title: rails_admin ArgumentError when trying to edit or create a new entry
body: <p>I'm getting this error while trying to create a new or edit a Limitation (model below)</p>
  
  <p>ArgumentError in Rails_admin/main#new</p>
  
  <p>Showing /Users/deini/.rvm/gems/ruby-2.0.0-p195/gems/rails_admin-0.4.9/app/views/rails_admin/main/_form_filtering_select.html.haml where line #11 raised:</p>
  
  <p>wrong number of arguments (0 for 1+)</p>
  
  <p>Extracted source (around line #11):  (selected_id = field.selected_id)</p>
  
  <pre><code>    selected_id = selected.send(field.associated_primary_key)
       selected_name = selected.send(field.associated_object_label_method)
     else
       selected_id = field.selected_id
       selected_name = field.formatted_value
     end
  </code></pre>
  
  <p>System.rb</p>
  
  <pre><code>class System &lt; ActiveRecord::Base
    has_many :attachments
  
    has_many :limitations
    has_many :companies, :through =&gt; :limitations
  
    accepts_nested_attributes_for :attachments
    accepts_nested_attributes_for :companies
  
    attr_accessible :conf_type, :version, :hardware_type, :name, :attachments_attributes, :company_ids, :companies_attributes
  
    rails_admin do
      list do
        exclude_fields :created_at, :updated_at
      end
    end
  
  end
  </code></pre>
  
  <p>company.rb</p>
  
  <pre><code>  class Company &lt; ActiveRecord::Base
    belongs_to :distributor
    has_many :users, dependent: :destroy
  
    has_many :limitations
    has_many :systems, :through =&gt; :limitations
  
    accepts_nested_attributes_for :limitations
  
    attr_accessible :distributor_id, :name, :system_ids, :email, :limitations_attributes
  
    rails_admin do
      list do
        exclude_fields :custom_url, :created_at, :updated_at
      end
  
      edit do
        exclude_fields :custom_url, :users
      end
    end
  
  end
  </code></pre>
  
  <p>limitation.rb</p>
  
  <pre><code>class Limitation &lt; ActiveRecord::Base
    belongs_to :company
    belongs_to :system
    attr_accessible :company_id, :system_id, :version_limit
  end
  </code></pre>
  
  <p>I haven't modify anything in my rails_admin, any ideas of what I'm doing wrong?</p>