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You Don't Know Query (WordCamp Netherlands 2012)

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WordCamp Netherlands 2012

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Andrew Nacin
Core Developer of WordPress
and Tech Ninja at Audrey Capital

@nacin on Twitter
nacin@wordpress.org

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You Don’t Know Query

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You Don't Know Query (WordCamp Netherlands 2012)

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An update to a talk I gave at WordCamp Portland 2011, "You Don't Know Query" is an advanced development talk from March 25, 2012, in Utrecht, Netherlands.

An update to a talk I gave at WordCamp Portland 2011, "You Don't Know Query" is an advanced development talk from March 25, 2012, in Utrecht, Netherlands.

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You Don't Know Query (WordCamp Netherlands 2012)

  1. 1. WordCamp Netherlands 2012
  2. 2. Andrew Nacin Core Developer of WordPress and Tech Ninja at Audrey Capital @nacin on Twitter nacin@wordpress.org
  3. 3. You Don’t Know Query
  4. 4. What do you know?
  5. 5. Conditional Tags is_author( ), is_home( ), etc.
  6. 6. query_posts( )
  7. 7. Ways to query query_posts( ) new WP_Query( ) get_posts( )
  8. 8. The Loop while ( have_posts( ) ) : the_post( ); endwhile;
  9. 9. A secondary loop $query = new WP_Query( … ); while ( $query->have_posts( ) ) : $query->the_post( ); endwhile;
  10. 10. An array of posts $result = get_posts( … ); foreach ( $result as $post_obj ) { }
  11. 11. What don’t you know?
  12. 12. Every query object has its own methods is_author( ) is the same as calling $wp_query->is_author( )
  13. 13. function is_author( ) { global $wp_query; return $wp_query->is_author( ); }
  14. 14. With the regular loop while ( have_posts( ) ) : the_post( ); if ( is_author( ) ) echo "An author query."; endwhile;
  15. 15. With the regular loop while ( have_posts( ) ) : the_post( ); if ( $wp_query->is_author( ) ) echo "An author query."; endwhile;
  16. 16. A secondary loop $query = new WP_Query( … ); while ( $query->have_posts( ) ) : $query->the_post( ); if ( $query->is_author( ) ) echo "An author query."; endwhile;
  17. 17. A secondary loop $query = new WP_Query( … ); while ( $query->have_posts( ) ) : $query->the_post( ); if ( $query->is_author( ) ) echo "An author query."; endwhile;
  18. 18. A secondary loop $query = new WP_Query( … ); while ( $query->have_posts( ) ) : $query->the_post( ); if ( $query->is_author( ) ) echo "An author query."; endwhile;
  19. 19. 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( );
  20. 20. 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?
  21. 21. What is the main query, and why should I care?
  22. 22. 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';
  23. 23. Let's look in the bootstrap: $wp_the_query = new WP_Query(); $wp_query =& $wp_the_query;
  24. 24. Quick lesson on PHP references $a = 4; $b =& $a; $b = 2; var_dump( $a ); // int(2) $a = 6; var_dump( $b ); // int(6)
  25. 25. So: So the real main query is in $wp_the_query. And a live copy of it is stored in $wp_query.
  26. 26. 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';
  27. 27. 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';
  28. 28. What is that wp( ) call? function wp( $query_vars = '' ) { global $wp; $wp->main( $query_vars ); }
  29. 29. Holy $!@?, what just happened?
  30. 30. In the bootstrap: $wp = new WP( ); So there's a wp( ) function, and a WP class.
  31. 31. class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .
  32. 32. class WP { . . . function main( ) { $this->init( ); $this->parse_request( ); $this->send_headers( ); $this->query_posts( ); $this->handle_404( ); $this->register_globals( ); . . .
  33. 33. 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 ); }
  34. 34. 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
  35. 35. 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';
  36. 36. Before we get to the theme, we have your posts. Got it?
  37. 37. Then why do we do this? query_posts( 'author=-5' ); get_header( ); while( have_posts( ) ) : the_post( ); endwhile; get_footer( );
  38. 38. That's running 2* queries! One, the query WordPress thought we wanted. Two, this new one you're actually going to use.
  39. 39. * Actually, WP_Query doesn't run just one query. It usually runs four.
  40. 40. 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.
  41. 41. (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, ) );
  42. 42. </aside>
  43. 43. PROTIP ‘Measure twice, cut once’ is bad for performance.
  44. 44. Other problems with query_posts( )
  45. 45. Pagination breaks. WordPress calculated paging using the query it did, not the query you did.
  46. 46. query_posts( array( 'author' => -5, 'posts_per_page' => 25, ) ); This will not work well.
  47. 47. You easily mess up globals. This can break widgets and more.
  48. 48. query_posts( ) is bad. Do we agree?
  49. 49. Introducing pre_get_posts class WP_Query { . . . function &get_posts() { $this->parse_query(); // Huzzah! do_action_ref_array( 'pre_get_posts', array( &$this ) ); . . .
  50. 50. 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' );
  51. 51. Still with us? Good, ‘cause here’s where things get complicated.
  52. 52. '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.
  53. 53. What if I just want it on the main query?
  54. 54. $wp_the_query makes a triumphant return.
  55. 55. 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' );
  56. 56. 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( ).
  57. 57. query_posts( 'author=-5' ); while ( have_posts( ) ) : the_post( ); endwhile; wp_reset_query( );
  58. 58. query_posts( 'author=-5' ); while ( have_posts( ) ) : the_post( ); endwhile; wp_reset_query( );
  59. 59. function query_posts( $query ) { // Break the reference to $wp_the_query unset( $wp_query ); $wp_query =& new WP_Query( $query ); return $wp_query; }
  60. 60. query_posts( 'author=-5' ); while ( have_posts( ) ) : the_post( ); endwhile; wp_reset_query( );
  61. 61. 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( ); }
  62. 62. 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.
  63. 63. 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.
  64. 64. What about page templates?
  65. 65. /* Template: My Template */ query_posts( $query_string . '&author=-5&posts_per_page=25' ); get_header( ); while ( have_posts( ) ) : the_post( ); endwhile;
  66. 66. 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' );
  67. 67. 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( ).
  68. 68. And Finally 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( )
  69. 69. Thanks! Questions? @nacin

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