game_customize_core_example.php

<?php
add_action( 'rest_api_init', 'rest_register_game_meta' );

function rest_register_game_meta() {

    //For generic fields that should never allow front-end updates
		$restArgsDisplay = array(
				'get_callback'    => 'rest_get_game_meta',
				'update_callback' => null,
				'schema'          => null,
		);

    //For generic fields that can be update in all cases.
    $restArgsEditGameMeta = array(
				'get_callback'    => 'rest_get_game_meta',
				'update_callback' => 'rest_update_game_meta',
				'schema'          => null,
		);

    register_rest_field( 'game', 'game_date',                 $restArgsDisplay );
    register_rest_field( 'game', 'game_location',             $restArgsDisplay );
    register_rest_field( 'game', 'game_center_score',         $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_center_assists',       $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_shootguard_score',     $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_shootguard_assists',   $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_pointguard_score',     $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_pointguard_assists',   $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_powerforward_score',   $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_powerforward_assists', $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_smallforward_score',   $restArgsEditGameMeta );
    register_rest_field( 'game', 'game_smallforward_assists', $restArgsEditGameMeta );

}

//Used in register_post_meta callbacks to retrieve meta data
function rest_get_game_meta( $object, $field_name, $request ) {
    return get_post_meta( $object[ 'id' ], $field_name );
}

//Ensure score / assist are sent in right format and resource before storing to meta field
function rest_update_game_meta( $value, $object, $field_name ) {
  //Validate basic format
  if ( ! is_string( $value ) ) {
      return new WP_Error( 'rest_count_invalid', esc_html__( 'The score/assist provided does not match format criteria', 'game' ), array( 'status' => 406 ) );;
  }

  //Validate access level
  if ( current_user_can('volunteer') ) {
		//Additional logic to confirm volunteer is assigned to this game, otherwise
		return new WP_Error( 'rest_access_invalid', esc_html__( 'Resource is not assigned to edit requested data', 'game' ), array( 'status' => 403 ) );

	}

  //Validate score / assist is not LBJ level
  $number = (int) abs( strip_tags( $value ) );
  if( $number < 0 || $number > 150 ) {
    return new WP_Error( 'rest_count_invalid', esc_html__( 'The count provided does not match format criteria', 'game' ), array( 'status' => 406 ) );
  }

  return update_post_meta( $object->ID, $field_name, $number );
}


//Default example update_post_meta that has only basic field validation
function rest_update_any_meta( $value, $object, $field_name ) {
    if ( ! $value || ! is_string( $value ) ) {
        return;
    }

    return update_post_meta( $object->ID, $field_name, strip_tags( $value ) );
}