rewardhistory.diff

diff --git a/src/models/task.js b/src/models/task.js
index 4b99640..62e2803 100644
--- a/src/models/task.js
+++ b/src/models/task.js
@@ -83,7 +83,8 @@ var TodoSchema = new Schema(
 
 var RewardSchema = new Schema(
   _.defaults({
-    type: {type:String, 'default': 'reward'}
+    type: {type:String, 'default': 'reward'},
+    purchaseHistory: Array // [{taskId, timestamp}]
   }, TaskSchema)
   , { _id: false, minimize:false }
 );