Project.create({ /* */ }).then(function(project) {
  return User.create({ /* */ }).then(function(user) {
    return project.hasUser(user).then(function(result) {
      // result would be false
      return project.addUser(user).then(function() {
        return project.hasUser(user).then(function(result) {
          // result would be true
        })
      })
    })
  })
})