title: ActiveAdmin with has_many problem; undefined method 'new_record?'
body: <p>I'm trying to customise a ActiveAdmin form for a Recipe model that has a has_many relationship with Step.</p>
  
  <pre><code>class Recipe &lt; ActiveRecord::Base
    has_many :steps
  end
  
  class Step &lt; ActiveRecord::Base
    acts_as_list :scope =&gt; :recipe
  
    belongs_to :recipe
  end
  </code></pre>
  
  <p>I have the following in my ActiveAdmin file with relation to this:</p>
  
  <pre><code>form do |f|
    f.has_many :steps do |ing_f|
      ing_f.inputs
    end
  end
  </code></pre>
  
  <p>The following error is thrown when I try to load the form:</p>
  
  <blockquote>
    <p>undefined method `new_record?' for nil:NilClass</p>
  </blockquote>
  
  <p>I've isolated it so far to the has_many method but I'm lost past this. Any advice and help would be appreciated!</p>