public function decrease_usage_count( &$coupon, $used_by = '' ) {
		global $wpdb;
		$new_count = $this->update_usage_count_meta( $coupon, 'decrease' );
		if ( $used_by ) {
			/**
			 * We're doing this the long way because `delete_post_meta( $id, $key, $value )` deletes.
			 * all instances where the key and value match, and we only want to delete one.
			 */
			$meta_id = $wpdb->get_var(
				$wpdb->prepare(
					"SELECT meta_id FROM $wpdb->postmeta WHERE meta_key = '_used_by' AND meta_value = %s AND post_id = %d LIMIT 1;",
					$used_by,
					$coupon->get_id()
				)
			);
			if ( $meta_id ) {
				delete_metadata_by_mid( 'post', $meta_id );
				$coupon->set_used_by( (array) get_post_meta( $coupon->get_id(), '_used_by' ) );
			}
		}

		do_action( 'woocommerce_decrease_coupon_usage_count', $coupon, $new_count, $used_by );

		return $new_count;
	}