diff --git a/mongo-query.coffee b/mongo-query.coffee
index e06cd2d..e91de12 100644
--- a/mongo-query.coffee
+++ b/mongo-query.coffee
@@ -71,6 +71,7 @@ testExpressions = (obj, expressions, lop) ->
     else throw new Error('Unknown logical operator')
 
 
+# generate field projection from a query
 projection = (query, proj) ->
   for k,v of query
     if k.charAt(0) == '$' # this is a logical operator
@@ -79,6 +80,17 @@ projection = (query, proj) ->
     else
       proj[k] = 1
 
+# optimize projection by removing overlaps
+optimizeProjection = (obj) ->
+  keys = Object.keys(obj).sort()
+  for i in [1 ... keys.length]
+    a = keys[i-1]
+    b = keys[i]
+    if common.startsWith(b, a)
+      if b.charAt(a.length) == '.' or b.charAt(a.length - 1) == '.'
+        delete obj[b]
+
 
 exports.test = test
 exports.projection = projection
+exports.optimizeProjection = optimizeProjection
diff --git a/presets.coffee b/presets.coffee
index 61a7fc7..1533901 100644
--- a/presets.coffee
+++ b/presets.coffee
@@ -84,6 +84,7 @@ exports.assertPresets = (deviceId, presetsHash, callback) ->
           else
             throw new Error('Unknown configuration type')
 
+    mongoQuery.optimizeProjection(projection)
     db.devicesCollection.findOne({'_id' : deviceId}, projection, (err, device) ->
       devicePresets = []
       for p in presets