WordPress Alternating main loop (pre_get_posts filter)


Syntax

  • add_action( 'pre_get_posts', 'callback_function_name' );
  • function callback_function_name( $query ) {}
  • // for PHP 5.3.0 or above
  • add_action( 'pre_get_posts', function( $query ){} );

Parameters

ParameterDetails
$query(WP_Query) Loop object

Remarks

If you are using PHP 5.3.0 or above, you can use closures (anonymous functions)

add_action( 'pre_get_posts', function( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;

    // this code will run only if
    // - this query is main query
    // - and this is not admin screen
});

Even more specific loop targeting

Let's say we want to change main loop, only for specific taxonomy, or post type.

Targeting only main loop on book post type archive page.

add_action( 'pre_get_posts', 'my_callback_function' );

function my_callback_function( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;
    if( !is_post_type_archive( 'book' ) ) return;

    // this code will run only if
    // - this query is main query
    // - and this is not admin screen
    // - and we are on 'book' post type archive page
}

You can also check for category, tag or custom taxonomy archive page using is_category(), is_tag() and is_tax().

You can use any conditional tag available in WordPress.

Show posts from only one category

add_action( 'pre_get_posts', 'single_category' );

function single_category( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;

    $query->set( 'cat', '1' );
    return;
}

Pre get posts filter basic usage

Sometimes you would like to change main WordPress query.

Filter pre_get_posts is the way to go.

For example using pre_get_posts you can tell main loop to show only 5 posts. Or to show posts only from one category, or excluding any category etc.

add_action( 'pre_get_posts', 'my_callback_function' );

function my_callback_function( $query ) {
    // here goes logic of your filter
}

As you can see, we are passing main loop query object into our callback function argument.

Important note here: we are passing argument as a reference. It means that we do not need to return query or set any globals to get it working. As $query is a reference to the main query object, all changes we make on our object are immediately reflected in the main loop object.

Exclude category from posts list edit share

add_action( 'pre_get_posts', 'single_category_exclude' );

function single_category_exclude( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;

    $query->set( 'cat', '-1' );
    return;
}

Change posts_per_page for main loop

All we need to do is to use set() method of $query object.

It takes two arguments, first what we want to set and second what value to set.

add_action( 'pre_get_posts', 'change_posts_per_page' );

function change_posts_per_page( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;

    $query->set( 'posts_per_page', 5 );
    return;
}

Targeting only main WordPress loop

WordPress is applying pre_get_posts filter to literally any loop it generates. It means that all changes we are making in our callback function are applied to all exiting loops.

Obviously it is not what we want in most scenarios.

In most cases we would like to target only main loop, and only for non-admin screens.

We can use is_main_query() method and is_admin() global function to check if we are in the right place.

add_action( 'pre_get_posts', 'my_callback_function' );

function my_callback_function( $query ) {
    if( !$query->is_main_query() || is_admin() ) return;

    // this code will run only if
    // - this query is main query
    // - and this is not admin screen
}