Problem: I have a one-to-many post relationship between two custom post types. In the template for the child post type, I would like to display "Next" and "Previous" links to paginate between the sibling posts.
Solution: Use a custom shortcode like this example that implements the post relationships API to create pagination links:
// https://toolset.com/forums/topic/next-and-previous-sibling-posts-in-a-one-to-many-post-relationship/
// get a link to the next or previous sibling post in child post content template
// example: [tssupp-next-prev-child current="[wpv-post-id]" step="1" relslug="book-chapter"][/tssupp-next-prev-child]
// current: current post id, step: 1 for next or -1 for previous, relslug: slug of the post relationship
function tssupp_next_prev_child($atts) {
$a = shortcode_atts( array(
'current' => 0,
'step' => '1',
'relslug' => '',
), $atts );
$link = '';
// get all child posts
$relationship_slug = $a['relslug'];
$current_child_id = $a['current'];
$parent_id = toolset_get_related_post( $current_child_id, $relationship_slug );
$sibling_args = array(
'query_by_role'=>'parent',
'limit'=>1000,
'role_to_return'=>'child',
'orderby'=>'title'
);
$siblings = toolset_get_related_posts( $parent_id, $relationship_slug, $sibling_args );
// loop over child posts and get index of the current post
foreach($siblings as $i=>$sibling) {
if( $sibling == $current_child_id ) {
break;
}
}
// increment or decrement index for next or previous sibling
$i += $a['step'];
// create link to next/previous sibling
if(isset($siblings[$i])){
$perm = get_the_permalink( $siblings[$i] );
$title = get_the_title( $siblings[$i] );
$link .= $a['step']=='1' ? "Next: " : "Previous: ";
$link .= "<a href='" . $perm . "'>" . $title . "</a>";
}
// output the link, or empty string if not set
return $link;
}
add_shortcode( 'tssupp-next-prev-child', 'tssupp_next_prev_child' );
You'll use the shortcode like this to generate a "Next" link (if necessary):
[tssupp-next-prev-child step="1" relslug="book-chapter" current="[wpv-post-id]"][/tssupp-next-prev-child]
Replace book-chapter with the slug of the post relationship.
You'll change the step value to -1 to generate a "Previous" link (if necessary):
[tssupp-next-prev-child step="-1" relslug="book-chapter" current="[wpv-post-id]"][/tssupp-next-prev-child]
Again, replace book-chapter with the slug of the post relationship.
If you want to modify the links, you should only need to edit this part of the code:
// create link to next/previous sibling
if(isset($siblings[$i])){
$perm = get_the_permalink( $siblings[$i] );
$title = get_the_title( $siblings[$i] );
$link .= $a['step']=='1' ? "Next: " : "Previous: ";
$link .= "<a href='" . $perm . "'>" . $title . "</a>";
}
Relevant Documentation:
https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/#toolset_get_related_post
https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/#toolset_get_related_posts
