WordPress - pre_get_posts in place of query_posts on pages

Question

My situation is somewhat complex, I'll try to explain it as succinctly as possible.

I'm currently using query_posts to modify the main query on custom pages on my site, which as far as I can tell works quite well, though I've read that using query_posts is bad practice for a number of different reasons.

So, why am I using query_posts and not creating a WP_Query object you may ask?

It's because I'm using the infinite-scroll plugin, infinite-scroll doesn't play nice with WP_query, but it works absolutely fine when you simply modify the main query with query_posts. For example, pagination doesn't work using infinite scroll + WP_query (main concern).

On one page, I'm modifying the query to get most viewed posts.

<?php $paged = get_query_var( 'paged' ) ? get_query_var( 'paged' ) : 1; ?>     
<?php query_posts( array( 'meta_key' => 'wpb_post_views_count', 'orderby' => 'meta_value_num', 'order' => 'DESC' ,  'paged' => $paged, ) ); ?>     


<?php if (have_posts()) : ?>

<?php while ( have_posts() ) : the_post() ?>

    <?php if ( has_post_format( 'video' )) {
            get_template_part( 'video-post' );
        }elseif ( has_post_format( 'image' )) {
            get_template_part( 'image-post' );
        } else {
           get_template_part( 'standard-post' );
        }

    ?>

<?php endwhile;?>

<?php endif; ?>

So after a lot of reading I gather that my other option to modify the main query is using pre_get_posts, though I'm somewhat unsure as to how to go about this.

Take this for example:-

function textdomain_exclude_category( $query ) {
    if ( $query->is_home() && $query->is_main_query() ) {
        $query->set( 'cat', '-1,-2' );
    }
}
add_action( 'pre_get_posts', 'textdomain_exclude_category' );

Alright, so simple enough - if it's the home page, modify the main query and exclude two categories.

What I'm confused about and can't figure out is:-

1. the use case scenario for custom page templates. With my query_posts modification I can just drop in the array before if (have_posts()), select my page template, publish it and away I go. With pre_get_posts I can't figure out how to say for example $query->most-viewed etc

2. array( 'meta_key' => 'wpb_post_views_count', 'orderby' => 'meta_value_num', 'order' => 'DESC' , 'paged' => $paged, ) );

How the heck do I do that with pre_get_posts and make sure it's paginated, ie. works with infinite scroll? In all the examples I've seen with pre_get_posts there's no arrays.

Answer 1

How to use the pre_get_posts hook to display list of posts on a page, through a custom page template?

I've been playing with the pre_get_posts hook and here's one idea

Step #1:

Ceate a page called for example Show with the slug:

example.com/show

Step #2:

Create a custom page template:

tpl_show.php

located in the current theme directory.

Step #3:

We construct the following pre_get_posts action callback:

function b2e_pre_get_posts( $query )
{
    $target_page = 'show';                             // EDIT to your needs

    if (    ! is_admin()                               // front-end only
         && $query->is_main_query()                    // main query only
         && $target_page === $query->get( 'pagename' ) // matching pagename only
    ) {
        // modify query_vars:
        $query->set( 'post_type',      'post'                 );  // override 'post_type'
        $query->set( 'pagename',       null                   );  // override 'pagename'
        $query->set( 'posts_per_page', 10                     );
        $query->set( 'meta_key',       'wpb_post_views_count' );
        $query->set( 'orderby',        'meta_value_num'       );
        $query->set( 'order',          'DESC'                 );

        // Support for paging
        $query->is_singular = 0;

        // custom page template
        add_filter( 'template_include', 'b2e_template_include', 99 );
    }
}

add_action( 'pre_get_posts', 'b2e_pre_get_posts' );

where

function b2e_template_include( $template )
{
    $target_tpl = 'tpl_show.php'; // EDIT to your needs

    remove_filter( 'template_include', 'b2e_template_include', 99 );

    $new_template = locate_template( array( $target_tpl ) );

    if ( ! empty( $new_template ) )
        $template = $new_template; ;

    return $template;
}

This should also give us pagination:

example.com/show/page/2
example.com/show/page/3

etc.

Notes

I updated the answer and removed the query-object part modification, based on the suggestion from @PieterGoosen, since it could e.g. break the breadcrumbs on his setup.

Also removed the is_page() check within the pre_get_posts hook, since it might still give some irregularities in some cases. The reason is that the query-object is not always available. This is being worked on, see e.g. #27015. There are workarounds possible if we want to use the is_page() or is_front_page().

I constructed the following table, just to get a better overview of some of the properties and query varaiables of the main WP_Query object, for a given slug:

It's interesting to note that the pagination in WP_Query depends on the nopaging not being set and the current page not being singular (from the 4.4 source):

// Paging
if ( empty($q['nopaging']) && !$this->is_singular ) {
    $page = absint($q['paged']);
    if ( !$page )
        $page = 1;

    // If 'offset' is provided, it takes precedence over 'paged'.
    if ( isset( $q['offset'] ) && is_numeric( $q['offset'] ) ) {
        $q['offset'] = absint( $q['offset'] );
        $pgstrt = $q['offset'] . ', ';
    } else {
        $pgstrt = absint( ( $page - 1 ) * $q['posts_per_page'] ) . ', ';
    }
    $limits = 'LIMIT ' . $pgstrt . $q['posts_per_page'];
}

where we can see that the LIMIT part of the generated SQL query is within the conditional check. This explains why we modify the is_singular property above.

We could have used other filter/hooks, but here we used pre_get_posts as mentioned by the OP.

Hope this help.