Re: iOS Core Data complex predicate.
Re: iOS Core Data complex predicate.
- Subject: Re: iOS Core Data complex predicate.
- From: Andreas Grosam <email@hidden>
- Date: Thu, 17 Mar 2011 23:49:59 +0100
On Mar 17, 2011, at 8:15 PM, Sandro Noël wrote:
> Greetings!
>
> I am facing a problem with a complicated predicate I'm building.
>
...
> then this predicate is fed to a fetch Request which in turn is fed to a fetched Result Controller.
> the fetch is configured to [fetchRequest setReturnsDistinctResults:YES];
>
> The problem as stated in the introduction this complete predicate returns multiple instances of the same record and it should not.
About setReturnsDistinctResults:
"If YES, the request returns only distinct values for the *fields specified by propertiesToFetch*."
That is, setReturnDistinctResults: makes only sense IFF you also specified the set of properties you want to fetch (via -setPropertiesToFetch). And this requires that you specify NSDictionaryResultType for the resultType property of the fetch request. As a result, with setReturnDistinctResults:YES you get a set of dictionaries whose values are distinct.
In other words, you cannot use -setReturnsDistinctResults:YES to make your result set distinct if this array contains *managed objects*.
You might use a NSSet which you initialize from your original array of managed objects in order to get a unique set.
Alternatively, you might fetch just objectID properties using a NSDictionaryResultType. This, however, is a bit elaborated:
When you return (unique) dictionaries as objects in your result array, and if you want these dictionaries having a key "objectID" which value corresponds the actual managed object of this dictionary instance, you need to create an appropriate NSExpressionDescription: and include this property description in the array which you pass to -setPropertiesToFetch:
NSExpressionDescription* objectIdDescription = [[NSExpressionDescription alloc] init];
[objectIdDescription setExpression:[NSExpression expressionForEvaluatedObject]];
[objectIdDescription setExpressionResultType:NSObjectIDAttributeType];
Note: NSExpressionDescription is subclassed from NSPropertyDescription.
Then you use objectIdDescription as one of the properties (NSPropertyDescription) you want to fetch:
[myFetchRequest setPropertiesToFetch:[NSArray arrayWithObject: objectIdDescription]];
Don't forget to set the result type (after you set the entity):
[myFetchRequest setResultType: NSDictionaryResultType];
[myFetchRequest setReturnsDistinctResults:YES];
...
The next step would be to extract the objectIDs from the array of dictionaries and store them in an array for convenience.
Regards
Andreas
> if any of you see a flaw in the logic or anything, please comment, I'm currently so deep into it, I cant see it anymore. :)
>
> I hope I have not missed nay details.
> Thank you!!!
> Sandro.
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