db={"posts":[{title:"Demo",content:"Demo",headerImage:"Test",tags:[1,2,3]}],"tags":[{_id:1,name:"t",description:"S",creationDate:""}]}

db.posts.aggregate([{$addFields:{TagCount:{$size:"$tags"}}},{$lookup:{from:"tags",localField:"tags",foreignField:"_id",as:"tagList"}}])