https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/database/command/Command.expr.html

const _ = db.command
const $ = _.aggregate
db.collection('items').where(_.expr(
  $.lt(
    $.cond({
      if: $.gte('$price', 10),
      then: $.multiply(['$price', '0.5']),
      else: $.multiply(['$price', '0.8']),
    })
    ,
    8
  )
).get()