[{_id:1,hobbies:["eat","read","swim"]},{_id:2,hobbies:["eat","sleep","swim"]},{_id:3,hobbies:["code","read","eat"]}]

db.collection.aggregate([{$unwind:{path:"$hobbies"}},{$group:{_id:null,result:{$addToSet:"$hobbies"}}},{$set:{result:{$sortArray:{input:"$result",sortBy:1}}}}])