aggregate.push({
        $lookup: {
          from: 'examnum',
          let: { userId: '$_id' },
          pipeline: [
            {
              $match: {
                $expr: {
                  $eq: [ '$exambatchId', mongoose.Types.ObjectId(data.exambatchId) ],
                },
              },
            },
            {
              $match: {
                $expr: {
                  $eq: [ '$userId', '$$userId' ],
                },
              },
            },
            {
              $sort: { _id: 1 },

            }, {
              $limit: 1,
            },
          ],
          as: 'examnum',
        },
      });

TAGS：nodejs npm mongoose $lookup