public function register_meta_fields() {
			// Add meta fields to our post type.
			foreach ( INVP::keys_and_types( true ) as $key_arr ) {

				if ( empty( $key_arr['name'] ) ) {
					continue;
				}
				$key = apply_filters( 'invp_prefix_meta_key', $key_arr['name'] );
				register_post_meta(
					INVP::POST_TYPE,
					$key,
					array(
						'show_in_rest' => true,
						'single'       => 'options_array' !== $key_arr['name'],
						'type'         => $key_arr['type'] ?? 'string',
					)
				);
			}

			// Add a couple fields that are used on media attachments.
			$attachment_keys   = array();
			$attachment_keys[] = array(
				'name' => 'hash',
				'type' => 'string',
			);
			$attachment_keys[] = array(
				'name' => 'photo_number',
				'type' => 'integer',
			);
			$attachment_keys[] = array(
				'name' => 'vin',
				'type' => 'string',
			);

			// Add meta fields to attachments.
			foreach ( $attachment_keys as $key_arr ) {
				$key = apply_filters( 'invp_prefix_meta_key', $key_arr['name'] );
				register_post_meta(
					'attachment',
					$key,
					array(
						'sanitize_callback' => 'sanitize_text_field',
						'show_in_rest'      => true,
						'single'            => true,
						'type'              => $key_arr['type'],
					)
				);
			}
		}