How can I retrieve data from alternate database using wpdb class?

Question

I'm attempting to retrieve data from a separate WordPress installation on the same server. I'm able to use the wpdb class to successfully connect to the alternate database and get results, however, I can't seem to use the class methods with my newly created database.

For clarification: I have 2 Wordpress installations. I am attempting to retrieve posts from one database into a page from the other installation. The new database is another basic Wordpress database.

This works and brings me data:

$query = "
SELECT *
FROM wp_posts
WHERE post_type='post'
AND post_status='publish'
";

$rows = $newdb->get_results($query, ARRAY_A);

This fails to get results:

$query = "
SELECT *
FROM $newdb->posts
WHERE post_type='post'
AND post_status='publish'
";

$rows = $newdb->get_results($query, ARRAY_A);

I'm using the following to connect the database:

$newdb = new wpdb('username','password','database','localhost');

All of the material I've found uses the wpdb methods to interact with the new database and I'm having trouble making my queries more complex without the ability to use them.

What am I missing?

Answer 1

After instantinating a wpdb the instace vars holding the table names are still not present. You need to call the set_prefix() method, in order to setup all table names according to your other wordpress table prefix. (Very likely this will be wp_ in your case).

$newdb = new wpdb( DB_USER , DB_PASSWORD , DB_NAME , DB_HOST );
var_dump($newdb->posts); // NULL = not good

$newdb->set_prefix('wp_');
var_dump($newdb->posts); // 'wp_posts' yeah!