Got proportionalTo working with gems as well as points. Probably also got it working for LevelSessions, but this should be tested.

server/achievements/EarnedAchievement.coffee

@@ -27,18 +27,20 @@ EarnedAchievementSchema.statics.createForAchievement = (achievement, doc, origin
     achievementName: achievement.get 'name'
     earnedRewards: achievement.get 'rewards'
 
-  worth = achievement.get('worth') ? 10
+  pointWorth = achievement.get('worth') ? 10
+  gemWorth = achievement.get('rewards')?.gems ? 0
   earnedPoints = 0
+  earnedGems = 0
+  
   wrapUp = (earnedAchievementDoc) ->
     # Update user's experience points
-    update = {$inc: {points: earnedPoints}}
+    update = {$inc: {points: earnedPoints, 'earned.gems': earnedGems}}
     for rewardType, rewards of achievement.get('rewards') ? {}
-      if rewardType is 'gems'
-        update.$inc['earned.gems'] = rewards if rewards
-      else if rewards.length
+      continue if rewardType is 'gems'
+      if rewards.length
         update.$addToSet ?= {}
         update.$addToSet["earned.#{rewardType}"] = $each: rewards
-    User.update {_id: userID}, update, {}, (err, count) ->
+    User.update {_id: mongoose.Types.ObjectId(userID)}, update, {}, (err, count) ->
       log.error err if err?
       done?(earnedAchievementDoc)
 
@@ -54,21 +56,25 @@ EarnedAchievementSchema.statics.createForAchievement = (achievement, doc, origin
       expFunction = achievement.getExpFunction()
       earned.notified = false
       earned.achievedAmount = newAmount
-      earned.earnedPoints = (expFunction(newAmount) - expFunction(originalAmount)) * worth
+      earnedPoints = earned.earnedPoints = (expFunction(newAmount) - expFunction(originalAmount)) * pointWorth
+      earnedGems = earned.earnedGems = (expFunction(newAmount) - expFunction(originalAmount)) * gemWorth
       earned.previouslyAchievedAmount = originalAmount
       EarnedAchievement.update {achievement: earned.achievement, user: earned.user}, earned, {upsert: true}, (err) ->
         return log.debug err if err?
 
-      earnedPoints = earned.earnedPoints
       #log.debug earnedPoints
-      wrapUp()
+      wrapUp(new EarnedAchievement(earned))
+    else
+      done?()
 
   else # not alreadyAchieved
     #log.debug 'Creating a new earned achievement called \'' + (achievement.get 'name') + '\' for ' + userID
-    earned.earnedPoints = worth
+    earned.earnedPoints = pointWorth
+    earned.earnedGems = gemWorth
     (new EarnedAchievement(earned)).save (err, doc) ->
       return log.error err if err?
-      earnedPoints = worth
+      earnedPoints = pointWorth
+      earnedGems = gemWorth
       wrapUp(doc)
 
   User.saveActiveUser userID, "achievement"

server/achievements/earned_achievement_handler.coffee

@@ -65,6 +65,10 @@ class EarnedAchievementHandler extends Handler
         return @sendNotFoundError(res, 'Could not find achievement.')
       else if not trigger
         return @sendNotFoundError(res, 'Could not find trigger.')
+      else if achievement.get('proportionalTo')
+        EarnedAchievement.createForAchievement(achievement, trigger, trigger.unchangedCopy, (earnedAchievementDoc) =>
+          @sendCreated(res, (earnedAchievementDoc or earned)?.toObject())
+        )
       else if earned
         achievementEarned = achievement.get('rewards')
         actuallyEarned = earned.get('earnedRewards')
@@ -82,8 +86,6 @@ class EarnedAchievementHandler extends Handler
             return @sendDatabaseError(res, err) if err
             return @sendSuccess(res, earned.toObject())
           )
-      else if achievement.get('proportionalTo')
-        return @sendBadInputError(res, 'Cannot currently do this to repeatable docs...')
       else
         EarnedAchievement.createForAchievement(achievement, trigger, null, (earnedAchievementDoc) =>
           @sendCreated(res, earnedAchievementDoc.toObject())

test/server/functional/achievement.spec.coffee

@@ -21,6 +21,8 @@ repeatable =
   userField: '_id'
   proportionalTo: 'simulatedBy'
   recalculable: true
+  rewards:
+    gems: 1
 
 diminishing =
   name: 'Simulated2'
@@ -169,19 +171,23 @@ describe 'Achieving Achievements', ->
 
   it 'verify that a repeatable achievement has been earned', (done) ->
     unittest.getNormalJoe (joe) ->
-      EarnedAchievement.find {achievementName: repeatable.name}, (err, docs) ->
-        expect(err).toBeNull()
-        expect(docs.length).toBe(1)
-        achievement = docs[0]
 
-        if achievement
-          expect(achievement.get 'achievement').toBe repeatable._id
-          expect(achievement.get 'user').toBe joe._id.toHexString()
-          expect(achievement.get 'notified').toBeFalsy()
-          expect(achievement.get 'earnedPoints').toBe 2 * repeatable.worth
-          expect(achievement.get 'achievedAmount').toBe 2
-          expect(achievement.get 'previouslyAchievedAmount').toBeFalsy()
-        done()
+      User.findById(joe.get('_id')).exec (err, joe2) ->
+        expect(joe2.get('earned').gems).toBe(2)
+        
+        EarnedAchievement.find {achievementName: repeatable.name}, (err, docs) ->
+          expect(err).toBeNull()
+          expect(docs.length).toBe(1)
+          achievement = docs[0]
+  
+          if achievement
+            expect(achievement.get 'achievement').toBe repeatable._id
+            expect(achievement.get 'user').toBe joe._id.toHexString()
+            expect(achievement.get 'notified').toBeFalsy()
+            expect(achievement.get 'earnedPoints').toBe 2 * repeatable.worth
+            expect(achievement.get 'achievedAmount').toBe 2
+            expect(achievement.get 'previouslyAchievedAmount').toBeFalsy()
+          done()
 
   it 'verify that the repeatable achievement with complex exp has been earned', (done) ->
     unittest.getNormalJoe (joe) ->
@@ -195,6 +201,17 @@ describe 'Achieving Achievements', ->
           expect(achievement.get 'earnedPoints').toBe (Math.log(.5 * (2 + .5)) + 1) * diminishing.worth
 
         done()
+        
+  it 'increases gems proportionally to changes made', (done) ->
+    unittest.getNormalJoe (joe) ->
+      User.findById(joe.get('_id')).exec (err, joe2) ->
+        joe2.set('simulatedBy', 4)
+        joe2.save (err, joe3) ->
+          expect(err).toBeNull()
+          User.findById(joe3.get('_id')).exec (err, joe4) ->
+            expect(joe4.get('earned').gems).toBe(4)
+            done()
+
 
 describe 'Recalculate Achievements', ->
   EarnedAchievementHandler = require '../../../server/achievements/earned_achievement_handler'
@@ -241,7 +258,8 @@ describe 'Recalculate Achievements', ->
             unittest.getNormalJoe (joe) ->
               User.findById joe.get('id'), (err, guy) ->
                 expect(err).toBeNull()
-                expect(guy.get 'points').toBe unlockable.worth + 2 * repeatable.worth + (Math.log(.5 * (2 + .5)) + 1) * diminishing.worth
+                # TODO: fix this to properly handle recalculating proportionalTo recalculation
+                expect(guy.get 'points').toBe unlockable.worth + 2 * repeatable.worth + (Math.log(.5 * (4 + .5)) + 1) * diminishing.worth
                 done()
 
   it 'cleaning up test: deleting all Achievements and related', (done) ->