ruby on rails - How does new method know the errors in the create method? -

a sample code taken http://guides.rubyonrails.org/getting_started.html

controller

def new   @article = article.new end  def create   @article = article.new(article_params)    if @article.save     redirect_to @article   else     render 'new'   end end  private   def article_params     params.require(:article).permit(:title, :text)   end 

new.html.erb

<%= form_for :article, url: articles_path |f| %>    <% if @article.errors.any? %>     <div id="error_explanation">       <h2>         <%= pluralize(@article.errors.count, "error") %> prohibited         article being saved:       </h2>       <ul>         <% @article.errors.full_messages.each |msg| %>           <li><%= msg %></li>         <% end %>       </ul>     </div>   <% end %>    <p>     <%= f.label :title %><br>     <%= f.text_field :title %>   </p>    <p>     <%= f.label :text %><br>     <%= f.text_area :text %>   </p>    <p>     <%= f.submit %>   </p>  <% end %>  <%= link_to 'back', articles_path %> 

a statement link

the reason why added @article = article.new in articlescontroller otherwise @article nil in our view, , calling @article.errors.any? throw error.

my doubt: if there error while filling form, how @article in new know errors caused @article instance of create action? aren't both different variables? , when render 'new' shouldn't @artcle of create method goes out of scope holds error description , @article of new contain no error information?

when there error in @article creation, there no redirection rendering means render/display new action view i.e new.html.erb without going new action or more precisely without making call new action. see http://brettu.com/rails-daily-ruby-tip-28-whats-the-difference-between-redirect_to-and-render-in-rails/

on errors when render new.html.erb use @article object has errors these lines

 @article = article.new(article_params)  @article.save #save runs validations 

so after submit form, purpose of new action done. whole thing handled create action in on errors display errors , uses @article object initialized , saved in create action , on successful creation make call show action using redirect

hope makes sense.