Correct Implementation of Parent/Child Nsmanagedobjectcontext

Correct implementation of parent/child NSManagedObjectContext

The parent/child MOC model is a really powerful feature of Core Data. It simplifies incredibly the age-old concurrency problem we used to have to deal with. However, as you've stated, concurrency is not your issue. To answer your questions:

1. Traditionally, you use the NSMainQueueConcurrencyType for the NSManagedObjectContext associated with the main thread, and NSPrivateQueueConcurrencyTypes for child contexts. The child context does not need to match its parent. The NSConfinementConcurrencyType is what all NSManagedObjectContexts get defaulted to if you don't specify a type. It's basically the "I will managed my own threads for Core Data" type.
2. Without seeing your code, my assumption would be the scope within which you create the child context ends and it gets cleaned up.
3. When using the parent/child context pattern, you need to be using the block methods. The biggest benefit of using the block methods is that the OS will handle dispatching the method calls to the correct threads. You can use performBlock for asynchronous execution, or performBlockAndWait for synchronous execution.

You would use this such as:

- (void)saveContexts {
    [childContext performBlock:^{
        NSError *childError = nil;
        if ([childContext save:&childError]) {
            [parentContext performBlock:^{
                NSError *parentError = nil;
                if (![parentContext save:&parentError]) {
                    NSLog(@"Error saving parent");
                }
            }];
        } else {
            NSLog(@"Error saving child");
        }
    }];
}

Now, you need to keep in mind that changes made in the child context (e.g. Entities inserted) won't be available to the parent context until you save. To the child context, the parent context is the persistent store. When you save, you pass those changes up to the parent, who can then save them to the actual persistent store. Saves propogate changes up one level. On the other hand, fetching into a child context will pull data down through every level (through the parent and into the child)

4. You need to use some form of objectWithID on the managedObjectContext. They are the safest (and really only) way to pass objects around between contexts. As Tom Harrington mentioned in the comments, you may want to use existingObjectWithID:error: though because objectWithID: always returns an object, even if you pass in an invalid ID (which can lead to exceptions). For more details: Link

Child NSManagedObjectContext update from parent

Fetched results controller needs a main context.

Use this pattern to get rid of the "choppiness".

RootContext (private queue) - saves to persistent store
MainContext (main queue) child of RootContext - use for UI (FRC)
WorkerContext (private queue) - child of MainContext - use for updates & inserts

The your web query is finished, create a worker context and update the data model. When you save, the changes will be pushed up to the main context and your UI should update via the FRC delegate. Save the main and root context to persist.

Make sure you are using the block methods performBlock and performBlockAndWait throughout when dealing with child contexts.

NSManagedObjectContext Child/Parent - Child not removing registeredObjects

When you perform save operation on MOC, It will save the object to parent MOC of persistent store. But object will still retained by MOC in memory.

By default, the references between a managed object and its context are weak. The exception to this rule is that a managed object context maintains a strong reference to any changed (inserted, deleted, and updated) objects until the pending transaction is committed (with a save:) or discarded (with a reset or rollback).

If you feel that this object is no longer need full for current future or alarms are generated (on the fly in a memory store), you would like to trim the graph object by turning every thing into a fault using "refreshObject:object mergeChanges:NO"

Save object from child NSManagedObjectContext not available in parent

Alright, it's a bug! Known bug for many years, but only documented in StackOverflow. The answer is here
https://stackoverflow.com/a/11996957/2073793

One needs to obtain permanent object ids using
obtainPermanentIDs(for:)
before saving in the child context. Then, those permanent ObjectIds can be used to retrieve the objects from within the parent context.

Use NSManagedObject in child NSManagedObjectContext instead of its parent

This is where the objectID property of NSManagedObject will come in handy.

• Ask the object for its ID

  let objectID = myManagedObject.objectID
  

• Ask the child context for a managed object with that ID

  do {
      let childManagedObject = try childContext.existingObjectWithID(objectID)
      print("\(newObject)")
  } catch {
  
  }
  

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