One Quickie

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Manipulating a to-many relationship (Core Data->General)

Use mutableSetValueForKey: This returns a proxy that mutates the relationship and does KVO notifications. Think of the name as "[NS]MutableSet" "valueForKey" rather than "mutableSetValue" "forKey", because it returns a mutable set that you manipulate

NSMutableSet *employees;
employees = [department mutableSetValueForKey: @"employees"];
[employees addObject: newEmployee];
[employees removeObject: sackedEmployee];