# app/models/user.rb
class User < ActiveRecord::Base
    validates :username, :password, {presence: true, length: { in: 3..20 } }
    validates :username, uniqueness: true
end

# test/functional/users_controller_test.rb
require 'test_helper'

class UsersControllerTest < ActionController::TestCase
  setup do
    @user = users(:one)
    @user.username = "test"
    @user.password = "test"
  end

  test "should create user" do
    assert_difference('User.count') do
      post :create, user: @user.attributes
    end

    assert_redirected_to user_path(assigns(:user))
  end

  test "should update user" do
    put :update, id: @user, user: @user.attributes
    assert_redirected_to user_path(assigns(:user))
  end
end

# test/fixtures/users.yml
one:
  username: username
  password: password
  group: 1