title: ActiveAdmin deleting attached CarrierWave images on form submission
body: <p>I'm not sure what I'm doing wrong here. File uploads are working but if I submit the form without selecting a file to upload it deletes the previously attached image(s).</p>
  
  <p>Here's what the ActiveAdmin form looks like:</p>
  
  <pre><code>form do |f|
    f.inputs do
      f.input :model_number
      f.input :description
      f.input :slug
      f.input :categories
      f.has_many :product_images do |image|
        image.input :product_id, as: :hidden, id: :product_id, input_html: { value: "%i" }
        image.input :image
      end
    end
    f.actions
  end
  </code></pre>
  
  <p>And the relevant parts of the respective models:</p>
  
  <pre><code>class ProductImage &lt; ActiveRecord::Base
    belongs_to :product
  
    mount_uploader :image, ProductImageUploader
  
    validates :image, :product_id, presence: true
  end
  
  class Product &lt; ActiveRecord::Base
    has_many :product_images, dependent: :destroy
  
    accepts_nested_attributes_for :product_images
  
    validates_associated :product_images
  end
  </code></pre>
  
  <p>Any insights would be much appreciated. Thanks! </p>