You don’t know query - WordCamp UK Edinburgh 2012

You Don’t Know Query

WordCamp Edinburgh UK
14-15 July 2012

Scott Cariss aka Brady
• Lead developer at Philosophy Design.

• Moderator at WordPress Answers
(http://wordpress.stackexchange.com)

• WordPress plugin developer and enthusiast

scott@philosophydesign.com
@l3rady on Twitter

Conditional Tags
is_author(), is_home(), etc.

Who has ever heard of query_posts()?

Ways to query
query_posts()
new WP_Query()
get_posts()

The loop
if( have_posts() )
while( have_posts() ):
the_post();

endwhile();

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();
}

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( );

But why do we call things like
wp_reset_postdata( ) and
wp_reset_query( )?

What about using query_posts( )?

How can you alter a query? What about
the main query?

What is the main query, and why
should I care?

Let's dig in

wp-blog-header.php
// Load the WordPress bootstrap require
dirname( __FILE__ ) . '/wp-load.php';

// Decide which template ﬁles to load
ABSPATH . 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:
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

// Do magic
wp();

// Decide which template ﬁles to load

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( );
}
...
}

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 );
}

What do we get?
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

// Parse what to query, and query it.
wp();

// Load the theme.

Before we get to the theme, we have
your posts.
Are we clear so far?

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. Pull down all metadata for these posts.
4. Pull down all terms for these posts.

Instead of query_posts()?
We can use this:

// In WP::parse_request()

$this->query_vars = apply_ﬁlters(
'request', $this->query_vars );

We can modify query variables in mid
air:
function brady_filter_out_author( $qvs )
{
if( ! Isset( $qvs*‘author’+ ) )
$qvs*‘author’+ = ‘-5’;

return $qvs;
}
add_filter( “request”, “brady_filter_out_author”);

Powerful, but lacks context.
Problems:

1. Conditional tags don’t work yet.

2. Only works on the main query.

3. WP_Query is way cooler.

Introducing pre_get_posts
class WP_Query
{
...
function &get_posts()
{
$this->parse_query();
// OMG! Conditional tags are available!!
do_action_ref_array( 'pre_get_posts', array( &$this ) );
}
...
}

Lets kill off query_posts()!
function brady_alter_home( $query )
{
if ( $query->is_home( ) )
$query->set( 'author', '-5' );
}
add_action(
'pre_get_posts', ‘brady_alter_home' );

Still with us?
Good ‘cause here’s where things get hairy.

‘request’ fires for the main query only.

‘pre_get_posts’ fires for every post query:

• get_posts()
• new WP_Query()
• That random recent posts widget.
• Everything.

What if I just want it on the
main query?

$wp_the_query makes a
triumphant return.

Main query only!
function brady_alter_home( $query )
{
if ( $query->is_home( ) &&
$wp_the_query === $query )
$query->set( 'author', '-5' );
}
add_action(
'pre_get_posts', ‘brady_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( );

class WP_Query
{
...
function &query_posts( $query )
{
// Break the reference to $wp_the_query
unset( $wp_query );
$wp_query =& new WP_Query( $query );
...
}
...
}

class WP_Query
{
...
function wp_reset_query( )
{
// Restore the 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 and the globals.

Calling $my_query->the_post( )?
wp_reset_postdata( ) will reset the globals.

Since WordPress 3.3!
Rather than:
$wp_the_query === $other_query_object

You‘re able to call:
$other_query_object->is_main_query( )

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( ).

And finally
request is a nice hook. pre_get_posts is more
powerful and ﬂexible. Just use it properly.

Always check if you're modifying the main query
using $query->is_main_query( )

$query === $wp_the_query before 3.3

You don’t know query - WordCamp UK Edinburgh 2012

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