db={"teacher":[{"rating":4,"teacher_id":"123","course_count":1,},{"rating":5,"teacher_id":"456","course_count":4,},{"rating":5,"teacher_id":"no-exists","course_count":4,}],"course":[{"teacher_id":"123","advanced":true,},{"teacher_id":"456","advanced":false,}]}

db.course.aggregate([{"$match":{"advanced":true}},{"$lookup":{"localField":"teacher_id","from":"teacher","foreignField":"teacher_id","as":"teacher"}},{"$match":{"teacher.course_count":{"$lt":2}}},{"$project":{"_id":0,"advanced":1,"teacher_id":1}},{"$limit":10}])