Alternative to “containers.@count == 0” predicate. Still haven't got it filtering in query, though.

related_counts

// "name" is example: need to reference an attribute in containers that MUST NOT BE NIL
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"containers.name"];
NSExpression *countExpression = [NSExpression expressionForFunction:@"count:" arguments:@[keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: @"count"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];

NSExpression* selfExpression = [NSExpression expressionForEvaluatedObject];
NSExpressionDescription* selfDescription = [[NSExpressionDescription alloc] init];
[selfDescription setName:@"objectID"];
[selfDescription setExpression:selfExpression];
[selfDescription setExpressionResultType:NSManagedObjectIDResultType];

[request setPropertiesToFetch:@[expressionDescription, selfDescription]];
[request setResultType:NSDictionaryResultType];

NSArray *results = [context executeFetchRequest:request error:&error];
if ( results == nil ) {
	NSLog(@"ERROR: %@", error);
}

for ( NSDictionary* result in results ) {
	if ( [result[@"count"] integerValue] == 0 ) {
		NSManagedObject* object = [context existingObjectWithID:result[@"objectID"] error:&error];
		// Operate on it
		NSLog(@"object: %@", object);
	}
}