database as YAML in the Rails
4.2 version, if we load in the Rails 5.0 version it will miss the
@details instance variable so methods like #clear and #add will start to
fail.

1 files changed, 9 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 5ee9413cff..9df4ca51fe 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -419,16 +419,23 @@ module ActiveModel
       I18n.translate(key, options)
     end
 
-    def marshal_dump
+    def marshal_dump # :nodoc:
       [@base, without_default_proc(@messages), without_default_proc(@details)]
     end
 
-    def marshal_load(array)
+    def marshal_load(array) # :nodoc:
       @base, @messages, @details = array
       apply_default_array(@messages)
       apply_default_array(@details)
     end
 
+    def init_with(coder) # :nodoc:
+      coder.map.each { |k, v| instance_variable_set(:"@#{k}", v) }
+      @details ||= {}
+      apply_default_array(@messages)
+      apply_default_array(@details)
+    end
+
   private
     def normalize_message(attribute, message, options)
       case message