I'm having difficulty understanding this syntax:

Code:
<%= form_for([@post, @post.comments.build]) do |f| %>
usually, I see it as:

Code:
<%= form_for @post do |f| %>
what's with the extra stuff? what does @post.comments.build do? Things to note: posts is related to comments... as in posts has many comments and a comment belongs to a post. Igot this code for this tutorial:

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


Code:
<h2>Add a comment:</h2>
<%= form_for([@post, @post.comments.build]) do |f| %>
  <div class="field">
    <%= f.label :commenter %><br />
    <%= f.text_field :commenter %>
  </div>
  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>