[{posts:[{post:{id:"dkdkd",title:"hello",body:"hello world",tags:[1,2,3]}},{post:{id:"ccc",title:"hello2",body:"hello world2",tags:[3,4,5]}}]},{posts:[{post:{id:"dkdkd",title:"hello",body:"hello world",tags:[5,6,7]}},{post:{id:"ccc",title:"hello2",body:"hello world2",tags:[3,4,5]}}]}]

db.collection.aggregate([{$project:{_id:0,tags:{$reduce:{input:"$posts.post.tags",initialValue:[],in:{"$setUnion":["$$value","$$this"]}}}}},{$group:{_id:"Total",tags:{"$push":"$tags"}}},{$project:{_id:0,tags:{$reduce:{input:"$tags",initialValue:[],in:{"$setUnion":["$$value","$$this"]}}}}}])