WordPress – Get all posts where a meta key does not exist

Simple…

$search_values['meta_query'] = array(
'relation' => 'OR',
array(
'key' => 'psychedelics', //replace with the non existing field to query
'compare' => 'NOT EXISTS',
'value' => '' // Ignored, but needed to work...
),
array(
'key' => 'psychedelics', //replace with the non existing field to query
'value' => '2CB' //any value
)
)

Source:
http://wordpress.stackexchange.com/questions/80303/query-all-posts-where-a-meta-key-does-not-exist

AC.