title: Product variation WP_Query with a product category in Woocommerce
body: <p>With Woocommerce, I am trying to make a WP_Query for product variations post type with a product category 'Apple'.</p>
  
  <pre><code>$args = array(
      'product_cat'    =&gt; 'Apple',
      'post_type'      =&gt; array('product', 'product_variation'),
      'post_status'    =&gt; 'publish',
      'key'            =&gt; '_visibility',
      'value'          =&gt; 'visible',
      'posts_per_page' =&gt; 100,
      'taxonomy'       =&gt; 'pa_size',
      'meta_value'     =&gt; '39',
      'meta_query'     =&gt; array(
          array(
              'key'         =&gt; '_stock',
              'value'       =&gt; 0,
              'compare'     =&gt; '&gt;'
          )
      )
  );
  </code></pre>
  
  <p>But I can't get it work in this query. If I remove <code>'product_cat' =&gt; 'Apple'</code>, the query works. Why?</p>