title: OperationFailure: exception: invalid operator '$size'
body: <p>Hello I am using mongoDB 3.4.0</p>
  
  <p>and the query is:</p>
  
  <pre><code>db.collection.aggregate([
                  {"$project":{"_id": ObjectId("595c911093e2359c3af3fefb"),
                              "GROUP_count": { "$size": "$GROUPS" },
                              "SYSTEM_count": { "$size": "$IDUS" }
                              }
                           }])
  </code></pre>
  
  <p>But i an getting error "OperationFailure: exception: invalid operator '$size'"  </p>