title: Filter wordpress post by metavalue
body: <p>I want display <code>custom post type ='product'</code> if it has the <code>metakey='child'</code> in wordpress admin section:</p>
  
  <p>I used the following method but no products displaying after that.</p>
  
  <pre><code>add_filter( 'posts_where' , 'posts_where_statement' );
  $where = "AND wp_postmeta.metakey LIKE 'child' AND wp_posts.post_type = 'product' ";
  </code></pre>
  
  <hr/>
  
  <pre><code>AND wp_posts.post_type = 'product'   // This works just fine
  AND wp_postmeta.metakey LIKE 'child' // This doesn't work!
  </code></pre>
  
  <p>Please help me on this one. Thank you.</p>