How to add another condition to a specific active facet? I did try to add a condition using hook_views_pre_execute or hook_views_query_alter, but since Im usiong SearchAPI, theres is no point apparently in using these functions (no $query in views_query_alter, and no query too in views_pre_execute 

Maybe using hook__search_api_query_alter could d othe trick but I dont know how to implement it.

My question is : How can I achieve this ?

Knowing that I need to add anoteh condition only when items of a specific facet are active (among a few other facets).
The solution is quite simple :

function mymodule_search_api_query_alter(SearchApiQueryInterface $query) {
    if(preg_match('/PRETTYPATHID/',url($_GET['q']))){
        $base_filter = $query-&gt;createFilter('AND');
        $base_filter-&gt;condition('MYFIELD', NULL, "&lt;&gt;");
        $query-&gt;filter($base_filter);
    }

}