Can't query by meta_key

Question

For some reason. I do not have the ability to query by meta_key when I run a loop. What could cause this and what can I do to the diagnose the problem? I am using wordpress 3.4.1.

$args = array( 'meta_key' => 'slideshow_image');

$query = new WP_Query($args);

Answer 1

Thanks for @dunc and @helgatheviking I got the answer. Here's my code. You need the post type declared, otherwise it resorts to "post". I also couldn't do this unless I put meta_key and meta_value in a 'meta_query' multidimensional array.

$args = array(
         'post_type'    => 'tsa_events',
         'meta_query'   => array(
             array(
               'key'          => 'slideshow_image',
               'value'    => array(''),
               'compare'  => 'NOT IN'
                )
             )

         );

EDIT: you can also structure your query this way:

$args = array(
         'post_type'     => 'tsa_events',
         'meta_key'      => 'slideshow_image',
         'meta_value'    => array(''),
         'meta_compare'  => 'NOT IN'
);