Hello list,
a little while ago I posted a question regarding some strange behavior by an
NSFetchedResultsController (under the same subject line as this message). Since
then I've narrowed the problem down a bit more and now have a test project to
show it.
Here's what the goal is, in a nutshell.
I have a list of, say, countries. Some are "selected", others not. I need to
display all the countries in the same tableview, in the same section, and what
differentiates the selected ones from the deselected ones is their text color.
Moreover, the selected ones should be sorted ascending by their "index" (an
integer that's used to keep track of their position in the list) while the
deselected ones must be sorted ascending by their "name".
The test project's core data model has a single entity, "CountryCD", with the
attributes "name", "index", "selected" (representing a boolean), and a
transformable attribute "indexOrName" which returns the entity's index if it's
selected or its name if it's deselected.
- (id) indexOrName;
{
if ([self.selected isEqualToNumber: [NSNumber numberWithBool: YES]])
{
return self.index;
}
else
{
return self.name;
}
}
The fetched results controller is defined as usual but contains two sort
descriptors:
NSSortDescriptor* sortBySelected = [[NSSortDescriptor alloc]
initWithKey: @"selected" ascending: NO];
NSSortDescriptor* sortByIndexOrName = [[NSSortDescriptor alloc]
initWithKey: @"indexOrName" ascending: YES];
the idea being that entities get sorted first by their "selected" status and
then by their "indexOrName" attribute. Since that attribute returns the
entity's "index" or its "name", depending on the entity's "selected" status,
the fetched results controller *should* sort the entities as desired.
Should, but doesn't. And that's the problem I can't seem to solve.
In the test project, I create country entities in the following order
[self createCountryOfName: @"India"];
[self createCountryOfName: @"Honduras"];
[self createCountryOfName: @"Germany"];
[self createCountryOfName: @"Denmark"];
[self createCountryOfName: @"Brazil"];
[self createCountryOfName: @"Egypt"];
[self createCountryOfName: @"Australia"];
[self createCountryOfName: @"China"];
[self createCountryOfName: @"Finland"];
and make every second one deselected, which *should* result in them being
displayed as follows:
// (ordered by index)
// India
// Germany
// Brazil
// Australia
// Finland
// (ordered by name)
// China
// Denmark
// Egypt
// Honduras
Alas, they're displayed like this (or some other seemingly random order):
(selected)
Brazil
Germany
Australia
India
Finland
(deselected)
Honduras
Denmark
Egypt
China
Neither the selected ones are sorted by their "index" attribute nor the
deselected ones are sorted by their "name" attribute. In fact, the test project
shows that "indexOrName" is not accessed at all when the fetched results
controller does its fetch.
First I thought that there might be something wrong with the way I implemented
the fetched results controller, but if you replace "indexOrName" in the sort
descriptor with either "index" or "name", it works as would be expected.
So then I thought that using a transformable attribute is not the way to go in
order to achieve the result I need to achieve, but I can't see any other way to
do it.
If using a transformable attribute isn't the way to go, can anyone suggest an
alternative? If using a transformable attribute should work, then why doesn't
it as I've implemented it? It appears to me that NSFetchedResultsController
ignores a sort descriptor based on a transformable attribute.
The test project can be found here:
http://www.restlessbrain.com/FRC_Test.zip
I'd greatly appreciate any help. I've already spent several days trying to
figure this out but got nowhere and ran out of ideas.
Thanks in advance.
WT
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