added new tag batch change model

app/logical/moderator/tag_batch_change.rb

@@ -0,0 +1,24 @@
+module Moderator
+  class TagBatchChange
+    class Error < Exception ; end
+    
+    attr_reader :predicate, :consequent
+    
+    def initialize(predicate, consequent)
+      @predicate = predicate
+      @consequent = consequent
+    end
+    
+    def execute
+      raise Error.new("Predicate is missing") if predicate.blank?
+      
+      normalized_predicate = TagAlias.to_aliased(::Tag.scan_tags(predicate))
+      normalized_consequent = TagAlias.to_aliased(::Tag.scan_tags(consequent))
+
+      ::Post.tag_match(predicate).each do |post|
+        tags = (post.tag_array - normalized_predicate + normalized_consequent).join(" ")
+        post.update_attributes(:tag_string => tags)
+      end
+    end
+  end
+end