rails new user_login

# Use ActiveModel has_secure_password
gem 'bcrypt', '~> 3.1.7'

rails g model user name:string password_digest:string

# app/models/user.rb
class User < ActiveRecord::Base
 has_secure_password
end

rake db:migrate

rails g controller applicants new create

post 'applicants/create'

# app/controllers/applicants_controller.rb

class ApplicantsController < ApplicationController
 def new
  @user = User.new
 end

 def create
  @user = User.create(user_params)
  if @user.save
   redirect_to :sessions_new
  else
   render "new"
  end
 end

 private
  def user_params
   params.require(:user).permit(:name, :password, :password_confirmation)
  end
end

<!-- app/views/applicants/new.html.erb -->

<h1>注册</h1>

<% if @user.errors.any? %>
<ul>
  <% @user.errors.full_messages.each do |message| %>
  <li><%= message %></li>
  <% end %>
</ul>
<% end %>

<%= form_for @user, url: :applicants_create do |f| %>
  <p>
    <%= f.label :name %>
    <%= f.text_field :name %>
  </p>

  <p>
    <%= f.label :password %>
    <%= f.password_field :password %>
  </p>

  <p>
    <%= f.label :password_confirmation %>
    <%= f.password_field :password_confirmation %>
  </p>

  <p><%= f.button "提交" %></p>
<% end %>

rails g controller sessions new create

# config/routes.rb

post 'sessions/create'

# app/controllers/sessions_controller.rb

class SessionsController < ApplicationController
 def new
 end

 def create
  user = User.find_by(name: user_params[:name]).try(:authenticate, user_params[:password])
  if user
   render plain: sprintf("welcome, %s!", user.name)
  else
   flash.now[:login_error] = "invalid username or password"
   render "new"
  end
 end

 private
  def user_params
   params.require(:session).permit(:name, :password)
  end
end

<!-- app/views/sessions/new.html.erb -->

<h1>登录</h1>

<% if flash[:login_error] %>
  <p><%= flash[:login_error] %></p>
<% end %>

<%= form_for :session, url: :sessions_create do |f| %>

  <p>
    <%= f.label :name %>
    <%= f.text_field :name %>
  </p>

  <p>
    <%= f.label :password %>
    <%= f.password_field :password %>
  </p>

  <p><%= f.button "登录" %></p>

<% end %>