Database
[{_id:"60313c2de6ae000124b6626e",title:"<p>yyyyyyyyy</p>",description:"<p>yyyyyyyyyyyyyyyyyyaaaaaaaaaa</p>",authorId:"603000031f00ce4750e077ab",createDate:"2021-02-20T16:43:25.083+00:00",posts:[{_id:"95633628-a396-9621-7d4d-6ff5859f849c",title:"<p>sdfggfdsgdssgfd</p>",content:"",date:"2021-02-20T16:44:13.917Z"},{_id:"another id"},{_id:"not shown"}]}]
Query
db.collection.aggregate([{"$match":{"_id":"60313c2de6ae000124b6626e"}},{"$project":{"posts":{"$filter":{"input":"$posts","as":"post","cond":{"$eq":["$$post._id","95633628-a396-9621-7d4d-6ff5859f849c"]}}}}}])