Here is a little code snippet that will explain you how to get/list elgg entities based on some particular metadata_value. For example you may have 10 or 20 entities with a particular metadata called post_order and if you want to list these contents, then you can use the following option

$options = array( 	'types' => 'object',
					'subtype' => 'my_subtype',
					'container_guid' => $container_guid, 
					'limit' => 10,
					'offset' => 0, 
					'order_by_metadata' => array(	'name' => 'post_order', 
									'direction' => 'ASC', 
									'as' => 'integer'), 
					'full_view' => FALSE, 
					'pagination' => FALSE
				);

Now you can call this options in any elgg_get* or elgg_list* API’s like

• elgg_get_entities($options);
• elgg_list_entities($options);
• elgg_get_entities_from_metadata($options);