$ rails generate model User email:string password_digest:string

$ rails generate migration AddIndexToUsersEmail

class AddIndexToUsersEmail < ActiveRecord::Migration[6.1]
  def change
    # 以下を追記
    add_index :users, :email, unique: true
  end
end

$ rails db:migrate

class User < ApplicationRecord
  # 以下を追記
  before_save { self.email = email.downcase }
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, presence: true, length: { maximum: 255 },
                    format: { with: VALID_EMAIL_REGEX },
                    uniqueness: { case_sensitive: false }
  validates :password, presence: true, length: { minimum: 6 }
  has_secure_password
end

  before_save { self.email = email.downcase }
  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
  validates :email, presence: true, length: { maximum: 255 },
                    format: { with: VALID_EMAIL_REGEX },
                    uniqueness: { case_sensitive: false }

  validates :password, presence: true, length: { minimum: 6 }
  has_secure_password

# Use Active Model has_secure_password
gem 'bcrypt', '~> 3.1.7'

$ rails generate Controller Users

class UsersController < ApplicationController
  # 以下を追記
  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to profile_path(@user)
    else
      render 'new'
    end
  end

  def show
    @user = User.find(params[:id])
  end

  def destroy
    user = User.find(params[:id])
    user.destroy
    redirect_to signup_path
  end

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

<h1>ユーザー登録</h1>

<%= form_with model: @user, url: signup_path, local: true do |form| %>
  <% if @user.errors.any? %>
    <strong><%= @user.errors.count %>個のエラーがあります。</strong>
    <ul>
      <% @user.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
    </ul>
  <% end %>

  <%= form.label :email, 'メールアドレス' %>
  <%= form.email_field :email %>

  <%= form.label :password, 'パスワード' %>
  <%= form.password_field :password %>

  <%= form.label :password_confirmation, 'パスワード(確認)' %>
  <%= form.password_field :password_confirmation %>

  <%= form.submit '登録' %>
<% end %>

<h1>ユーザー情報</h1>

<p>メールアドレス：<%= @user.email %></p>
<p><%= link_to '登録解除', unsubscribe_path(@user), method: :delete, data: { confirm: '本当に登録解除しますか？' } %></p>

Rails.application.routes.draw do
  # 以下を追記
  get '/signup', to: 'users#new'
  post '/signup', to: 'users#create'
  get '/users/:id', to: 'users#show', as: 'profile'
  delete '/users/:id', to: 'users#destroy', as: 'unsubscribe'
end

$ rails console
> User.find(1)
=> <User id: 1, email: "test@example.com", password_digest: [FILTERED], created_at: "2021-11-29 14:38:21.222271000 +0000", updated_at: "2021-11-29 14:38:21.222271000 +0000">

$ rails generate controller Sessions

class SessionsController < ApplicationController
  # 以下を追記
  before_action :require_login, only: [:destroy]

  def new
  end

  def create
    user = User.find_by(email: params[:email].downcase)
    if user && user.authenticate(params[:password])
      log_in(user)
      redirect_to profile_path(user)
    else
      flash.now[:danger] = 'メールアドレスかパスワードが間違っています。'
      render 'new'
    end
  end

  def destroy
    log_out
    redirect_to login_path
  end
end

module SessionsHelper
  # 以下を追記
  def log_in(user)
    session[:user_id] = user.id
  end

  def current_user
    if session[:user_id]
      @current_user ||= User.find_by(id: session[:user_id])
    end
  end

  def logged_in?
    !current_user.nil?
  end

  def require_login
    redirect_to login_path if !logged_in?
  end

  def log_out
    session.delete(:user_id)
    @current_user = nil
  end
end

class ApplicationController < ActionController::Base
  # 以下を追記
  include SessionsHelper
end

  def log_in(user)
    session[:user_id] = user.id
  end

  def current_user
    if session[:user_id]
      @current_user ||= User.find_by(id: session[:user_id])
    end
  end

  def logged_in?
    !current_user.nil?
  end

  def require_login
    redirect_to login_path if !logged_in?
  end

  def log_out
    session.delete(:user_id)
    @current_user = nil
  end

<h1>ログイン</h1>

<%= form_with model: @session, local: true do |form| %>
  <%= flash[:danger] %>

  <%= form.label :email, 'メールアドレス' %>
  <%= form.email_field :email %>

  <%= form.label :password, 'パスワード' %>
  <%= form.password_field :password %>

  <%= form.submit 'ログイン' %>
<% end %>

<h1>ユーザー情報</h1>

<p>メールアドレス：<%= @user.email %></p>
<%# 以下を追記 %>
<p><%= link_to 'ログアウト', logout_path, method: :delete %></p>
<p><%= link_to '登録解除', unsubscribe_path(@user), method: :delete, data: { confirm: '本当に登録解除しますか？' } %></p>

Rails.application.routes.draw do
  # 以下を追記
  get '/login', to: 'sessions#new'
  post '/login', to: 'sessions#create'
  delete '/logout', to: 'sessions#destroy'
end

Rails.application.routes.draw do
  get '/signup', to: 'users#new'
  post '/signup', to: 'users#create'
  # 以下の2行を修正
  get '/profile', to: 'users#show'
  delete '/unsubscribe', to: 'users#destroy'
  get '/login', to: 'sessions#new'
  post '/login', to: 'sessions#create'
  delete '/logout', to: 'sessions#destroy'
end

class UsersController < ApplicationController
  # 以下を追記：コントローラーのリファクタリング①
  before_action :require_login, only: [:show, :destroy]

  ...

  def create
    # 以下を修正：コントローラーのリファクタリング②
    user = User.new(user_params)
    if user.save
      # 以下を追記：コントローラーのリファクタリング③
      log_in(user)
      redirect_to profile_path(user)
    else
      render 'new'
    end
  end

  ...

  def destroy
    # 以下を修正：コントローラーのリファクタリング④
    current_user.destroy
    redirect_to signup_path
  end

  ...
end

  before_action :require_login, only: [:show, :destroy]

    user = User.new(user_params)
    if user.save
      log_in(user)
      redirect_to profile_path(user)
    else
      render 'new'
    end

      log_in(user)

    current_user.destroy

<h1>ユーザー情報</h1>

<%# 以下を修正：ビューのリファクタリング① %>
<p>メールアドレス：<%= current_user.email %></p>
<p><%= link_to 'ログアウト', logout_path, method: :delete %></p>
<%# 以下を修正：ビューのリファクタリング② %>
<p><%= link_to '登録解除', unsubscribe_path, method: :delete, data: { confirm: '本当に登録解除しますか？' } %></p>

<p>メールアドレス：<%= current_user.email %></p>

<p><%= link_to '登録解除', unsubscribe_path, method: :delete, data: { confirm: '本当に登録解除しますか？' } %></p>