核心数据删除规则和多对多关系

级联规则将自动删除目标处的对象。因此,如果您删除一个部门,那么不管员工所在的部门数是多少,他们都将被删除。

听起来你想要的行为更加微妙,只删除那些“孤儿”雇员——也就是那些没有部门的雇员。删除部门时,找到这些部门的一个好方法是执行如下操作:

NSManagedObject *doomedDepartment = // get the department to be deleted

NSSet *employees = [doomedDepartment valueForKey:@"employees"];
NSSet *orphanedEmployees = [employees filteredSetUsingPredicate:[NSPredicate predicateWithFormat:@"departments.@count == 1"]];
for (NSManagedObject *orphanedEmployee in orphanedEmployees) {
    [managedObjectContext deleteObject:orphanedEmployee];
}    

[managedObjectContext deleteObject:doomedDepartment];

谢谢,亚历克斯。我可能会那样做。 与此同时,我发现了一种不同的方法:

1.)注册变更通知:

    [[NSNotificationCenter defaultCenter] addObserver:self 
            selector:@selector(managedObjectContextDidChange:) 
            name:NSManagedObjectContextObjectsDidChangeNotification 
            object:managedObjectContext];

2.)当发生变化并且员工得到更新时。我检查该对象是否与部门有0个关系,然后将其删除:

- (void)managedObjectContextDidChange:(NSNotification *)notification {
    NSSet *updatedObjects = [[notification userInfo] objectForKey:NSUpdatedObjectsKey];

for(NSManagedObject *obj in updatedObjects){        
    // walk through updated objects -> check for employees
    // check if they still contain departments and if not delete them
    if([obj.entity.name isEqualToString:@"Employee"]){
        NSLog(@"Employee changed!");
        if([[(Employee*)obj Departments] count]==0){
            NSLog(@"No more relations -> Delete Employee");
            [managedObjectContext deleteObject:obj];
        }
    }
}}

这也很有效,但是如果你有几个不同的实体来观察这种行为,那么可能会变得更复杂。