Re: Core Data, SQL stores, and predicate restrictions

FROM : Bill Dudney
DATE : Sat May 17 00:33:07 2008

HI Sean,

One example would be if you have a method that calculates a value. 
That value would not be in the DB so the compiled predicate (if it 
would even compile) would not work because there is not column 
corresponding to your calc'd value.

I think this is worth filing a bug against the docs to clear that up.

HTH,


-bd-
http://bill.dudney.net/roller/objc

On May 16, 2008, at 3:50 PM, Sean McBride wrote:

> 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
>
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