ts query

$m_query = array(
			    'relation'    => 'OR',
			    'mls_no_maine' => array(
			        'key'     => $prefix . 'mls_no_maine',
			        'compare' => 'EXISTS',
			    ),

			    'mls_no_maine' => array(
			        'key'     => $prefix . 'mls_no_maine',
			        'compare' => 'NOT EXISTS',
			    ),

			);


			$args = array(
				'post_type'              => array( 'ts-listings' ),
				'post_status'            => array( 'publish' ),
				'posts_per_page'		 => '-1',
				'meta_query'			 => $m_query,
				// 'meta_key'			 	 => $prefix . 'mls_no_maine',
				'orderby'				 => 'meta_value',
				'order'					 => 'ASC',
			);

			$q = new WP_Query($args);