Context already has a coordinator; cannot replace - ios

Question

I'm getting the error "Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Context already has a coordinator; cannot replace.'" while executing my app.
In the DataBaseManager.m I have the following code :
AppDelegate *sharedDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
NSManagedObjectContext *privateWriterContext = [sharedDelegate privateWriterContext];
NSManagedObjectContext *context = [sharedDelegate managedObjectContextChild];
context.parentContext = privateWriterContext;
NSManagedObjectContext *contextforThread = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
contextforThread.parentContext = context;
[contextforThread performBlock:^{
//things to do in background
NSError *error;
if (![contextforThread save:&error])
{
// handle error
}
[context performBlock:^{
NSError *error;
if (![context save:&error])
{
// handle error
}
[privateWriterContext performBlock:^{
NSError *error;
if (![privateWriterContext save:&error])
{
// handle error
}
}];
}];
}];
and in appDelegate.m I have the following code :
- (NSManagedObjectContext *)managedObjectContextChild
{
if (_managedObjectContextChild != nil) {
return _managedObjectContextChild;
}
_managedObjectContextChild = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
return _managedObjectContextChild;
}
- (NSManagedObjectContext *)privateWriterContext
{
if (_privateWriterContext != nil) {
return _privateWriterContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_privateWriterContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_privateWriterContext setPersistentStoreCoordinator:coordinator];
}
return _privateWriterContext;
}
Xcode shows the line
context.parentContext = privateWriterContext;
when error occurs.
How could I rectify this error?

Answer 1

At the top of - (NSManagedObjectContext *)managedObjectContextChild put a breakpoint and see where else that NSManagedObjectContext is being used.
The error is telling you that it already has a persistent store coordinator, so that's your line of attack.
You are accessing these as single instances so the other areas of the application need to know what to do with them (and what not to do!). You could also inspect persistentStoreCoordinator and parentContext property just before the error to help get a picture of what's happening.
It's nothing terrible and shouldn't be too difficult to find with a couple of sensibly placed breakpoints, however you may have to think about how you manage the various contexts more carefully to solve the issue.