On Apr 16, 2009, at 9:01 PM, 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'".
I am not really up to speed on NSPredicateEditor, but could you use -
ruleEditor:predicatePartsForCriterion:withDisplayValue:inRow: to
"convert" the display case of grade = A-F to a predicate that is
actually grade >= <= score range? I imagine you would need to
construct a compound predicate with subpredicates (sub-rows). Either
that or use the "BETWEEN" operator.
HTH,
Keary Suska
Esoteritech, Inc.
"Demystifying technology for your home or business"
_______________________________________________
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
