Core Data, SQL stores, and predicate restrictions

FROM : Sean McBride
DATE : Fri May 16 23:50:02 2008

Hi all,

After reading the docs and archives, it is clear to me now that with
Core Data's SQL store one is more limited in the types of predicates one
can use with fetches.

What I can't find is a list of what I can and can't do.  The best I
found was "[the] SQL store, on the other hand, compiles the predicate
[...] to SQL and evaluates the result in the database itself. [...] it
means that evaluation happens in a non-Cocoa environment, and so sort
descriptors (or predicates) that rely on Cocoa cannot work".  How do I
know if my predicate "relies on Cocoa"?

For example, given the Employees/Departments scenario: if I try to fetch
all employees that are in exactly 2 departments I use a fetch predicate
of "departments[SIZE] == 2" which works great everywhere except with the
SQL store.  I learnt this the hard way.

Thanks,

--
____________________________________________________________
Sean McBride, B. Eng                <email_removed>
Rogue Research                        www.rogue-research.com
Mac Software Developer              Montréal, Québec, Canada