JetEngine Relations: update items programmatically, register a relation programmatically

register-relation.php

<?php

add_filter( 'jet-engine/relations/raw-relations', function( $relations ) {
	$relations[999] = array(
		'name'                => 'User to post',
		'parent_rel'          => null,
		'type'                => 'many_to_many',
		'is_legacy'           => false,
		'id'                  => 999,
		
		'parent_object'       => 'mix::users',
		'child_object'        => 'posts::post',
		
		'parent_control'      => true,
  		'child_control'       => true,
		
		'parent_manager'      => true,
  		'child_manager'       => true,
		
		'parent_allow_delete' => true,
  		'child_allow_delete'  => true,
		
		'rest_get_enabled'    => true,
  		'rest_post_enabled'   => true,
		'rest_get_access'     => 'manage_options',
  		'rest_post_access'    => 'manage_options',
		
		'labels'              => array (
		  'name'                        => 'User to post',
		  'parent_page_control_title'   => 'Posts, connected to the user',
		  'parent_page_control_connect' => 'Connect posts',
		  'parent_page_control_select'  => 'Select posts',
		  'child_page_control_title'    => 'Users, connected to the post',
		  'child_page_control_connect'  => 'Connect users',
		  'child_page_control_select'   => 'Select users',
		  'child_page_control_create'   => 'Create user',
		  'parent_page_control_create'  => 'Create post',
		),
	);
	return $relations;
} );

//relation with meta fields and CCT-specific controls
add_filter( 'jet-engine/relations/raw-relations', function( $relations ) {
	$relations[1000] = array (
		'id'                  => '1000',
		'db_table'            => true,
		'parent_control'      => true,
		'child_control'       => true,
		'parent_manager'      => true,
		'child_manager'       => true,
		'parent_allow_delete' => true,
		'child_allow_delete'  => true,
		'is_legacy'           => false,
		'rest_get_enabled'    => true,
		'rest_post_enabled'   => true,
		'name'                => '',
		'parent_object'       => 'cct::blum_cct',
		'child_object'        => 'posts::blum-cpt',
		'parent_rel'          => NULL,
		'type'                => 'many_to_many',
		'legacy_id'           => '',
		'rest_get_access'     => 'manage_options',
		'rest_post_access'    => 'manage_options',
		'cct'                 => 
		array (
			'cct::blum_cct' => 
			array (
				'title_field'   => 'f1',
				'create_fields' => 
				array (
					'f2',
					'rep_test',
					'num3',
					'num1',
				),
			),
		),
		'labels'              => array (
			'name'                        => 'cct to cpt',
			'parent_page_control_title'   => 'Parent Object: label of relation box',
			'parent_page_control_connect' => 'Parent Object: label of connect button',
			'parent_page_control_select'  => 'Parent Object: label of select item control',
			'parent_page_control_create'  => 'Parent Object: label of create button',
			'child_page_control_title'    => 'Child Object: label of relation box',
			'child_page_control_connect'  => 'Child Object: label of connect button',
			'child_page_control_select'   => 'Child Object: label of select item control',
			'child_page_control_create'   => 'Child Object: label of create button',
		),
		'meta_fields'         => 
		array (
			array (
				'title'       => 't1',
				'name'        => 't1',
				'object_type' => 'field',
				'width'       => '100%',
				'options'     => array (),
				'type'        => 'text',
			),
			array (
				'title'       => 't2',
				'name'        => 't2',
				'object_type' => 'field',
				'width'       => '100%',
				'options'     => array (),
				'type'        => 'text',
			),
		),
	);
	return $relations;
} );

update-related.php

<?php

//call these methods not earlier than on 'init' action with the priority of 12

//get relation by ID
$relation = jet_engine()->relations->get_active_relations( $rel_id );

//set relation update context - 'parent' or 'child'
$relation->set_update_context( 'parent' );

//update relation
$relation->update( $parent_object_id, $child_object_id );

For anyone who's wondering how to create the relationship permanently (i.e. saved in the database), rather than inserting via a hook, I thought it's worth posting this as an example:

$rel_props = [
	'name' => 'Parent to child post',
	'parent_object' => 'posts::parent',
	'child_object' => 'posts::child',
	'type' => 'one_to_many',
	'child_control' => false,
	'parent_control' => true,
	'labels' => [
		'name' => 'Parent to child post',
		'parent_page_control_title' => 'Connected child posts',
		'parent_page_control_connect' => 'Connect a child post',
		'parent_page_control_select' => 'Select a child post',
	],
	'meta_fields' => [],
	'rest_post_access' => 'manage_options'
];
jet_engine()->relations->data->set_request($rel_props);
$rel_id = jet_engine()->relations->data->create_item(false);

// If you need to use the new relation immediately, you need to instantiate it
// as it won't be loaded into the manager until the next request
if (! class_exists('\Jet_Engine\Relations\Relation')) {
	// JetEngine won't load the class file if there are no relations defined
	require_once jet_engine()->relations->component_path('relation.php');
}

$relation = new \Jet_Engine\Relations\Relation($rel_id, $rel_props);
jet_engine()->relations->add_active_relation($rel_id, $relation);