title: How to add custom filter to Active Admin?
body: <p>Active Admin allows me to <a href="http://activeadmin.info/docs/3-index-pages.html" rel="noreferrer">define filters</a> that are displayed on the index page like so:</p>
  
  <pre><code>ActiveAdmin.register Promo do
  
    filter :name
    filter :address
    filter :city
    filter :state
    filter :zip
  
  end
  </code></pre>
  
  <p>I would like to combine all the fields above into one, so that I can search for Promos that contain the search string in name or full address. My model already has a named scope that I can use:</p>
  
  <pre><code>class Promo &lt; ActiveRecord::Base
    scope :by_name_or_full_address, lambda { |q| where('name LIKE :q OR address LIKE :q OR city LIKE :q OR state LIKE :q OR zip LIKE :q', :q =&gt; "%#{q}%") }
  end
  </code></pre>