Question

I’m trying to see if I have a correct understanding of prefetching with Core Data. I’m executing a fetch request with a batchSize of 20 for a Message object, with has a to-one relationship with a MBDate property. I prefetch this property:

[fetchRequest setRelationshipKeyPathsForPrefetching:
           [NSArray arrayWithObjects:@"date", nil]];

The prefetch command goes through, as shown by this output:

CoreData: annotation: Prefetching with key 'date'.  Got 9 rows.
CoreData: annotation: total fetch execution time: 0.0094s for 20 rows

However, in just the first batch, if I do

if(message.date.isFault) NSLog(@"isFault");,

it always outputs isFault, even though I prefetched it! And if I try to access any of the properties of the MBDate object, a fault request fires:

CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZDATE, t0.ZDATESTRING FROM ZMBDATE t0 WHERE  t0.Z_PK = ? 
CoreData: annotation: sql connection fetch time: 0.0008s
CoreData: annotation: total fetch execution time: 0.0014s for 1 rows.
CoreData: annotation: fault fulfilled from database for : 0x85d22b0 <x-coredata://736D04C4-7DB3-40B7-B208-79B5DFD68260/MBDate/p61>

Why is the prefetch not going through? Why are my objects still faults?
I know it’s hard to give any definite answers, but any tips or ideas would be great!

Answer 1

Can you try this same thing without setting batch size?

There is a curious note in NSFetchRequest Class Reference under fetchBatchSize:

If you set a non-zero batch size, the collection of objects returned
when the fetch is executed is broken into batches. When the fetch is
executed, the entire request is evaluated and the identities of all
matching objects recorded, but no more than batchSize objects’ data
will be fetched from the persistent store at a time. The array
returned from executing the request will be a proxy object that
transparently faults batches on demand.

It’s as if you set batchSize the objects returned will be faulted no matter what.