You Don't Know Query (WordCamp Netherlands 2012)
Post on 17-May-2015
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WordCamp Netherlands 2012
Andrew Nacin
Core Developer of WordPress and Tech Ninja at Audrey Capital
@nacin on Twitter nacin@wordpress.org
You Don’t Know Query
What do you know?
Conditional Tags
is_author( ), is_home( ), etc.
query_posts( )
Ways to query
query_posts( ) new WP_Query( ) get_posts( )
The Loop
while ( have_posts( ) ) :
the_post( ); endwhile;
A secondary loop
$query = new WP_Query( … ); while ( $query->have_posts( ) ) :
$query->the_post( ); endwhile;
An array of posts
$result = get_posts( … ); foreach ( $result as $post_obj ) { }
What don’t you know?
Every query object has its own methods
is_author( ) is the same as calling $wp_query->is_author( )
function is_author( ) {
global $wp_query;
return $wp_query->is_author( ); }
With the regular loop
while ( have_posts( ) ) :
the_post( ); if ( is_author( ) ) echo "An author query.";
endwhile;
With the regular loop
while ( have_posts( ) ) :
the_post( ); if ( $wp_query->is_author( ) ) echo "An author query.";
endwhile;
A secondary loop
$query = new WP_Query( … ); while ( $query->have_posts( ) ) :
$query->the_post( ); if ( $query->is_author( ) ) echo "An author query.";
endwhile;
A secondary loop
$query = new WP_Query( … ); while ( $query->have_posts( ) ) :
$query->the_post( ); if ( $query->is_author( ) ) echo "An author query.";
endwhile;
A secondary loop
$query = new WP_Query( … ); while ( $query->have_posts( ) ) :
$query->the_post( ); if ( $query->is_author( ) ) echo "An author query.";
endwhile;
If you do: $my_query = new WP_Query( $query ); You can do: while ( $my_query->have_posts( ) ) : $my_query->the_post( ); endwhile; wp_reset_postdata( );
Why do we call functions like wp_reset_postdata( ) and wp_reset_query( )? What about using query_posts( )? How can you alter a query? How can you alter the main query?
What is the main query, and why should I care?
wp-blog-header.php // Load the WordPress bootstrap require './wp-load.php'; // Do magic wp( ); // Decide which template files to load require WPINC . '/template-loader.php';
Let's look in the bootstrap: $wp_the_query = new WP_Query(); $wp_query =& $wp_the_query;
Quick lesson on PHP references
$a = 4; $b =& $a; $b = 2; var_dump( $a ); // int(2) $a = 6; var_dump( $b ); // int(6)
So: So the real main query is in $wp_the_query. And a live copy of it is stored in $wp_query.
wp-blog-header.php // Load the WordPress bootstrap require './wp-load.php'; // Do magic wp( ); // Decide which template files to load require WPINC . '/template-loader.php';
wp-blog-header.php // Load the WordPress bootstrap require './wp-load.php'; // Do magic wp( ); // Decide which template files to load require WPINC . '/template-loader.php';
What is that wp( ) call?
function wp( $query_vars = '' ) { global $wp;
$wp->main( $query_vars );
}
Holy $!@?, what just happened?
In the bootstrap:
$wp = new WP( ); So there's a wp( ) function, and a WP class.
class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .
class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .
WP::parse_request( ) — Parses the URL using WP_Rewrite — Sets up query variables for WP_Query WP::query_posts( ) {
global $wp_the_query; $wp_the_query->query( $this->query_vars );
}
Boom. SELECT SQL_CALC_FOUND_ROWS
wp_posts.* FROM wp_posts WHERE 1=1
AND wp_posts.post_type = 'post' AND wp_posts.post_status = 'publish'
ORDER BY wp_posts.post_date DESC LIMIT 0, 10
wp-blog-header.php // Load WordPress. require './wp-load.php'; // Parse what to query. Then query it. wp( ); // Load the theme. require WPINC . '/template-loader.php';
Before we get to the theme, we have your posts.
Got it?
Then why do we do this?
query_posts( 'author=-5' ); get_header( ); while( have_posts( ) ) : the_post( ); endwhile; get_footer( );
That's running 2* queries! One, the query WordPress thought we wanted. Two, this new one you're actually going to use.
* Actually, WP_Query doesn't run just one query. It usually runs four.
1. Get me my posts: SELECT SQL_CALC_FOUND_ROWS … FROM wp_posts LIMIT 0, 10
2. How many posts exist? SELECT FOUND_ROWS( )
3. Get all metadata for these posts. 4. Get all terms for these posts.
(You can turn these off selectively…)
$my_query = new WP_Query( array( 'no_found_rows' => true, 'update_post_meta_cache' => false, 'update_post_term_cache' => false,
) );
</aside>
PROTIP ‘Measure twice, cut once’ is bad for performance.
Other problems with query_posts( )
Pagination breaks. WordPress calculated paging using the query it did, not the query you did.
query_posts( array( 'author' => -5, 'posts_per_page' => 25,
) ); This will not work well.
You easily mess up globals.
This can break widgets and more.
query_posts( ) is bad. Do we agree?
Introducing pre_get_posts class WP_Query {
. . . function &get_posts() { $this->parse_query(); // Huzzah! do_action_ref_array( 'pre_get_posts', array( &$this ) ); . . .
A truly awesome hook. function nacin_alter_home( $query ) {
if ( $query->is_home( ) ) $query->set( 'author', '-5' );
} add_action( 'pre_get_posts', 'nacin_alter_home' );
Still with us?
Good, ‘cause here’s where things get complicated.
'pre_get_posts' fires for every post query: — get_posts( ) — new WP_Query( ) — That random recent posts widget your client installed without you knowing. — Everything.
What if I just want it on the main query?
$wp_the_query makes a triumphant return.
Main query only!
function nacin_alter_home( $query ) { global $wp_the_query; if ( $wp_the_query === $query && $query->is_home() ) $query->set( 'author', '-5' );
} add_action( 'pre_get_posts', 'nacin_alter_home' );
Hmm. How does this work? $wp_the_query should never be modified. It holds the main query, forever. $wp_query keeps a live reference to $wp_the_query, unless you use query_posts( ).
query_posts( 'author=-5' ); while ( have_posts( ) ) :
the_post( ); endwhile; wp_reset_query( );
query_posts( 'author=-5' ); while ( have_posts( ) ) :
the_post( ); endwhile; wp_reset_query( );
function query_posts( $query ) { // Break the reference to $wp_the_query unset( $wp_query ); $wp_query =& new WP_Query( $query ); return $wp_query;
}
query_posts( 'author=-5' ); while ( have_posts( ) ) :
the_post( ); endwhile; wp_reset_query( );
function wp_reset_query( ) { // Restore reference to $wp_the_query unset( $wp_query ); $wp_query =& $wp_the_query;
// Reset the globals, too. wp_reset_postdata( );
}
Calling the_post( )? wp_reset_query( ) will reset $wp_query and the globals.
Calling $my_query->the_post( )?
wp_reset_postdata( ) will reset the globals.
New in WordPress 3.3!
Rather than: $wp_the_query === $other_query_object
You can call:
$other_query_object->is_main_query( ) is_main_query( ), the function, will act on $wp_query, like any other conditional tag.
What about page templates?
/* Template: My Template */ query_posts( $query_string .
'&author=-5&posts_per_page=25' ); get_header( ); while ( have_posts( ) ) :
the_post( ); endwhile;
function nacin_my_template( $query ) { if ( ! $query->is_main_query( ) ) return; if ( ! is_page_template( 'my-template.php' ) ) return; $query->set( 'author', '-5' ); $query->set( 'posts_per_page', '25' );
} add_action( 'pre_get_posts',
'nacin_my_template' );
Some Lessons
Every WP_Query object has methods that mimic the global conditional tags. The global conditional tags apply to $wp_query, the main or current query. $wp_query is always the main query, unless you use query_posts( ). Restore it with wp_reset_query( ).
pre_get_posts is a powerful and flexible hook. Just use it properly. Always check if you're modifying the main query using $query->is_main_query( )
And Finally
Thanks! Questions?
@nacin
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