Question

How to correctly configure CoreData datamodel and NSManagedObjects for a to-many relationship?
I feel i have misconfigured the xcdatamodeld file, but I can’t find what’s wrong.
I generated a new single view project with CoreData selected. The following code is a simple as I can explain the problem.

Parent *parent = [NSEntityDescription insertNewObjectForEntityForName:@"Parent"      inManagedObjectContext:[self managedObjectContext]];

Child *child = [NSEntityDescription insertNewObjectForEntityForName:@"Child" inManagedObjectContext:[self managedObjectContext]];    

child.parent = parent;

When i call

[parent.children count];

Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘-[Child count]: unrecognized selector sent to instance 0x7450370’

When I call

NSMutableSet *children = [parent mutableSetValueForKey:@"children"];

‘NSManagedObjects of entity ‘Parent’ do not support -mutableSetValueForKey: for the property ‘children”

Also, there doesn’t seem to be any generated add/remove methods being generated to modify the children collection such as

- (void)addChildObject:(Child *)value;
- (void)removeChildObject:(Child *)value;

xcdatamodeld

Entities

• Child
  relationship: parent
  destination: Parent
  inverse: children

• Parent
  relationship: children
  destination: Child
  inverse: parent

• Configurations (Default)
  entity: Child
  class: Child

  entity: Parent
  class: Parent

Parent.h

@interface Parent : NSManagedObject
  @property (nonatomic, retain) NSSet *children;
@end

Parent.m

@implementation Parent
  @dynamic children;
@end

Child.h

@interface Child : NSManagedObject
  @property (nonatomic, retain) NSManagedObject *parent;
@end

Child.m

@implementation Child
  @dynamic parent;
@end

Answer 1

You should use “Editor -> Create NSManagedObject Subclass…” in Xcode to create the managed object subclass files. This ensures that you have the correct accessor functions. Another advantage is that Xcode then knows the accessor functions and does autocompletion.

Make also sure that the children relationship is defined as “To-Many Relationship” in the Core Data Inspector.