If it helps, the reason for Apple only supporting searching of
persistent properties is for performance. If you're using the SQLite
store for example, the predicate is not evaluated against the in-
memory objects, but against the individual bits of data in the SQLite
table. Of course, why Apple couldn't have then added automatic support
for in-memory matching as the second step I don't know…
On 17 Apr 2009, at 04:01, Jerry Krinock wrote:
The fact that Core Data "cannot fetch using a predicate based on
transient properties" [1] seems to greatly limit the utility of the
NSPredicateEditor view, and makes me very sad.
For example, say that my objects are student test results with a
'score' attribute and two dozen other properties. I could give the
user an NSPredicateEditor and let them have oodles of fun
constructing complex predicates.
But what if I need the user to be able to set a predicate with a
left-side-expression of "letter grade" and a right-side-expression
popup menu showing 'A' - 'F'. If I could fetch based on a transient
'letterGrade' attribute, I could implement some custom accessors
which would calculate 'letterGrade' from 'score' as needed, the
predicate emitted from the NSPredicateEditor would "just work", and
life would be sweet.
But since I can't use transient properties in my predicate,
providing a popup like that in NSPredicateEditor seems to mean that
I'm going to have to somehow deconstruct the compound predicate
which Apple put so many man-years of engineering into, have Core
Data do "sub-fetches", then do my own filtering and put the results
back together. I fear that writing bug-free code to handle the
general compound predicate would be very time-consuming, and also it
would be MVC hell with my NSPredicateEditor subclass (view) code
wanting to have model logic such as "if score > 93, letterGrade =
'A'".
Does anyone have any suggestions for a "least worst" workaround?
Thanks,
Jerry
[1] http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdFetching.html#/
/apple_ref/doc/uid/TP40002484
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