[{"id":"631664448cb20310bc25c89d","tags":["database","middle-layer"]},{"id":"6316945f8995f05ac71d3b22","tags":["relational","database"]},]

db.collection.aggregate([{$unwind:"$tags"},{$group:{_id:"_id",tags:{"$addToSet":"$tags"}}},{$project:{tags:1,_id:0,}},])