Building an inbox system using Express/MongoDB

I have been developing an app for renting flats for a personal project of mine. I have been wondering about how to build/implement an inbox system to the current app which uses mongodb as database.

What does matter in such a feature? Would it be possible to define a schema called messages ?

var MessageSchema = new mongoose.Schema({
  message: String,
  sender : {
        id : {
            type : mongoose.Schema.Types.ObjectId,
            ref: "User"
          },
         username: String
    }
  recipient : {
        id : {
            type : mongoose.Schema.Types.ObjectId,
            ref: "User"
          },
         username: String
    }
)};

If I were to send a message to user y then it would be stored as recipient and the x value would be the sender. When I want to render a page called inbox could i do Messages.find({req.user._id) to sort all messages ? I am a bit lost in regards to the schema design.

via Ozan