iOS - Core Data - Delete records using relationships and fetch request

Question

Overview:

I have an iOS project in which I am using core data

• I have an Employees entity and a Department entity.
• 1 department can contain many employees
• So the entity Department has a "to many" relationship with the entity Employees, the relationship is called employees and the reverse relationship is called whichDepartment

Aim-1:

I want to delete all the employees in a specific department

Questions:

a) is the following correct, or would it cause mutation or some problems ?

b) is this is the correct way to do it ?

Pls Note - removeEmployees is a method that was auto generated while creating the subclasses of the entities

- (void) deleteAllEmployeesForDepartment: (Department*) requestedDepartment
{
    [requestedDepartment removeEmployees:requestedDepartment.employees];
}

Aim-2:

• I want to delete the employees based on some condition
• I am deleting objects inside a fast enumeration loop for the fetched records

Questions:

c) Is the following correct, or would it cause some mutation ?

d) Is it like modifying the object in fast enumeration ?

e) Is there a better way to do it ?

Pls Note - removeEmployees is a method that was auto generated while creating the subclasses of the entities

- (void) deleteAllType1EmployeesWithDepartment: (Department*) requestedDepartment
{ 
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"type == %i AND whichDepartment ==%i", 1, requestedDepartment.departmentID];

    NSError *error;
    NSArray *listOfEmployeesToBeDeleted = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];    

    for(Employees *currentEmployee in listOfEmployeesToBeDeleted)
    {       
       [self.managedObjectContext deleteObject:currentEmployee];
    }
}

Answer 1

Firstly, in your deleteAllEmployeesForDepartment: this is fine to remove objects that way.

If in addition you want to delete the Employees objects from Core Data then you should add another rule, a delete rule set to cascade, meaning that when an Employee "losses" a Department (the relationship is broken, either by the Department being deleted, or the Department removing the Employee), it (the Employee) also is deleted.

Your second question is a little more interesting.

What I recommend is adding another method directly to the Department subclass of NSManagedObeject, you could call it - clearEmployeesOfType: passing a type number.

Since your Department has a reference to an NSSet of Employees via the to-many relationship you could use that NSSet and it's filteredSetUsingPredicate: method to filter out the ones you want.

The returning set can be used to message the removeEmployees: method on your Department, a little like the following (warning, code not tested).

- (void) clearEmployeesOfType:(NSUInteger)type
{ 
    NSPredicate * predicate = [NSPredicate predicateWithFormat:@"SELF.type == %d", type];
    NSSet * firedEmployees = [self.employees filteredSetUsingPredicate:predicate];

    [self removeEmployees:firedEmployees];
}

I would recommend this solution rather then loading objects and removing them one by one, whenever you can, rely on relationships and delete rules in Core Data.