use of pre_get_posts()

Introducing pre_get_posts :-
------------------------------
class WP_Query{
    function &get_posts(){
        $this->parse_query();
		//pre_get_posts()
		do_action_ref_array( 'pre_get_posts', array( &$this ) );
    }
}

This is used something like this
function nacin_alter_home( $query ){
    if( $query->is_home() ){
	    $query->set( 'author', '-5' );
	}
}
add_action( 'pre_get_posts', 'nacin_alter_home' );

'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


so, What Can be done if we want it just on the main query
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' );

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 we use query_posts()

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


What query_posts( 'author=-5' ); does ?

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


What wp_reset_query() does
function wp_reset_query(){
    //Restore reference to $wp_the_query
	unset( $wp_query );
	$wp_query = &$wp_the_query;
	//Reset the goals, 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

in wordpress 3.3
Rather than:
$wp_the_query === $other_query_object
we can call:
$other_query_object->is_main_query()

is_main_query(), the function, will act on $wp_query, like any other conditional tag

On Page 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;
	$query->set( 'author', '-5' );
	$query->set( 'posts_per_page', '25' );	
}
add_action( 'pre_get_posts', 'nacin_my_template' );

Five most recent posts by author with ID 15 in the toast
<?php
$foo = new WP_Query( array( 'posts_per_page' => 5, 'author' => 15, 'category_name' => 'toast' ) )
?>
using tax_query :-
<?php
$foo = new WP_Query( array( 'post_per_page' => 5, 'author' => 15, 'tax_query' => array( array( 'taxonomy' => 'category', 'field' => 'slug', 'terms' => 'toast' ) ) ) );
?>

Querying two categories:-
<?php
$foo = new WP_Query( array( 'post_per_page' => 5, 'author' => 15, 'tax_query' => array( array(
    'taxonomy' => 'category',
	'field' => 'slug',
	'terms' => array( 'toast', 'breakfast' ),
	'operator' => 'AND'
) ) ) )
?>

Querying a category and a tag:-
<?php
$foo = new WP_Query( 'posts_per_page' => 5, 'author' => 15, 'tax_query' => (
    'relation' => 'AND',
	array(
	    'taxonomy' => 'category',
		'field' => 'slug',
		'terms' => 'toast',
	),
	array(
	    'taxonomy' => 'post_tag',
		'field' => 'slug',
		'terms' => 'wheat'
	)
) )
?>
Querying a category and a tag, excluding subcategories:
<?php
    $foo = new WP_Query( array(
	    'posts_per_page' => 5,
		'author' => 15,
		'tax_query' => array(
		    'relation' => 'AND',
			array(
			    'taxonomy' => 'category',
				'field' => 'slug',
				'terms' => 'toast',
				'include_childdren' => false
			),
			array(
			    'taxonomy' => 'post_tag',
				'field' => 'slug',
				'terms' => 'wheat'
			)
		)
	) )
?>

Using the meta_query:--
->Five posts with feature images
<?php
$foo = new WP_Query( array( 'posts_per_page' => 5, 'meta_query' => array( array( 'key' => '_thumbnail_id' ) ) ) );
?>
->Five posts using the same thumbnail ID 100
<?php
    'posts_per_page' => 5,
	'meta_query' => array( array( 'key' => '_thumbnail_id', 'value' => 100, 'type' => 'NUMERIC' ) )
?>

Using the meta query : multiple queries
->Five posts with featured images AND flagged as 'featured' post
<?php
    $foo = new WP_Query( array( 'posts_per_page' => 5, 'meta_query' => array( array( 'key' => '_thumbnail_id' ), array( 'key' => '_featured' ) ) ) );
?>

->Five posts with featured images OR flagged as 'featured' posts
<?php
    $foo = new WP_Query( array( 'posts_per_page' => 5, 'meta_query' => array( 'relation' => 'OR', array( 'key' => '_thumbnail_id' ), array( 'key' => '_featured' ) ) ) );
?>

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 fleexible hook Just use it properly

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

comments:-

It's not possible to alter the loop on a template by checking (is_page_template()) inside pre_gest_posts. if you run multiple queries on the template it fails.