Have POST /db/prepaid/:id/redeemers set user.coursePrepaidID property

app/schemas/models/user.coffee

@@ -319,6 +319,9 @@ _.extend UserSchema.properties,
     courseID: c.objectId({})
     courseInstanceID: c.objectId({})
   }
+  coursePrepaidID: c.objectId({
+    description: 'Prepaid which has paid for this user\'s course access'
+  })
 
 c.extendBasicProperties UserSchema, 'user'
 

server/prepaids/prepaid_handler.coffee

@@ -78,14 +78,26 @@ PrepaidHandler = class PrepaidHandler extends Handler
         Prepaid.count {'redeemers.userID': userID}, (err, count) =>
           return @sendDatabaseError(res, err) if err
           return @sendSuccess(res, @formatEntity(req, prepaid)) if count
-          redeemers = _.clone(prepaid.get('redeemers') or [])
-          redeemers.push({ date: new Date(), userID: userID })
-          prepaid.set('redeemers', redeemers)
-          # Not worrying about race conditions. Worst case: overwrite one user with another.
-          # You can't end up with more than maxRedeemers in the list of redeemers.
-          prepaid.save (err, prepaid) =>
+
+          query =
+            _id: prepaid.get('_id')
+            'redeemers.userID': { $ne: req.user.get('_id') }
+            $where: "this.redeemers.length < #{prepaid.get('maxRedeemers')}"
+          update = { $push: { redeemers : { date: new Date(), userID: userID } }}
+          Prepaid.update query, update, (err, nMatched) =>
             return @sendDatabaseError(res, err) if err
-            @sendSuccess(res, @formatEntity(req, prepaid))      
+            if nMatched is 0
+              @logError(req.user, "POST prepaid redeemer lost race on maxRedeemers")
+              return @sendForbiddenError(res)
+              
+            user.set('coursePrepaidID', prepaid.get('_id'))
+            user.save (err, user) =>
+              return @sendDatabaseError(res, err) if err
+              # return prepaid with new redeemer added locally
+              redeemers = _.clone(prepaid.get('redeemers') or [])
+              redeemers.push({ date: new Date(), userID: userID })
+              prepaid.set('redeemers', redeemers)
+              @sendSuccess(res, @formatEntity(req, prepaid))      
 
   createPrepaid: (user, type, maxRedeemers, properties, done) ->
     Prepaid.generateNewCode (code) =>

test/server/functional/prepaid.spec.coffee

@@ -53,7 +53,12 @@ describe '/db/prepaid', ->
             request.post {uri: url, json: redeemer }, (err, res, body) ->
               expect(body.redeemers.length).toBe(1)
               expect(res.statusCode).toBe(200)
-              done()
+              prepaid = Prepaid.findById body._id, (err, prepaid) ->
+                expect(err).toBeNull()
+                expect(prepaid.get('redeemers').length).toBe(1)
+                User.findById  otherUser.id, (err, user) ->
+                  expect(user.get('coursePrepaidID').equals(prepaid.get('_id'))).toBe(true)
+                  done()
               
     it 'does not allow more redeemers than maxRedeemers', (done) ->
       loginNewUser (user1) ->