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POFetched Properties, cross store relationships
 

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    <p>I've got a store that is synchronized externally and a store that is unique to the application instance, so in order to cleanly differentiate the two I want to have some join entities between them and then resolve through to the entities between using Fetched Properties, as "discussed" in the Core Data Programming Guide:</p> <p>developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html#//apple_ref/doc/uid/TP40001857-SW5</p> <p>I think I just don't really "get" how Fetched Properties are supposed to be used - and I've spent a fair number of hours looking for examples with no real luck.</p> <p>The way I think of it is,</p> <p>I have the following Entities each in a different store <strong>Foo</strong> with attribute <em>relatedBarName</em> in store <strong>A</strong> <strong>Bar</strong> with attribute <em>barName</em> in store <strong>B</strong></p> <p>I need to create a fetched property on Foo named <em>findRelatedBar</em> that relates <strong>Foo</strong> to <strong>Bar</strong> loosely through <em>barName</em> = <em>relatedBarName</em>.</p> <p>However, I don't understand how since <strong>Foo</strong> and <strong>Bar</strong> are in different stores how to declare any relationship of any sort, whether through the fetched property or not, from <strong>Foo</strong> to <strong>Bar</strong>? </p> <p>The predicate builder in XCode seems to want a Destination entity. If they are in different schemas, how can you declare the destination? If you don't declare a destination, how do you at runtime indicate that <em>findRelatedBar</em> on <strong>Foo</strong> is describing <strong>Bar</strong>?</p> <p>Otherwise, do they need to be in the same schema but just stored in different stores?</p> <ul> <li>In crafting this question, I thought of these questions and answered them myself by more focused examination of the documentation. I assume if I found it confusing, others might as well, so I'll inline them with this post to make it easier to find related answers to fetched properties / core data stores.</li> </ul> <p>Q) If a store coordinator have more than one store associated with it of the same schema, how do insertions know which store to insert to?</p> <p>A) <a href="http://developer.apple.com/documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObjectContext_Class/NSManagedObjectContext.html#//apple_ref/occ/instm/NSManagedObjectContext/assignObject:toPersistentStore:" rel="nofollow noreferrer" title="You use the assignObject method on the managed object context">You use the assignObject:toPersistentStore: method on the managed object context.</a></p> <p>Q) What does <strong>FETCH_SOURCE</strong> refer to in specific?</p> <p>A) It's simply the managed object which has the fetched property associated with it. Sort of like "<strong>self</strong>"</p> <p>Q) What does <strong>FETCHED_PROPERTY</strong> refer to in specific?</p> <p>A) It is a reference to the fetched property description instance you are using to query with - you can use this to insert per query variable substitution. By setting a property (as in the Core Data Programming example) on the userInfo of the property description instance you're using, you can inject that value into the expression.</p> <p>Thanks!!!!</p>
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