text: ALTER TABLE `Post` ADD COLUMN `authorId` char(30) CHARACTER SET utf8;
  UPDATE `Post`, `_PostToUser` SET `Post`.`authorId` = `_PostToUser`.B where `_PostToUser`.A = `Post`.`id`;
  ALTER TABLE `Post` ADD FOREIGN KEY (`authorId`) REFERENCES `User`(`id`);
  DROP TABLE `_PostToUser`;