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

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.