migrate custom validation

app/models/impediment.rb

@@ -3,6 +3,8 @@ class Impediment < Task
 
   after_save :update_blocks_list
 
+  validate :validate_blocks_list
+
   safe_attributes "blocks_ids",
                   :if => lambda {|impediment, user|
                             (impediment.new_record? && user.allowed_to?(:create_impediments, impediment.project)) ||
@@ -56,10 +58,6 @@ class Impediment < Task
     }
   end
 
-  def validate
-    validate_blocks_list
-  end
-
   def validate_blocks_list
     if blocks_ids.size == 0
       errors.add :blocks_ids, :must_block_at_least_one_issue