云函数如何根据用户openid 获取云数据库的自身数据？wxContext.OPENID怎么不能赋值

const cloud = require('wx-server-sdk')
cloud.init()
const db = cloud.database()
const MAX_LIMIT = 100

//.doc(this.data.id_)
exports.main = async(event, context) => {
  const wxContext = cloud.getWXContext()
 
  return await db.collection("userlist").where({
    _id: wxContext.OPENID
  }).orderBy('time', 'desc').get({
    success: function(res) {
      // return res
    }
  });

  return {
    //  data,
    event,
    openid: wxContext.OPENID,
    appid: wxContext.APPID,
    unionid: wxContext.UNIONID,
  }
}