// index.js
var schemas = {
  users: { id:1, name:'', email:'', groups:[] },
  groups: { id:1, name:'' },
};

var database = new Database(schemas, {
  users: [
    { id:1, name:'joe', email:'joe@gmail.com', groups:[1] },
    { id:2, name:'joe Blow', email:'joe.blow@example.com', groups:[1, 2] },
  ],
  groups: [
    { id:1, name:'The Joes' },
    { id:2, name:'The Blows' },
  ],
});

var app = ApplicationFactory(require('express')(), database);

// ApplicationFactory.js
module.exports = function(app, database) {

  app.get('/groups/:id', function(req, res) {
    // this piece of code should be reusable
    database.get('groups', { id:Number(req.params.id) }).then(function(groups) {
      if (groups[0]) {
        database.get('users', { 'groups': [ groups[0].id ] }).then(function(users) {
          groups[0].users = users;
          res.send(groups[0]);
        });
      } else {
        res.sendStatus(404);
      }
    });
  });

  app.post('/groups/:id/users', function(req, res) {
    var groupId = Number(req.params.id);
    var user = req.body;
    database.get('users', { 'id': user.id }).then(function(users) {
      var groups = users[0].groups;
      if (groups.indexOf(groupId) == -1) { groups.push(groupId) }
      database.set('users', { 'groups' : groups }, user.id).then(function() {

        // this piece of code should be reusable
        database.get('groups', { id:groupId }).then(function(groups) {
          database.get('users', { 'groups': [ groups[0].id ] }).then(function(users) {
            groups[0].users = users;
            res.send(groups[0]);
          });
        });

      });
    });
  });

  return app;
}