[{_id:1,username:"xy",text:"asdf",comments:[{username:"User1",text:"hi",},{username:"User2",text:"hi1",},{username:"User3",text:"hi2",},{username:"User4",text:"hi3",}]},{_id:2,username:"xy",text:"asdf",comments:[{username:"User2",text:"hi1",},{username:"User3",text:"hi2",},{username:"User4",text:"hi3",}]}]

db.collection.aggregate([{"$project":{"comments":1,"existsUser":{"$cond":{"if":{"$gt":[{"$size":{"$setIntersection":["$comments.username",["User1"]]}},0]},"then":true,"else":false}}}}])