在猫鼬中找到子文档的_id

Question

我的模型的架构是：-

var socialSchema = new mongoose.Schema({
  friend_names:[String],
  friend_pics:[String]
})

var userSchema= new mongoose.Schema({
    name: {
      type: String,
    },
    username: {
      type: String,
      index:true
    },
    email: {
      type: String,
    },
    password: {
      type: String,

    },

    social:[socialSchema]

  })

考虑以下猫鼬查询：-

module.exports.friendAccept= function(req, res){
  var friend_name= req.body.frnames;
  var friend_pic = req.body.frpics;
  var user= req.user;
  User
  .findOne({name:user.name})
  .select('social')
  .exec(function(err,user){

    console.log(user);
})
}

上面的控制台日志的输出看起来像这样：-

{ _id: 5bc6205168bf4e52a8c1aa26,
  social:
   [ { friend_names: [Array],
       friend_pics: [Array],
       _id: 5bc73e145d2a2d3808f3cd71 } ] }
{ _id: 5bc6205168bf4e52a8c1aa26,
  social:
   [ { friend_names: [Array],
       friend_pics: [Array],
       _id: 5bc73e145d2a2d3808f3cd71 } ] }

上面声明的friend_name和friend_pic变量包含这些子文档之一中存在的值。我如何获取子文档的_id（例如5bc6205168bf4e52a8c1aa26或5bc73e145d2a2d3808f3cd71的示例），该子文档实际上保存着这些值friend_name和friend_pic