Two tables with bidirectional relationships

[Magnus Müller]

Hello,

i am new two MySQL and i am trying to setup my first project which is based on an existing DB (non MySQL).

Now i face a problem.

I have two tables: Departments and Persons. Both tables have unique primary keys.

Each department has a person as head of the department. 1:N This relationship formed no problem.

The second relationship says that every person can belong to a department. But this is not mandatory. So this relationship is 1:N but in the opposite direction.

When i try to send my model to the DB i get a nice little error message (as many i suppose):

Executing SQL script in server
ERROR: Error 6125: Failed to add the foreign key constraint. Missing unique key for constraint 'fk_tbl_persons_tbl_departments1' in the referenced table 'tbl_departments'
SQL Code:
ALTER TABLE `mydb`.`tbl_persons`
ADD CONSTRAINT `fk_tbl_persons_tbl_addresses1`
FOREIGN KEY (`per_adr_index`)
REFERENCES `mydb`.`tbl_addresses` (`adr_index`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
ADD CONSTRAINT `fk_tbl_persons_tbl_departments1`
FOREIGN KEY (`per_dep_index`)
REFERENCES `mydb`.`tbl_departments` (`dep_index`)
ON DELETE NO ACTION
ON UPDATE NO ACTION

I am not sure if i am doing something here which is not possible or what the error might be or how i have to deal with this situation.