Errno 121, duplicate key on write or update?

This is likely because you have named at least one constraint with the same identifier as a column:

/* You already have a column named `restaurant` in this table, 
   but are naming the FK CONSTRAINT `restaurant` also... */
CONSTRAINT `restaurant`
    FOREIGN KEY (`restaurant` )
    REFERENCES `mydb`.`restaurants` (`id` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)

Should use a different identifier for the constraint like fk_restaurant as in :

CONSTRAINT `fk_restaurant`
    FOREIGN KEY (`restaurant` )
    REFERENCES `mydb`.`restaurants` (`id` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)

And same thing in the food table:

  /* Name it fk_food */
  CONSTRAINT `fk_food`
    FOREIGN KEY (`food` )
    REFERENCES `mydb`.`food` (`id` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  /* Name it fk_restaurant */
  CONSTRAINT `fk_restaurant`
    FOREIGN KEY (`restaurant` )
    REFERENCES `mydb`.`restaurants` (`id` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)

Those are the only three I see, but there could be others I missed.