title: ActiveAdmin nested form on #show page
body: <p>Is it possible to add nested form to #show page?</p>
  
  <p>Now i have my admin/posts.rb:</p>
  
  <pre class="lang-rb prettyprint-override"><code>ActiveAdmin.register Post do
    show do |post|
      h2 post.title
      post.comments.each do |comment|
        row :comment do comment.text end
      end
    end
  end
  </code></pre>
  
  <p>It lists all the comments for post.
  Now i need a form to add new comments.
  I'm trying to do like this:</p>
  
  <pre class="lang-rb prettyprint-override"><code>ActiveAdmin.register Post do
    show do |post|
      h2 post.title
      post.comments.each do |comment|
        row :comment do comment.text end
      end
  
      form do |f|
        f.has_many :comments do |c|
          c.input :text
        end
      end
    end
  end
  </code></pre>
  
  <p>and get an error:</p>
  
  <blockquote>
    <p>undefined method `has_many' for &lt;form&gt;&lt;/form&gt; :Arbre::HTML::Form</p>
  </blockquote>
  
  <p>Models for Post and Comments look like:</p>
  
  <pre class="lang-rb prettyprint-override"><code>class Post &lt; ActiveRecord::Base
    has_many :comments
    accepts_nested_attributes_for :comments
  end
  
  class Comment &lt; ActiveRecord::Base
    belongs_to :post
  end
  </code></pre>
  
  <p>How do i add that form to my show page?
  Thanks</p>