Wordpress - When to use WP_query(), query_posts() and pre_get_posts
You are right to say:
Never use
query_posts
anymore
pre_get_posts
pre_get_posts
is a filter, for altering any query. It is most often used to alter only the 'main query':
add_action('pre_get_posts','wpse50761_alter_query');
function wpse50761_alter_query($query){
if( $query->is_main_query() ){
//Do something to main query
}
}
(I would also check that is_admin()
returns false - though this may be redundant.). The main query appears in your templates as:
if( have_posts() ):
while( have_posts() ): the_post();
//The loop
endwhile;
endif;
If you ever feel the need to edit this loop - use pre_get_posts
. i.e. If you are tempted to use query_posts()
- use pre_get_posts
instead.
WP_Query
The main query is an important instance of a WP_Query object
. WordPress uses it to decide which template to use, for example, and any arguments passed into the url (e.g. pagination) are all channelled into that instance of the WP_Query
object.
For secondary loops (e.g. in side-bars, or 'related posts' lists) you'll want to create your own separate instance of the WP_Query
object. E.g.
$my_secondary_loop = new WP_Query(...);
if( $my_secondary_loop->have_posts() ):
while( $my_secondary_loop->have_posts() ): $my_secondary_loop->the_post();
//The secondary loop
endwhile;
endif;
wp_reset_postdata();
Notice wp_reset_postdata();
- this is because the secondary loop will override the global $post
variable which identifies the 'current post'. This essentially resets that to the $post
we are on.
get_posts()
This is essentially a wrapper for a separate instance of a WP_Query
object. This returns an array of post objects. The methods used in the loop above are no longer available to you. This isn't a 'Loop', simply an array of post object.
<ul>
<?php
global $post;
$args = array( 'numberposts' => 5, 'offset'=> 1, 'category' => 1 );
$myposts = get_posts( $args );
foreach( $myposts as $post ) : setup_postdata($post); ?>
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endforeach; wp_reset_postdata(); ?>
</ul>
In response to your questions
- Use
pre_get_posts
to alter your main query. Use a separateWP_Query
object (method 2) for secondary loops in the template pages. - If you want to alter the query of the main loop, use
pre_get_posts
.
There are two different contexts for loops:
- main loop that happens based on URL request and is processed before templates are loaded
- secondary loops that happen in any other way, called from template files or otherwise
Problem with query_posts()
is that it is secondary loop that tries to be main one and fails miserably. Thus forget it exists.
To modify main loop
- don't use
query_posts()
- use
pre_get_posts
filter with$query->is_main_query()
check - alternately use
request
filter (a little too rough so above is better)
To run secondary loop
Use new WP_Query
or get_posts()
which are pretty much interchangeable (latter is thin wrapper for former).
To cleanup
Use wp_reset_query()
if you used query_posts()
or messed with global $wp_query
directly - so you will almost never need to.
Use wp_reset_postdata()
if you used the_post()
or setup_postdata()
or messed with global $post
and need to restore initial state of post-related things.
There are legitimate scenarios for using query_posts($query)
, for example:
You want to display a list of posts or custom-post-type posts on a page (using a page template)
You want to make pagination of those posts work
Now why would you want to display it on a page instead of using an archive template?
It's more intuitive for an administrator (your customer?) - they can see the page in the 'Pages'
It's better for adding it to menus (without the page, they'd have to add the url directly)
If you want to display additional content (text, post thumbnail, or any custom meta content) on the template, you can easily get it from the page (and it all makes more sense for the customer too). See if you used an archive template, you'd either need to hardcode the additional content or use for example theme/plugin options (which makes it less intuitive for the customer)
Here's a simplified example code (which would be on your page template - e.g. page-page-of-posts.php):
/**
* Template Name: Page of Posts
*/
while(have_posts()) { // original main loop - page content
the_post();
the_title(); // title of the page
the_content(); // content of the page
// etc...
}
// now we display list of our custom-post-type posts
// first obtain pagination parametres
$paged = 1;
if(get_query_var('paged')) {
$paged = get_query_var('paged');
} elseif(get_query_var('page')) {
$paged = get_query_var('page');
}
// query posts and replace the main query (page) with this one (so the pagination works)
query_posts(array('post_type' => 'my_post_type', 'post_status' => 'publish', 'paged' => $paged));
// pagination
next_posts_link();
previous_posts_link();
// loop
while(have_posts()) {
the_post();
the_title(); // your custom-post-type post's title
the_content(); // // your custom-post-type post's content
}
wp_reset_query(); // sets the main query (global $wp_query) to the original page query (it obtains it from global $wp_the_query variable) and resets the post data
// So, now we can display the page-related content again (if we wish so)
while(have_posts()) { // original main loop - page content
the_post();
the_title(); // title of the page
the_content(); // content of the page
// etc...
}
Now, to be perfectly clear, we could avoid using query_posts()
here too and use WP_Query
instead - like so:
// ...
global $wp_query;
$wp_query = new WP_Query(array('your query vars here')); // sets the new custom query as a main query
// your custom-post-type loop here
wp_reset_query();
// ...
But, why would we do that when we have such a nice little function available for it?