title: Spree find product without any taxons
body: <p>How can I find all the products in spree which have no taxons added?</p>
  
  <p>If I run <code>Spree::Product.first.taxons</code> I get <code>ActiveRecord::Associations::CollectionProxy</code> with an array of the taxons which the product has but how would I find all the products where this array is empty?</p>
  
  <p>I attempted </p>
  
  <pre><code>Spree::Product.where(taxons: nil)
  </code></pre>
  
  <p>but this returned</p>
  
  <pre><code>ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column spree_products.taxon_id does not exist
  </code></pre>