Re: [Leopard] SUBQUERY predicate with CoreData
Re: [Leopard] SUBQUERY predicate with CoreData
- Subject: Re: [Leopard] SUBQUERY predicate with CoreData
- From: "Barry Wark" <email@hidden>
- Date: Mon, 4 Feb 2008 17:00:21 -0800
Thank you to Ben and Melissa for setting me straight. I notice that
the NSExpression documentation gives
"SUBQUERY(residents, $x, $x.firstname == "Jane" && $x.lastname ==
"Doe").count != 0)"
as an example subquery for a similar use case. Besides the missing
opening '(', this example has the, as I know now, incorrect use of
.count != 0 pattern. Is this well known, or worth filing a bug for the
documentation?
Thanks,
Barry
On Feb 4, 2008 3:29 PM, Ben Trumbull <email@hidden> wrote:
> >[fetch setPredicate:[NSPredicate
> >predicateWithFormat:@"(SUBQUERY(self.keyValues, $kv, $kv.key like
> >'key1' && $kv.floatValue==1.0).count > 0) && fooProp like
> >'fooProp1'"]];
>
> A SUBQUERY expression produces an array of results, a lot like
> NSArray's -filteredArrayWithPredicate:
>
> Tacking on .count at the end is adding a keypath. Basically it's like
> [subqueryResults valueForKeyPath:@"count"]
>
> NSArray responds to the keypath .count by invoking -count on each
> element and giving you back an array of all the counts. This array
> is not happy being compared to a scalar 0.
>
> NSArray responds to the keypath "@count" in the manner you want. You
> can use either
>
> "(SUBQUERY(self.keyValues, $kv, $kv.key like 'key1' &&
> $kv.floatValue==1.0).@count > 0)"
>
> or
>
> "(SUBQUERY(self.keyValues, $kv, $kv.key like 'key1' &&
> $kv.floatValue==1.0)[size] > 0)"
>
> where the second form is using some of the built-in predicate
> operators for aggregates.
> --
>
> -Ben
>
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