Database
[{_id:"6291f69f811b414175d11b16",savesOnComment:[],comments:[{_id:"6297aa1112c9fe003a32ddeb",owner:"61152b692816b44ff246e07b",contentId:"6291f69f811b414175d11b16",replyId:null,saves:["61152b692816b44ff246e07b"]},{_id:"6297aa1512c9fe003a32dded",owner:"61152b692816b44ff246e07b",contentId:"6291f69f811b414175d11b16",replyId:null,saves:["61152b692816b44ff246e07b"]}],}]
Query
db.collection.aggregate([{"$set":{"users":{"$setUnion":["$savesOnComment","$comments.owner",{"$reduce":{"input":"$comments.saves","initialValue":[],"in":{"$concatArrays":["$$value","$$this"]}}}]}}}])