db={"orders":[{"id":1,"orderName":"a","seqId":100,"etc":[],"desc":[],},{"id":2,"orderName":"b","seqId":200,"etc":[],"desc":[]},{"id":3,"orderName":"c","seqId":100,"etc":[],"desc":[],},],"goods":[{"id":1,"title":"example1","items":[{"id":10,"details":[{"id":100},{"id":101,}]},{"id":20,"details":[{"id":102,},{"id":103,}]},]},{"id":2,"title":"example2","items":[{"id":30,"details":[{"id":200},{"id":201}]},{"id":40,"details":[{"id":202},{"id":203}]},]},]}

db.orders.aggregate([{"$match":{"etc":[],"desc":[]}},{"$lookup":{"from":"goods","localField":"seqId","foreignField":"items.details.id","pipeline":[{$project:{_id:0,title:1}}],"as":"goodsLookup"}},{"$unwind":"$goodsLookup"},{$group:{_id:"$goodsLookup.title",cnt:{$sum:1}}}])