keep track of the previous cid a topic was moved from

src/topics/data.js

@@ -78,6 +78,12 @@ module.exports = function(Topics) {
 		db.setObjectField('topic:' + tid, field, value, callback);
 	};
 
+
+	Topics.setTopicFields = function(tid, data, callback) {
+		callback = callback || function() {};
+		db.setObject('topic:' + tid, data, callback);
+	};
+
 	Topics.deleteTopicField = function(tid, field, callback) {
 		db.deleteObjectField('topic:' + tid, field, callback);
 	};

src/topics/tools.js

@@ -242,7 +242,10 @@ module.exports = function(Topics) {
 					categories.incrementCategoryFieldBy(cid, 'topic_count', 1, next);
 				},
 				function (next) {
-					Topics.setTopicField(tid, 'cid', cid, next);
+					Topics.setTopicFields(tid, {
+						cid: cid,
+						oldCid: oldCid
+					}, next);
 				}
 			], function(err) {
 				if (err) {