declare a variable ruby on rails

Question

In the example given at

http://guides.rubyonrails.org/getting_started.html

I got confused by variable "comment".

The original version is

<h2>Comments</h2>
<% @post.comments.each do |comment| %>
  <p>
    <strong>Commenter:</strong>
    <%= comment.commenter %>
  </p>

  <p>
    <strong>Comment:</strong>
    <%= comment.body %>
  </p>
<% end %>

In this version, I can understand "comment" is from |comment|

Then, in section 7, this part is changed into

<h2>Comments</h2>
<%= render @post.comments %>

In the app/views/comments/_comment.html.erb the code is

<p>
  <strong>Commenter:</strong>
  <%= comment.commenter %>
</p>

<p>
  <strong>Comment:</strong>
  <%= comment.body %>
</p>

Then I got confused. Where is "comment" declared?

Is there a tutorial introducing how variables declared in ruby on rails?

Thank you very much.

Answer 1

This article should clear it up. The core of it is that

<% render @post.comments %>

will do the same thing as

<% @post.comments.each do |comment| %>
  <%= render partial: 'comments/comment', locals: { comment: comment } %>
<% end %>

Your variable comment inside the partial template is declared by render, triggered by the locals hash. The article further explains the magic of how you get the latter from the former.