title: Rails select records with date bigger than or equal to nil
body: <p>I have a <code>Product</code> model with <code>expired_at</code> date field. I want query which return all products with <code>expired_at</code> value that bigger than some value or equals to nil.</p>
  
  <p>But I met a troubles here:</p>
  
  <pre><code>manufactured_at = Date.tomorrow
  Product.where('expired_at &gt; ? OR expired_at = NULL', manufactured_at)
  # =&gt; ActiveRecord::Relation []
  Product.all
  # =&gt; Product id: 1, expired_at: nil, Product id; 2, expired_at: nil, ...
  </code></pre>
  
  <p>How can I fetch these products in my query?</p>