此代码应该可以正常工作，但是。。。它有一些问题。让我们将其分解并添加一些评论。

function searchcategory($query) {
    if ($query->is_search) {
        // Checking only for is_search is very risky. It will be set to true
        // whenever param "s" is set. So your function will modify any wp_query with s,
        // for instance the one in wp-admin... But also any other, even if
        // the query isn\'t querying for posts...

        $query->set(\'cat\',\'37\');
        // You pass ID of term in here. It\'s a number. Passing it as string
        // is not a problem, but it\'s another bad practice, because it will
        // have to be converted to number.
    }
    return $query;

    // It\'s an action - you don\'t have to return anything in it. Your result 
    // will be ignored.
}
add_filter(\'pre_get_posts\',\'searchcategory\');

// pre_get_posts is and action and not a filter.
// Due to the way actions/filters are implemented,
// you can assign your callback using `add_filter`,
// but it isn\'t a good practice.

function searchcategory($query) {
    if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
        $query->set( \'cat\', 37 );
    }
}
add_action( \'pre_get_posts\', \'searchcategory\' );