mocha mongoose's with `--watch`

tets_coupons.js

// connect mongodb
var mongoose = require('mongoose');
// 见https://github.com/LearnBoost/mongoose/issues/1251
mongoose.models = {};
mongoose.modelSchemas = {};

var clearDB = function(done){
  async.parallel([
    function (cb) {
    Coupon.collection.remove(cb)
  },
  ], done);
}

describe('Coupon', function() {
  // before(function(done) {