Thanks for the reply Melissa - much appreciated.
I was beginning to toy with the idea of trying a function expression
(which I have used before for other purposes) to isolate an individual
attribute object and perform the several comparisons therein.
Presumably SUBQUERY, being designed for the job, benefits from some
performance advantages over the function approach(?).
Regards
Luke
On 8-Dec-08, at 2:07 PM, Melissa J. Turner wrote:
On Dec 6, 2008, at 01:45, Luke Evans wrote:
If there are any NSPredicate gurus out there, I'm scratching my
head on this one:
I have an entity that can optionally have a set of attributes (name-
value pairs). This is set up in the model as a optional to-many
relationship to an attribute entity.
Now, I want to fetch the list of managed objects of this entity
whose attributes exactly match a dictionary that is passed. The
predicate for this needs to filter the objects that have all the
required attributes; matching both the name, and value for each
attribute.
At this point I'm not seeing how you can code this in a predicate.
If I AND some clauses like:
...ANY attr.name == %@ AND ANY attr.value == [EMAIL PROTECTED]
for each attribute I'm testing for, then AFAICS I'm not testing
that the _same_ attr has the name and value.
I wondered about:
...ANY (attr.name == %@ AND attr.value == %@)...
but this doesn't appear to be legal syntax.
I'm still experimenting, but if anyone knows how to express a query
that allows multiple clauses on the _same_ entity across a to-many
relationship I'd appreciate a clue!
Cheers
Luke
If you're targetting 10.5 or later, there's a subquery expression in
the predicates that was added to support this kind of query. The
syntax for what you're doing would be:
SUBQUERY(toMany, $each, $each.name == %@ AND $each.value =
%@)[EMAIL PROTECTED] > 0
Note that this is on the list of expensive predicate operations, so
it should be towards the end of a compound predicate.
Cheers,
+Melissa
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