Database
db={tags:[{"taggedBookmarksCount":2,"taggedNotesCount":0,"_id":"627a80e6b12b0dc78b3a6d4b","name":"Article"},{"taggedBookmarksCount":0,"taggedNotesCount":0,"_id":"62797885b479b5906ef6ed43","name":"Client"},],bookmark:[{"_id":"627a814db12b0dc78b3a6d54","bookmarkTags":[{"tagId":"627a814db12b0dc78b3a6d55","tag":"Article"},{"tagId":"62797885b479b5906ef6ed43","tag":"to be read"}],"bookmarkTitle":"Please sorrow of work",}]}
Query
db.bookmark.aggregate([{"$unwind":"$bookmarkTags"},{"$lookup":{"from":"tags","localField":"bookmarkTags.tagId","foreignField":"_id","as":"btags"}},{"$unwind":{path:"$btags",preserveNullAndEmptyArrays:true}},{"$group":{"_id":"$bookmarkTags.tagId","docs":{"$addToSet":"$btags"}}},{"$project":{tag_id:"$_id","count":{"$size":"$docs"},_id:0}}])