pre_get_posts and set

Question

I have some functions into my pre_get_posts filter. So, the problem is that when I set a meta_query, all the menus disappear from the page.

function propersearchfilter($query) {
    $taxarray = array();
    $metaarray = array();

    array_push($metaarray , array(
        'key' => 'wpcf-market',
        'value' => 'forsell',
        'compare' => 'LIKE'
    ));

    $query->set('meta_query', $metaarray);
    return $query;
}
add_action('pre_get_posts', 'propersearchfilter', 1);

Answer 1

pre_get_posts triggers for every query (front and admin side). It also triggers when querying nav-menu items (which is what is happening here).

Most of the time you only want to filter particular queries - so you need to check if that query is one you wish to modify.

Usually - and I think your example might be one - you only want to modify the 'main query'.

function wpse72969_pre_get_posts( $query) {
    //If its not the 'main query' don't do antying.
    if( !$query->is_main_query() )
        return;

    return $query; 
}
add_action('pre_get_posts', 'wpse72969_pre_get_posts');

More generally all the conditionals are available to you (Note: you want want to use the methods and not the functions: e.g. $query->is_search() as opposed to is_search().

You can even set custom variables to identify queries.

Answer 2

That happens because pre_get_posts fires on every single instance of WP_Query. Posts get used for a lot of things in WordPress, including nav menus.

If you want to only modify the main query (the "loop") you can test for is_main_query.

<?php
add_action('pre_get_posts', 'wpse72969_modify_q');
function wpse72969_modify_q($q)
{
    if(!$q->is_main_query())
        return;

    // any calls to `set` here will modify ONLY the main query
}

You can also use any of the conditional tags as well:

<?php
add_action('pre_get_posts', 'wpse72969_modify_q2');
function wpse72969_modify_q2($q)
{
    if(!$q->is_main_query() || !$q->is_search())
        return;

    // any calls to `set` here will modify ONLY 
    // the main query on search pages.
}