text: 
  var mongoose = require('mongoose')
  var Schema = mongoose.Schema
  
  var FriendSchema = new Schema({
      sender : { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
      receiver : { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
      status: String
  })
  
  FriendSchema.index({ "sender": 1, "receiver": 1}, { "unique": true });
  
  module.exports = mongoose.model('Friend', FriendSchema)