db={"PostSchema":[{"_id":1,"content":"first","comments":["a","b","c"]}],"Comment":[{"_id":"a","comment":"1st comemnt"},{"_id":"b","comment":"2nd comemnt"},{"_id":"c","comment":"3rd comemnt"}]}

db.PostSchema.aggregate([{"$lookup":{"from":"Comment",let:{cId:"$comments"},"pipeline":[{$match:{$expr:{_id:{in:["$$cId"]}}}},{$limit:10}],"as":"comments"}}])