Mongodb，golang。 我可以在不将数据加载到内存的情况下计算切片/地图吗？

In my collection labs I have:

{
    "_id" : ObjectId("57e602ada35ea4db6e4eee27"),
    "areas" : [
        "nanotech",
        "robotics"
    ]
}

My query is: db.labs.find({"_id" : ObjectId("57e602ada35ea4db6e4eee27")},{areas:1})

What I want is to count the elements in the slice but not retrieve the whole slice, anyone know? I'm using the mgo package, which has a great correspondence with golang. Thanks

db.labs.aggregate([
  {$match: {"_id" : ObjectId("57e602ada35ea4db6e4eee27")}},
  {$project: {
      areasCount: {"$size": "$areas"}
    }
  }
])

</div>