How to force UNIDIRECTIONAL to-many relationship to persist

Question

There is a problem with core data when a to-many relationship has no inverse. Changes made to the related property do not persist. This is a problem many of us have faced, a

Answer 1

This is a great question.

ButFirst thing first:
It clearly state in the documentation:

"Important: You must define many-to-many relationships in both directions—that is, you must specify two relationships, each being the inverse of the other. You can’t just define a to-many relationship in one direction and try to use it as a many-to-many. If you do, you will end up with referential integrity problems."

Never the less, Lets describe the issue (resulting database)
When defining a to-many relationship, the resulting database does not add an additional table to map the relationship.
It only sets a property on the entity at one end of the to-many relationship equal to the last item that referenced it.

Example:

Model:
Entity: Department
Relationships: NONE
Properties: name (string)

Entity: Employee
Relationships: departments (to-many,no-action)
Properties: name

Resulting Database:
ZDEPARTMENT:
Z_PK
Z_ENT
Z_OPT
Z2DEPARTMENTS (int)
ZNAME

ZEMPLOYEE:
Z_PK
Z_ENT
Z_OPT
ZNAME

This structure will obviously result in data inconsistency.

The solution will be to hold an entity: DepartmentEmployee modeling the to-many relationship in both directions but one of them would be unidirectional (Department -> DepartmentEmployee):

DepartmentEmployee:
Relationships: department (to-one,no-action), employee (to-one,nullify)

and you will have to maintain the table upon deletion of a department object.

Hope this made some sense :)