有2个集合 tasks 和user

tasks:

{"_id":"d81cd5415f6afe94004ad6361d00aa46","openid":"oN6HT5FH-JO6yIkhlfAZEYRF8d1k","modelType":"1","time":"15:51","date":"2020-09-23","chongfu":"","remark":"213123","status":1,"iswait":1.0}

user:

{"_id":"d81cd5415f6af0000049c9dc0038204a","num":0.0,"openid":"oN6HT5FH-JO6yIkhlfAZEYRF8d1k"}

我想要实现 查询 tasks  left join user where( tasks.iswait = 1 and user.num > 0 ) 要实现这种效果。要怎么改呢

    db.collection('tasks').aggregate()

    .match({

      iswait: 1

    })

    // .lookup({

    //   from: 'user',

    //   localField: 'openid',

    //   foreignField: 'openid',

    //   as: 'touser',

    // })

    .lookup({

      from: 'user',

      let: {

        tasks_openid: '$openid',

      },

      pipeline: $.pipeline()

        .match(_.expr($.and([

          $.eq(['$openid', '$$tasks_openid']),

          $.gt(['$num', 0])

        ])))

        .project({

          _id: 0,

          openid: 1,

          num: 1,

        })

        .done(),

      

      as: 'touser',

    })

    .skip(i * MAX_LIMIT).limit(MAX_LIMIT)

    .end()

    .then((res) => {




    })