Database
db={"posts":[{title:"Demo",content:"Demo",headerImage:"Test",tags:[ObjectId("5dd4eb821ea12d053479d12b")]}],"tags":[{_id:ObjectId("5dd4eb821ea12d053479d12b"),name:"t",description:"S",creationDate:""}]}
Query
db.posts.aggregate([{$addFields:{TagCount:{$size:"$tags"}}},{$lookup:{from:"tags",localField:"tags",foreignField:"_id",as:"tagList"}}])