title: Add filter to acf relationship field
body: <p>I am using <code>wordpress 4.9.7</code> and I am using <code>advanced custom fields 4.4.12</code>. </p>
  
  <p>In my backend I have a post type that is called <code>coins</code>, which has a relationship field called <code>related_coins</code> and a text-field, which is called <code>algorithm</code>. Basically my relationship field creates a relationship to the custom post type <code>products</code>. So product can have a relationship with several coins.</p>
  
  <p>I currently only can filter by post type. However, I would like to filter by the custom field <code>algorithm</code> of the post type <code>coin</code>.</p>
  
  <p><a href="https://i.stack.imgur.com/x96mU.png" rel="noreferrer"><img src="https://i.stack.imgur.com/x96mU.png" alt="enter image description here"></a></p>
  
  <p>I tried the following:</p>
  
  <pre><code>function graphic_card_products_query( $args, $field, $post_id ) {
  
      $args['meta_query'] = array(
          array(
          'key' =&gt; 'algorithm', // name of custom field
          'value' =&gt; 'related_coins',
          'compare' =&gt; 'LIKE'
          )
      )
  
      // return
      return $args;
  
  }
  
  // filter for every field
  add_filter('acf/fields/relationship/query/name=related_coins', 'graphic_card_products_query', 10, 3);
  </code></pre>
  
  <p>Basically I am trying to get a list of all values that the custom field <code>algorithm</code> has and hand it as a filter option back to the relationship field <code>related_coin</code>.</p>
  
  <p>Currently I do not get anything back.</p>
  
  <p>Any suggestions what I am doing wrong?</p>