How to test the outcome of a wpdb query?

Question

I want to get all the data with the meta key of "project_id" from post meta table. I wrote a query, it should work in thoery, but I have no way to see what I get, and inject this query to my plugin doesn't generate anything. Please take a look at this query, and help me understand if it is correct, what's the formate of the outcome, How can I echo it to see what I get:

function mp_all_ids( ){
global $bp, $wpdb, $post;

$query = <<<QUERY
SELECT      project_id.post_id
FROM        $wpdb->postmeta project_id
LEFT JOIN  $wpdb->postmeta height 
            on height.post_id = project_id.post_id
            and height.meta_key = 'height'
LEFT JOIN  $wpdb->postmeta width 
            on width.post_id = project_id.post_id
            and width.meta_key = 'width' 
WHERE       project_id.meta_key = 'project_id'
ORDER BY    project_id.meta_value+(0) ASC
QUERY;
$data = $wpdb->get_results( $query );

return apply_filters( 'mp_ids', $data );
}

Edit: the var_dump gets the following result:

array(3) {
  [0]=>
  object(stdClass)#457 (1) {
    ["post_id"]=>
    string(3) "385"
  }
  [1]=>
  object(stdClass)#458 (1) {
    ["post_id"]=>
    string(3) "451"
  }
  [2]=>
  object(stdClass)#377 (1) {
    ["post_id"]=>
    string(3) "453"
  }
}

How can I access the 3 strings in each object?

Answer 1

First of all, you should look into using WP_Query for this:

http://scribu.net/wordpress/advanced-metadata-queries.html

Secondly, to see the results, just do a var_dump() before the return:

echo '<pre>';
var_dump( $data );

Answer 2

To test if the query is working you could output the $query string that you have created and manually running that query on the wordpress database via PhpMyAdmin or a similar interface.

If there is any errors in the query or the query returns no results than you will be able to tweak it to get your desired effect before putting it in your code.