Selection of random entries from a Core Data Store

Question

Is there a way to select a fixed number of random entries from a Core Data store? I am just getting started with Core Data and have been stuck on this problem for quite some time.

As a last resort, I could query a large selection of entries into memory and then randomly select a fixed number.

Also, is there a way to specify custom SQL statements to be executed on the Core Data store? I realize that this would be highly unlikely since the underlying implementation of the store could be an XML file as well.

Answer 1

Mmm... maybe

[[[managedObjectsContext registeredObjects] allObjects] objectsAtIndex:r]

where r is random int between 0 and the number of objects minus one? Not efficient at all but quick and easy.

EDIT: if you want to pick the random object between a selection of your objects then create a fetch request that describes your object selection and do the same as above with the query results:

[[[managedObjectsContext executeFetchRequest:request error:&error] objectAtIndex:r]

Answer 2

With regards to your second question, that is one of the points of Core Data, to abstract away the underlying data store. Using NSPredicate and NSExpressions to build a fetch request, or storing one in the data model is the only way to query the store.

Answer 3

Not sure, but I think Core Data puts data into NSSets for you. So you might be able to use -[NSSet anyObject]. I haven't tested that or anything.