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WordCamp Portland 2011September 17, 2011

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Andrew NacinCore Developer of WordPressTech Ninja at Audrey Capitalnacin@wordpress.org@nacin on Twitter

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You Dont Know Query

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What do you know?

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Conditional Tagsis_author( ), is_home( ), etc.

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Who has ever heard ofquery_posts( )?

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Ways to queryquery_posts( )new WP_Query( )get_posts( )

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The loopif ( have_posts( ) ) while ( have_posts( ) ) : the_post( ); endwhile( );

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What dont you know?

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Every query object has itsown methodsis_author( ) is the same as calling$wp_query->is_author( )

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function is_author( ) { global $wp_query; return $wp_query->is_author( );}

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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( );

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But why do we call things likewp_reset_postdata( ) andwp_reset_query( )?What about using query_posts( )?How can you alter a query? Whatabout the main query?

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

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Lets dig in.

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wp-blog-header.php// Load the WordPress bootstraprequire dirname( __FILE__ ) . /wp-load.php;// Do magicwp();// Decide which template files to loadABSPATH . WPINC . /template-loader.php;

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Lets look in the bootstrap:$wp_the_query = new WP_Query();$wp_query =& $wp_the_query;

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Quick lesson on PHP references$a = 4;$b =& $a;$b = 2;var_dump( $a ); // int(2)$a = 6;var_dump( $b ); // int(6)

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So:So the real main query is in$wp_the_query.And a live copy of it is stored in$wp_query.

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wp-blog-header.php// Load the WordPress bootstraprequire dirname( __FILE__ ) . /wp-load.php;// Do magicwp();// Decide which template files to loadABSPATH . WPINC . /template-loader.php;

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wp-blog-header.php// Load the WordPress bootstraprequire dirname( __FILE__ ) . /wp-load.php;// Do magicwp( );// Decide which template files to loadABSPATH . WPINC . /template-loader.php;

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What is that wp( ) call?function wp( $query_vars = ) { global $wp; $wp->main( $query_vars );}

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Holy $!@?, what justhappened?

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In the bootstrap:$wp = new WP( );So theres a wp( ) function,and a WP class.

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class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .

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class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .

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WP::parse_request( )— Parses the URL using WP_Rewrite— Sets up query variables for WP_QueryWP::query_posts( ) { global $wp_the_query; $wp_the_query->query( $this->query_vars );}

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Boom.SELECT SQL_CALC_FOUND_ROWS wp_posts.*FROM wp_postsWHERE 1=1 AND wp_posts.post_type = post AND wp_posts.post_status = publishORDER BY wp_posts.post_date DESCLIMIT 0, 10

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wp-blog-header.php// Load WordPress.require dirname(__FILE__) . /wp-load.php;// Parse what to query, and query it.wp();// Load the theme.ABSPATH . WPINC . /template-loader.php;

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Before we get to the theme,we have your posts.Got it?

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Then why do we do this?query_posts( author=5 );get_header( );while( have_posts( ) ) : the_post( );endwhile;get_footer( );

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Thats running 2* queries!One, the query WordPressthought we wanted.Two, this new one youreactually going to use.

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* Actually, WP_Querydoesnt run just one query.It usually runs four.

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1. Get me my posts: SELECT SQL_CALC_FOUND_ROWS … FROM wp_posts LIMIT 0, 102. How many posts exist? SELECT FOUND_ROWS()3. Slurp all metadata for these posts.4. Slurp all terms for these posts.

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PROTIP‘Measure twice, cut once’is bad for performance.

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(A note, 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,) );

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So. Instead of this:query_posts( author=5 );get_header( );while ( have_posts( ) ) : the_post( );endwhile;get_footer( );

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We can use this:// In WP::parse_request()$this->query_vars = apply_filters( request, $this->query_vars );

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We can modify queryvariables in mid air:function nacin_filter_out_author( $qvs ) { if ( ! isset( $qvs[author] ) ) $qvs[author] = -5; return $qvs;}

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Powerful, but lacks context.

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Powerful, but lacks context.Problem 1: Conditional tags dont work yet.

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Powerful, but lacks context.Problem 1: Conditional tags dont work yet.Problem 2: Only works on the main query.

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Powerful, but lacks context.Problem 1: Conditional tags dont work yet.Problem 2: Only works on the main query.Problem 3: WP_Query is waaay cooler.

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

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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 );

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Still with us?Good, ‘cause here’s wherethings get hairy.

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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.

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What if I just want it on themain query?

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$wp_the_query makes atriumphant return.

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

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Hmm. How does this work?$wp_the_query should never be modified. Itholds the main query, forever.$wp_query keeps a live reference to$wp_the_query, unless you use query_posts().

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query_posts( author=-5 );while ( have_posts( ) ) : the_post( );endwhile;wp_reset_query( );

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query_posts( author=-5 );while ( have_posts( ) ) : the_post( );endwhile;wp_reset_query( );

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

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query_posts( author=-5 );while ( have_posts( ) ) : the_post( );endwhile;wp_reset_query( );

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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( ); . . .

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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.

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New thing for core in 3.3!Rather than: $wp_the_query === $other_query_object Youll be able to call: $other_query_object->is_main_query( ) is_main_query( ), the function, will act on$wp_query, like any other conditional tag.

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Some LessonsEvery WP_Query object has methods thatmimic the global conditional tags.The global conditional tags apply to$wp_query, the main or current query.$wp_query is always the main query, unlessyou use query_posts( ). Restore it withwp_reset_query( ).

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And Finallyrequest is a nice hook. pre_get_posts is morepowerful and flexible. Just use it properly.Always check if youre modifying the main queryusing $query === $wp_the_query$query->is_main_query( ) in 3.3!

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Thanks! Questions?@nacin