title: How do I set a default url for images[0] in Carrierwave?
body: <p>I have a standard image uploader using Carrierwave. I am also using Postgres. So this is what my migration looks like for adding images as JSON:</p>
  
  <pre><code>class AddImagesToListings &lt; ActiveRecord::Migration[5.1]
    def change
      add_column :listings, :images, :json
      remove_column :listings, :image
    end
  end
  </code></pre>
  
  <p>I want to make images[0] always have some image, but it seems like the Carrierwave documentation only covers this for single file uploads. Right now, here is my default_url method:</p>
  
  <pre><code>def default_url(*args)
      ActionController::Base.helpers.asset_path("default/" + ["default.jpg"].compact.join('_'))
  end
  </code></pre>
  
  <p>This was working when I only had :image, but now it isn't. Is there any way to set a default for images[0] so that I get a valid images[0].url for every listing I have (despite whether or not a user adds an image to the listing)?</p>