Question

How do I handle the deletion of the following objects? What should my delete rules look like?

Here’s what my object graph looks like:

Boss

• Boss-Department has a many-to-many-relationship
• If a Boss is deleted,
  the Departments belonging to that Boss should NOT be deleted (but the
  Departments’ relationship to this Boss should be deleted)

Department

• Department-Employee has a many-to-many relationship
• Department-Boss has a many-to-many relationship
• If a Department
  is deleted, the Employees belonging to that Department should be
  deleted IF they don’t have a relationship to any other Department

Employee

• Employee-Department has a many-to-many relationship
• Employees will never be deleted directly (only through the deletion of a department). Oh happy life!

Answer 1

In this situation the Delete rules for your objects should be handled like this

Boss-Department -> Nullify

Department-Employee -> Deny

Department-Boss -> Nullify

Here is what Apple explains the Delete Rules as

Deny: If there is at least one object at the relationship destination,
then the source object cannot be deleted. For example, if you want to
remove a department, you must ensure that all the employees in that
department are first transferred elsewhere (or fired!) otherwise the
department cannot be deleted.

Nullify: Set the inverse relationship for
objects at the destination to null. For example, if you delete a
department, set the department for all the current members to null.
This only makes sense if the department relationship for an employee
is optional, or if you ensure that you set a new department for each
of the employees before the next save operation.

Cascade: Delete the
objects at the destination of the relationship. For example, if you
delete a department, fire all the employees in that department at the
same time.

No Action: Do nothing to the object at the destination of
the relationship. For example, if you delete a department, leave all
the employees as they are, even if they still believe they belong to
that department.

For more information check out the CoreData Programming Guide