https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/database/aggregate/Aggregate.lookup.html 描述：在数据库里执行“高级操作”，里面想测试lookup语句，按照示例写了pipeline语句，但是执行失败，报错显示$.pipeline() 的写法是错误的，不知道该怎么修正 db.collection('schedule') .aggregate() .lookup({ from:'teacher', localField: 'teacher', foreignField: '_id', as: 'teacherList', }) .lookup({ from: 'class', let: { schedule_teacher: '$teacher', schedule_monday: '$monday', }, pipeline: $.pipeline() .match( _.expr($.and([ $.eq(['$teacher', '$$schedule_teacher']), $.eq(['$monday', '$$schedule_monday']) ])) ) .project({ _id: 0 }) .done(), as: 'classList', }) .end() RuntimeError: cannot read property 'pipeline' (from line 16, col 17 to line 16, col 25) 14 | schedule_monday: '$monday', 15 | }, > 16 | pipeline: $.pipeline() | ^^^^^^^^ 17 | .match( 18 | _.expr($.and([ 19 | $.eq(['$teacher', '$$schedule_teacher']),