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POCore Data: Fetching Relationship Attributes
 

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  1. POCore Data: Fetching Relationship Attributes
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    <p>Can't seem to figure out the trick to fetch an attribute from a relationship when limiting the fetch to a number of properties. In the code below "thumbnail" is a transformable UIImage attribute of the entity "Photo". That always comes back fine in the array of dictionaries with the key "thumbnail". What I cannot figure out is the second property/attribute where "previewImage" is a relationship in Photo to another entity named "Image" that has a transformable attribute "uiImage" (which is a UIImage).</p> <p>I've tried this:</p> <pre><code>[fetchRequest setPropertiesToFetch:@[@"thumbnail", @"previewImage.uiImage"]]; </code></pre> <p>but I got this error:</p> <pre><code>-[NSExpressionDescription valueTransformerName]: unrecognized selector sent to instance 0x13078920 </code></pre> <p>and I've tried this:</p> <pre><code>[fetchRequest setPropertiesToFetch:@[@"thumbnail", @"previewImage"]]; </code></pre> <p>where I do get two items in each dictionary</p> <pre><code>{ previewImage = "0xbabba90 &lt;x-coredata://03FF7129-1345-4BAE-A74E-2FF5A2B38CDC/Image/p5&gt;"; thumbnail = "&lt;UIImage: 0xbabbc60&gt;"; } </code></pre> <p>but when I try to use that "Image" object as below</p> <pre><code>NSDictionary *dict = [self.photos objectAtIndex:0]; Image *theImage = (Image *)[dict objectForKey:@"previewImage"]; [self.delegate photoCollectionViewController:self didSelectPreviewImage:theImage.uiImage]; </code></pre> <p>I get this error</p> <pre><code>"[_NSObjectID_48_0 uiImage]: unrecognized selector sent to instance 0xbabba90" </code></pre> <p>And I've even thrown this in the request</p> <pre><code>[fetchRequest setRelationshipKeyPathsForPrefetching:@[@"previewImage.uiImage"]]; </code></pre> <p>So, I guess I'm flailing and not getting this. Any help would be appreciated. Thanks!</p> <pre><code>NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Photo" inManagedObjectContext:self.managedObjectContext]; [fetchRequest setEntity:entity]; [fetchRequest setResultType:NSDictionaryResultType]; </code></pre>
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    1. USuser1139479
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    1. COThe reference to `_NSObjectID_48_0` suggests that you used `NSManagedObjectIDResultType` somewhere, though your code says `NSDictionaryResultType`. Did you ask for object IDs somewhere?
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      1. POCore Data: Fetching Relationship Attributes
      1. USTom Harrington
    2. CONo. I searched my entire project for "NSManagedObjectIDResultType" and nothing there. I didn't even know you could change the result type until I tried to limit fetched properties :) BTW, I have your book ;)
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      1. USuser1139479
 

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