form_validation_update_destroy.rb

class SongsController < ApplicationController
    before_action :set_song, only: [:show, :edit, :update, :destroy]
    
  
     def index
       @songs = Song.all
     end
  
     def new
       @song = Song.new
     end
  
     def create
       @song = Song.new(song_params)
       if @song.valid?
         @song.save
         redirect_to song_path(@song)
       else
         render :new
     end
    end
  
     def edit
     end
  
     def show
     end
  
     def update
       if  @song.update(song_params)
         @song.save
         redirect_to song_path(@song)
       else
         render :edit
       end
     end
  
     def destroy
       @song.destroy
       redirect_to songs_path
     end
  
     private
  
     def song_params
       params.require(:song).permit(:title, :released, :release_year, :artist_name, :genre)
     end
  
     def set_song
       @song = Song.find(params[:id])
     end
  
   end
---------------------------------------------------
#index

<h1>Song Index</h1>
<table>
  <tbody>
    <%= @songs.each do |song| %>
    <tr><td><%= song.title %></td></tr>
    <% end %>
  </tbody>
</table>   
---------------------------------------------------
  #new

<%= form_for (@song) do |f| %>
  <%= f.text_field :title %>
  <%= f.number_field :release_year %>
  <%= f.check_box :released %>
  <%= f.text_field :genre %>
  <%= f.text_field :artist_name %>
  <%= f.submit %>
<% end %>
  ----------------------------------------
    #show
  
  <h1>Show Song</h1>
<h1><%= @song.title %></h2>
<h2><%= @song.released %></h2>
<h2><%= @song.release_year %></h2>
<h2><%= @song.artist_name %></h2>
<h2><%= @song.genre %></h2>
------------------------------------------
    #edit
  
  <%= form_for @song do |f| %>
  <%= f.text_field :title %>
  <%= f.number_field :release_year %>
  <%= f.check_box :released %>
  <%= f.text_field :genre %>
  <%= f.text_field :artist_name %>
  <%= f.submit %>
<% end %>
-------------------------------------
      #Song 
    
    class Song < ActiveRecord::Base
    validates :title, presence: true
    validates :title, uniqueness: {scope: :release_year}
    validates :artist_name, presence: true
    validates :release_year, numericality: { less_than_or_equal_to: 2018}, if: :released
    validates :release_year, presence: true, if: :released
  end