title: Rails how to get a particular error message on association
body: <p>I'm actually having a hard time figure out how to get a first error message related to a particular association.
  In my form, I must display errors under every related fields such as : </p>
  
  <pre><code>= f.text_field :price, class: 'form-control'
    - if @item.errors.has_key? :price
      = content_tag :span, @item.errors[:price].first, class: 'form-error'
  </code></pre>
  
  <p>This works fine on model base attributes. However, my Item model is linked to another model via a polymorphic association. When displaying <code>errors.full_messages</code>, I get <code>["Attachments name doit être rempli(e)"]</code>. Here is the relation : </p>
  
  <pre><code>has_many :attachments, as: :attachable, dependent: :destroy
  accepts_nested_attributes_for :attachments
  validates :name, presence: true
  </code></pre>
  
  <p>How can I do in order to get the error returned by this association?</p>