added new tag batch change model

test/unit/moderator/tag_batch_change_test.rb

@@ -0,0 +1,33 @@
+require "test_helper"
+
+module Moderator
+  class TagBatchChangeTest < ActiveSupport::TestCase
+    context "a tag batch change" do
+      setup do
+        @user = Factory.create(:moderator_user)
+        CurrentUser.user = @user
+        CurrentUser.ip_addr = "127.0.0.1"
+        @post = Factory.create(:post, :tag_string => "aaa")
+      end
+  
+      teardown do
+        CurrentUser.user = nil
+        CurrentUser.ip_addr = nil
+      end
+
+      should "execute" do
+        tag_batch_change = TagBatchChange.new("aaa", "bbb")
+        tag_batch_change.execute
+        @post.reload
+        assert_equal("bbb", @post.tag_string)
+      end
+      
+      should "raise an error if there is no predicate" do
+        tag_batch_change = TagBatchChange.new("", "bbb")
+        assert_raises(TagBatchChange::Error) do
+          tag_batch_change.execute
+        end
+      end
+    end
+  end
+end