In this example, we have created a custom shortcode you can use to display pagination links like “Next: Next Sibling Post Title” and “Previous: Previous Sibling Post Title” to navigate among sibling child posts in a one-to-many post relationship. You can place this shortcode in the child post template to generate the appropriate link programmatically and display it only if necessary.

// 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' );

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>";
  }