Question

Here’s an example of my data model:

• There are mailboxes
• Each mailbox has many messages
• Each message has a flag to indicate if it is unread

How can I count the number of unread messages for a specific mailbox?

I tried creating a fetch request in the data model editor, but I can’t add a condition for the mailbox although the message entity has a relationship defined for its owning mailbox.

I know I’m not supposed to think in SQL when dealing with Core Data, but it feels so natural so say:

SELECT count(*) FROM Messages WHERE unread = 1 AND mailboxId = 12345

Is there another way to get the unread message count other than looping through each message for a specific mailbox?

Answer 1

You want to count without fetching the objects, as that would take longer:

- (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error

Returns the number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:

You will also need to add a predicate to your fetch request (which is the WHERE clause).

e.g

NSFetchRequest * request = [[NSFetchRequest alloc] init];
[request setEntity: [NSEntityDescription entityForName:@"Messages" inManagedObjectContext:context]];
[request setPredicate:[NSPredicate predicateWithFormat: @"(unread == 1 && mailboxId == %i)", mailboxId]];

(I may have my predicate syntax wrong…. but you should get the idea)