* @return array
	 */
	public function get_item_schema() {

		if ( isset( $this->schema ) ) {
			// Additional fields are not cached in the schema, @see https://core.trac.wordpress.org/ticket/47871#comment:5.
			return $this->add_additional_fields_schema( $this->schema );
		}

		$schema = $this->get_item_schema_base();

		// Add custom fields registered via `register_meta()`.
		$schema = $this->add_meta_fields_schema( $schema );

		// Allow the schema to be filtered.
		$schema = $this->filter_item_schema( $schema );

		/**
		 * Set `$this->schema` here so that we can exclude additional fields
		 * already covered by the base, filtered, schema.
		 *
		 * Additional fields are added through the call to add_additional_fields_schema() below,