Shows how to set up a field that holds a literal list of relations for the Participants Database Multi-Relational plugin

pdb_literal_relations_field.php

<?php

/**
 * Plugin Name: PDb Multi-Relational Literal Relations Field
 * Description: Maintains a field in the database with a list of the record's relations
 * Version: 0.1
 */

/*
 * this is designed to maintain a field in the record that can be used for simple filtering
 * 
 * without this plugin and the field it maintains, it is not possible to filter 
 * based on a record's relations, for example to show a list of all the students 
 * for a particular teacher.
 * 
 * to use this plugin you must have a text type field set up to recieve the names of the 
 * related records. 
 * The name of that field is assigned to the $fieldname variable
 */

class pdb_literal_relations_field {
  
  /**
   * @var string name of the literal relations field
   */
  private $fieldname = 'teachers';
  
  /**
   * @var string name of the relation to mirror
   */
  private $typename = 'teacher';
  
  /**
   * 
   */
  public function __construct()
  {
    add_action( 'pdb-after_submit_update', [ $this, 'update_literal_field'] );
  }
  
  /**
   * updates the literal relations field in the record
   * 
   * @param array $record the updated record data
   */
  public function update_literal_field( $record )
  {
    /* this class must be available by having the 
     * PDb Multi-Relational Utility Class plugin activated
     * https://gist.github.com/xnau/fb9cc360c58eba270c456dcdf3c6cb4c
     */
    if ( !class_exists( 'pdbmrdb_record' ) )
    {
      return;
    }
    
    $record_relations = new pdbmrdb_record( $record['id'] );
    
    $relations_list = $record_relations->get_relations( $this->typename );
    
    $relations_titles = [];
    
    foreach( $relations_list as $id => $title )
    {
      $relations_titles[] = $title;
    }
    
    $relations_string = implode( ', ', $relations_titles );
    
    Participants_Db::write_participant( [ $this->fieldname => $relations_string ], $record['id']);
  }
  
}

new pdb_literal_relations_field();

This is meant to be installed as a plugin: How to Install a WordPress Plugin from a Gist

You must also install and activate this plugin: PDb Multi-Relational Utility Class