Rails: Instance variable can be accessed as a local variable in view

Question

posts_controller.rb
    class PostsController < ApplicationController
      def new
        @post = Post.new
      end
    end


_form.html.erb
<%= form_with(model: post, local: true) do |form| %>
  <div class="field">
    <%= form.label :title %>
    <%= form.text_field :title %>
  </div>
<% end %>

Above is my code. Why is an instantiation of the class Post as @post in the controller can be accessed as a local variable in the view as post? Hoping I could get an answer from this concern.

Hey! You've missed an important path in your question: the new.html.erb view file. It should have a code construction similar to render partial: "form", locals: { post: @post }, which passes the local variable to the underlying view _form.html.erb. Take a look at this documentation for the more detailed explanation: guides.rubyonrails.org/… — zinovyev
– zinovyev, Commented Sep 8, 2020 at 21:34

Sebastián Palma · Accepted Answer · 2020-09-05 17:55:58Z

1

The post local variable isn't the one defined in the controller action, but the one defined in the view file which renders the partial _form.html.erb.

The variable defined in the controller is an instance variable, while the one used in the partial is a local one.

You can see ActionController::Renderer#render for more details.

answered Sep 5, 2020 at 17:55

Sebastián Palma

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