[{title:"title 1",body:"body 2",user:{username:"a",avatar:"avatar 1"}},{title:"title 2",body:"body 2",user:{username:"b",avatar:"avatar 2"}},{title:"title 3",body:"body 3",user:{username:"b",avatar:"avatar 2"}}]

db.collection.aggregate([{"$sort":{title:1}},{"$group":{_id:"$user.username",body:{$last:"$body"},title:{$last:"$title"},username:{$first:"$user.username"},avatar:{$first:"$user.avatar"},}},{$project:{_id:0}}])