Re: iOS Core Data complex predicate.
Re: iOS Core Data complex predicate.
- Subject: Re: iOS Core Data complex predicate.
- From: Sandro Noël <email@hidden>
- Date: Fri, 18 Mar 2011 14:23:22 -0400
Andreas
First off, thank you for taking the time to read my essay.
I did not know the setReturnDistinctResults: was dependant on setPropertiesToFetch:
your explanation is outstanding.
That did the trick, and I thank you very much.
Sandro.
On 2011-03-17, at 6:49 PM, Andreas Grosam wrote:
>
> 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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