Let\u2019s have a look at a typical day for Mr. WordPress–hereafter referred to as \u201cBig W.\u201d Usually, Big W gets requests from web visitors, figures out what they want, fetches the right information from the database, and then serves it up in the right theme template. The scene goes something like this:<\/p>\n
Web Visitor:<\/strong> I\u2019d like to see http:\/\/yourdomain.com\/good_page\/, please.<\/p>\n Big W:<\/strong> Sure, let me get that for you.<\/p>\n Big W steps away to:<\/p>\n Usually this is a smooth process Big W performs quickly. After serving up the data in the right template, he simply moves on to the next request.<\/p>\n\n But wait–trouble lurks around that template corner. Sometimes, developers add query_posts<\/a> calls or build a new WP_Query<\/a>\u00a0 in their templates. They don\u2019t mean any harm–they just want something special done with the data before it\u2019s served up in the template. Little do they know, they\u2019re causing headaches for Big W. Here\u2019s how that chapter unfolds:<\/p>\n Gasp! The theme developer waited until Big W got to the page template before specifying the page should only show posts containing a custom field named \u201ccolor\u201d with a value of \u201cblue!\u201d Let\u2019s see if we can get a word from Big W on his way to the new query.<\/p>\n Big W:<\/strong> You know, I realize some templates need a second query–maybe to feature a special section of featured posts–that sort of thing. But in this case, it\u2019s the default post we\u2019re looking at! Why couldn\u2019t they just tell me they only wanted blue posts before I ran the query? I mean–this is just rude!<\/p>\n\n A short time passes, and Big W returns with the new query results.<\/p>\n Big W:<\/strong> Okay, here are your new results. But you know what? I feel like you take me for granted, and don\u2019t appreciate the work I do for you. I didn\u2019t bother:<\/p>\n If that screws up your pagination or some widgets–it\u2019s not my problem! Good day to you!<\/p>\n We need a way to communicate with WordPress to ask for special results before the default query is run. Big W will not have to redo his work, and our site visitor gets their pages faster.<\/p>\n If some theme template of yours needs to alter the default query to get the right data, we need a way to let Big W know before he comes knocking on the template file\u2019s door. Using query_posts in the template file is like leaving a note on the door that says \u201cOh, I forgot to ask you to get a gallon of milk, too–please go back to the store.<\/em>\u201d So how do we put that note in Big W\u2019s pocket, instead of on the template file\u2019s front door? This is where pre_get_posts comes in.<\/p>\n The hook \u201cpre_get_posts\u201d is called before the default query is executed. We can assign a function to pre_get_posts, then, to provide Big W with any special instructions before running that default query. Here\u2019s how it works.<\/p>\n Great! Now we restrict our posts to only those with a custom field \u201ccolor\u201d whose value is \u201cblue.\u201d But wait–we just royally screwed up most of our site. The way we wrote this, every query anywhere on the site will use this new restriction. We didn\u2019t want that! We only wanted that restriction when the visitor asked for it.<\/p>\n We need to be more specific in our note to Big W, specifying this custom field restriction should only happen in certain situations. I\u2019ll cut to the chase and spit out the correct code here, with appropriate comments.<\/p>\n The following code will properly alter the default query, but only when the URL includes parameters to ask for a specific custom field and value. This code would work well in your theme’s functions.php file.<\/p>\n When your page needs loops in addition to the default, it\u2019s fine to ask Big W for a new query. After all–you aren\u2019t throwing away his original work, you\u2019re just asking for some additional data.<\/p>\n Example:<\/strong> Your page shows a list of posts tagged \u201cfeature\u201d at the top, as a navigational aid. Below this, the normal posts are to be displayed. In this case, you create a brand new query for the featured posts, use the results, and use wp_reset_postdata<\/a> to get globals back for the default query results.<\/p>\n\n\n
Somebody poisoned the waterhole<\/h2>\n
\n
\nquery_posts( array( 'meta_key' => 'color', 'meta_value' => 'blue' );<\/code><\/li>\nWhere are your manners?<\/h2>\n
Using query_posts incorrectly has more consequences<\/h2>\n
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How to improve our working relationship with Big W.<\/h2>\n
Enter the powerful and dangerous pre_get_posts<\/h2>\n
\/\/ our function that alters the default query
\nfunction note_to_bigw( $query ) {
\n\/\/ make changes to the default query parameters
\n$query->set( 'meta_key', \u2018color\u2019 );
\n$query->set( 'meta_value', \u2018blue\u2019 );
\n}
\n\/\/ we have to tell pre_get_posts to leave the note for Big W
\nadd_action( \u2018pre_get_posts\u2019, \u2018note_to_bigw\u2019 );<\/code>
\n<\/p>\nOne size does not fit all<\/h2>\n
Be careful what you wish for<\/h2>\n
function note_to_bigw( $query ) {
\n\/\/ Do not affect queries for admin pages
\nif( $query->is_admin == 1 ) {
\nreturn;
\n}
\n\/\/ stop if wp is working on anything but the main query
\nif( !$query->is_main_query() ) {
\nreturn;
\n}
\n\/\/ stop if we are not working on an archive view
\nif( !$query->is_archive == 1 ) {
\nreturn;
\n}
\n\/\/ retrieve field name \/ value from URL if exist
\n$custom_field = ( $_GET['field'] ) ? stripslashes( $_GET['field'] ) : '';
\n$custom_value = ( $_GET['value'] ) ? stripslashes( $_GET['value'] ) : '';
\nif( $custom_field ) {
\n\/\/ add meta key requirement -- we\u2019ll return all posts with some \u2018color\u2019
\n$query->set( 'meta_key', $custom_field );
\nif( $custom_value ) {
\n\/\/ build meta value requirement -- we\u2019ll return only blue
\n$query->set( 'meta_value', $custom_value );
\n}
\n}
\n\/\/ we have to tell pre_get_posts to leave the note for Big W
\nadd_action( \u2018pre_get_posts\u2019, \u2018note_to_bigw\u2019 );<\/code><\/p>\nSometimes a new WP_Query is okay<\/h2>\n
Credits<\/h2>\n