CODEWITHSUNDEEP
ioscocoasortingcore-datansfetchrequest
Colas
Colas

Reputation: 3573

What are the limits of making a sorted fetchRequest with Core Data?

In short :

Fetching entities and sorting them according to an @property which is not an attribute will cause a bug (with NSDictionaryMapNode involved) at the second run.

Is it normal or a bug ? Do you have any comment or help about that?

In long :

The situation is the following.

I have an Entity with two attributes attribute1 and attribute2.

I have generated [1] a classe Entity.m and added a @property to it called myProperty. Hence, myProperty is a @property of my class Entity.m but not an attribute of the entity Entity. By the way, myProperty is readonly (let's say it is something like attribute1 concatenated with attribute2.

Now, I do the following :

NSManagedObjectContext * myContext = ... ;
NSFetchRequest * myRequest = [NSFetchRequest fetchRequestWithEntityName:@"Entity"];
NSSortDescriptor * mySortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"myProperty"
                                                                    ascending:YES] ;
NSError *error ;

[myRequest setSortDescriptors:@[mySortDescriptor]] ;

NSArray * result  = [myContext executeFetchRequest:myRequest
                                             error:&error] ;

I understand that the problem comes from myProperty not being an attribute of Entity. The aim of my post is to raise this situation and to know if you would have any comment about this situation.

[1] With mogenerator (https://github.com/rentzsch/mogenerator).

Upvotes: 2

Views: 340

Answers (1)

Wain
Wain

Reputation: 119031

This is known. You can't sort on anything that isn't persistent in the store. It should never work (to work the first time means that you don't have a MOC or something like that).

You should add a new attribute to the entity that is persistent. Do something like implement willSave or observe the dependent keys and use that trigger to update the value of the new attribute (myProperty).

See section Fetch Predicates and Sort Descriptors of Core Data Programming Guide

Upvotes: 2

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