index: function(req, res) {
    User.findOne({
      _id: req.params.id
    })
    .populate({path: "roommates", match: {status: {$eq: "active"}}})
    .exec(function(err, user) {
      console.log(user.roommates);
    })
  }

[ { _id: 57f2e5e02d58f51a8284bc11,
    balance: 0,
    requests: [],
    status: 'pending' } ]

var UserSchema = new Schema({
  name: String,
  username: {
    type: String,
    required: [true, "Please enter a username"],
    minlength: [6, "Username must be at least 6 characters"],
    maxlength: [15, "Username cannot exceed 15 characters"],
    unique: true
  },
  password: {
    type: String,
    required: [true, "Please enter a password"],
    minlength: [6, "Password must be at least 6 characters"],
    maxlength: [17, "Password cannot exceed 17 characters"],
  },
  roommates: [{roommate: {type: Schema.Types.ObjectId, ref: "User", unique: true}, status: {type: String, default: "pending"}, requests: [{type: Schema.Types.ObjectId, ref: "Request"}], balance: {type: Number, default: 0}}]
})