title: ActiveAdmin nested form doesn't work
body: <p>I am using ActiveAdmin in my rails application.My setup is the following:</p>
  
  <p><em>app/models/translation_type.rb</em></p>
  
  <pre><code>class TranslationType &lt; ActiveRecord::Base
      has_many :translation_details
      accepts_nested_attributes_for :translation_details, allow_destroy: true
  end
  </code></pre>
  
  <p><em>app/models/translation_detail.rb</em></p>
  
  <pre><code>class TranslationDetail &lt; ActiveRecord::Base
    belongs_to :translation_type
    attr_accessor :id, :language_from, :language_to, :price
  end
  </code></pre>
  
  <p><em>app/admin/translation_type.rb</em></p>
  
  <pre><code>ActiveAdmin.register TranslationType do
    permit_params :translation_type, translation_details_attributes: [:id, :language_from, :language_to, :price, :_destroy]
  
    index do
      column :translation_type
      column :translation_details
      column :created_at
      column :updated_at
      actions
    end
  
    filter :translation_type
    filter :created_at
  
    form do |f|
      f.inputs "Translation Type Details" do
        f.input :translation_type
        f.input :created_at
        f.input :updated_at
      end
      f.inputs do
        f.has_many :translation_details, allow_destroy: true do |tran_det|
          tran_det.input :language_from, :collection =&gt; ['Gr', 'En', 'Fr', 'De']
          tran_det.input :language_to, :collection =&gt; ['Gr', 'En', 'Fr', 'De']
          tran_det.input :price
        end
      end
      f.actions
    end
  
    controller do
      before_filter { @page_title = :translation_type } 
    end
  </code></pre>
  
  <p>I don't need a section for <code>translation_detail</code> so I have omited <code>app/admin/translation_detail.rb</code></p>
  
  <p>I want to have nested forms of <code>translation_detail</code> in my <code>translation_type</code> form. Using the code above creates the nested form but after submit the attributes of <code>translation_detail</code> are not saved. Furthermore when I try to update or delete a <code>translation_detail</code> this message is shown</p>
  
  <pre><code>Couldn't find TranslationDetail with ID=* for TranslationType with ID=*
  </code></pre>
  
  <p>How can this be fixed?</p>