get_adjacent_post()

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get_adjacent_post( bool $in_same_term = falsearray|string $excluded_terms = bool $previous = truestring $taxonomy = ‘category’ )

Retrieves the adjacent post.

Description 

Can either be next or previous post.

Parameters 

$in_same_term

(bool) (Optional) Whether post should be in a same taxonomy term.

Default value: false

$excluded_terms

(array|string) (Optional) Array or comma-separated list of excluded term IDs.

Default value: ”

$previous

(bool) (Optional) Whether to retrieve previous post. Default true

Default value: true

$taxonomy

(string) (Optional) Taxonomy, if $in_same_term is true.

Default value: ‘category’


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Return 

(null|string|WP_Post) Post object if successful. Null if global $post is not set. Empty string if no corresponding post exists.


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Source 

File: wp-includes/link-template.php

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function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
    global $wpdb;
 
    $post = get_post();
    if ( ! $post || ! taxonomy_exists( $taxonomy ) ) {
        return null;
    }
 
    $current_post_date = $post->post_date;
 
    $join     = '';
    $where    = '';
    $adjacent = $previous ? 'previous' : 'next';
 
    if ( ! empty( $excluded_terms ) && ! is_array( $excluded_terms ) ) {
        // Back-compat, $excluded_terms used to be $excluded_categories with IDs separated by " and ".
        if ( false !== strpos( $excluded_terms, ' and ' ) ) {
            _deprecated_argument(
                __FUNCTION__,
                '3.3.0',
                sprintf(
                    /* translators: %s: The word 'and'. */
                    __( 'Use commas instead of %s to separate excluded terms.' ),
                    "'and'"
                )
            );
            $excluded_terms = explode( ' and ', $excluded_terms );
        } else {
            $excluded_terms = explode( ',', $excluded_terms );
        }
 
        $excluded_terms = array_map( 'intval', $excluded_terms );
    }
 
    /**
     * Filters the IDs of terms excluded from adjacent post queries.
     *
     * The dynamic portion of the hook name, `$adjacent`, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 4.4.0
     *
     * @param array $excluded_terms Array of excluded term IDs.
     */
    $excluded_terms = apply_filters( "get_{$adjacent}_post_excluded_terms", $excluded_terms );
 
    if ( $in_same_term || ! empty( $excluded_terms ) ) {
        if ( $in_same_term ) {
            $join  .= " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";
            $where .= $wpdb->prepare( 'AND tt.taxonomy = %s', $taxonomy );
 
            if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) ) {
                return '';
            }
            $term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );
 
            // Remove any exclusions from the term array to include.
            $term_array = array_diff( $term_array, (array) $excluded_terms );
            $term_array = array_map( 'intval', $term_array );
 
            if ( ! $term_array || is_wp_error( $term_array ) ) {
                return '';
            }
 
            $where .= ' AND tt.term_id IN (' . implode( ',', $term_array ) . ')';
        }
 
        if ( ! empty( $excluded_terms ) ) {
            $where .= " AND p.ID NOT IN ( SELECT tr.object_id FROM $wpdb->term_relationships tr LEFT JOIN $wpdb->term_taxonomy tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id) WHERE tt.term_id IN (" . implode( ',', array_map( 'intval', $excluded_terms ) ) . ') )';
        }
    }
 
    // 'post_status' clause depends on the current user.
    if ( is_user_logged_in() ) {
        $user_id = get_current_user_id();
 
        $post_type_object = get_post_type_object( $post->post_type );
        if ( empty( $post_type_object ) ) {
            $post_type_cap    = $post->post_type;
            $read_private_cap = 'read_private_' . $post_type_cap . 's';
        } else {
            $read_private_cap = $post_type_object->cap->read_private_posts;
        }
 
        /*
         * Results should include private posts belonging to the current user, or private posts where the
         * current user has the 'read_private_posts' cap.
         */
        $private_states = get_post_stati( array( 'private' => true ) );
        $where         .= " AND ( p.post_status = 'publish'";
        foreach ( (array) $private_states as $state ) {
            if ( current_user_can( $read_private_cap ) ) {
                $where .= $wpdb->prepare( ' OR p.post_status = %s', $state );
            } else {
                $where .= $wpdb->prepare( ' OR (p.post_author = %d AND p.post_status = %s)', $user_id, $state );
            }
        }
        $where .= ' )';
    } else {
        $where .= " AND p.post_status = 'publish'";
    }
 
    $op    = $previous ? '<' : '>';
    $order = $previous ? 'DESC' : 'ASC';
 
    /**
     * Filters the JOIN clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, `$adjacent`, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     * @since 4.4.0 Added the `$taxonomy` and `$post` parameters.
     *
     * @param string  $join           The JOIN clause in the SQL.
     * @param bool    $in_same_term   Whether post should be in a same taxonomy term.
     * @param array   $excluded_terms Array of excluded term IDs.
     * @param string  $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
     * @param WP_Post $post           WP_Post object.
     */
    $join = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms, $taxonomy, $post );
 
    /**
     * Filters the WHERE clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, `$adjacent`, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     * @since 4.4.0 Added the `$taxonomy` and `$post` parameters.
     *
     * @param string  $where          The `WHERE` clause in the SQL.
     * @param bool    $in_same_term   Whether post should be in a same taxonomy term.
     * @param array   $excluded_terms Array of excluded term IDs.
     * @param string  $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
     * @param WP_Post $post           WP_Post object.
     */
    $where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s $where", $current_post_date, $post->post_type ), $in_same_term, $excluded_terms, $taxonomy, $post );
 
    /**
     * Filters the ORDER BY clause in the SQL for an adjacent post query.
     *
     * The dynamic portion of the hook name, `$adjacent`, refers to the type
     * of adjacency, 'next' or 'previous'.
     *
     * @since 2.5.0
     * @since 4.4.0 Added the `$post` parameter.
     * @since 4.9.0 Added the `$order` parameter.
     *
     * @param string $order_by The `ORDER BY` clause in the SQL.
     * @param WP_Post $post    WP_Post object.
     * @param string  $order   Sort order. 'DESC' for previous post, 'ASC' for next.
     */
    $sort = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1", $post, $order );
 
    $query     = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
    $query_key = 'adjacent_post_' . md5( $query );
    $result    = wp_cache_get( $query_key, 'counts' );
    if ( false !== $result ) {
        if ( $result ) {
            $result = get_post( $result );
        }
        return $result;
    }
 
    $result = $wpdb->get_var( $query );
    if ( null === $result ) {
        $result = '';
    }
 
    wp_cache_set( $query_key, $result, 'counts' );
 
    if ( $result ) {
        $result = get_post( $result );
    }
 
    return $result;
}


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Changelog 

Changelog
VersionDescription
2.5.0Introduced.

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