[{"id":1,"comments":[{"id":1,"text":"Comment 1","likedBy":["Tomato","Potato"],},{"id":2,"text":"Comment 2","likedBy":["Carrot"],},],},]

db.collection.update({id:1,"comments.id":1,},[{$set:{"comments.likedBy":{$cond:[{$in:["Potato","$comments.likedBy"]},{$setDifference:["$comments.likedBy",["Potato"]]},{$concatArrays:["$comments.likedBy",["Potato"]]}],},},}])