title: WP Query: How to get posts from a specific author OR those with a specific meta value
body: <p>I would like to retrieve all those posts whose author (post table field) is a given one OR those which has a given meta value (postmeda table field).</p>
  
  <p>If "author" was a meta value, I know I could use a meta_query to achieve it. The thing here is that it is not... so I think I cannot use the "author" field within a meta_query and use the "relation" key.</p>
  
  <p>I'm looking for something like:</p>
  
  <pre><code>$args = array(
      'post_type'  =&gt; array('post'),
      'orderby'    =&gt; 'ASC',
      'order'      =&gt; 'date',
      'meta_query' =&gt; array(
          'relation' =&gt; 'OR',
          array(
              'relation'    =&gt; 'AND',
              array(
                  'field'   =&gt; 'author',
                  'value'   =&gt; $author_id,
                  'compare' =&gt; '==',
              ),
              array(
                  'key'     =&gt; '_meta_field_name',
                  'compare' =&gt; 'NOT EXISTS',
              ),
          ),
          array(
              'relation'    =&gt; 'AND',
              array(
                  'key'     =&gt; '_meta_field_name',
                  'compare' =&gt; 'EXISTS',
              ),
              array(
                  'key'     =&gt; '_meta_field_name',
                  'value'   =&gt; $meta_field_value,
                  'compare' =&gt; '==',
              ),
          ),
      ),
  );
  
  $data = new WP_Query( $args );
  </code></pre>
  
  <p>Any suggestion on how to achieve that using WP_Query?</p>
  
  <p>Thanks!</p>