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POInverse relation not set (in KVO handler)
 

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    <p>I have a very strange problem with inverse relations in Core Data, and I have managed to reduce my problem to a minimal example, starting from a new project in xcode based on the window template with support for Core Data (i.e., there's very little there). </p> <p>Suppose we have a Core Data model with three entities: Department, Employee, and DepartmentSummary (some sort of entity representing some statistics about the department). For simplicity's sake, we have only one-to-one relations:</p> <pre><code>DepartmentSummary Department Employee --------------------------------------------------------- employee &lt;----&gt; department department &lt;----&gt; summary </code></pre> <p>This is all there is in the model. In <code>application:didFinishLaunchingWithOptions:</code> we create an employee and a department and set up KVO:</p> <pre><code>NSManagedObject* employee = [NSEntityDescription insertNewObjectForEntityForName:@"Employee" inManagedObjectContext:[self managedObjectContext]]; [employee addObserver:self forKeyPath:@"department" options:0 context:nil]; NSManagedObject* department = [NSEntityDescription insertNewObjectForEntityForName:@"Department" inManagedObjectContext:[self managedObjectContext]]; [department setValue:employee forKey:@"employee"]; </code></pre> <p>The purpose of the KVO handler is to create a summary for the department as soon as the employee's department is set:</p> <pre><code>- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { [self createSummary:object]; } </code></pre> <p><code>createSummary</code> is simple: it creates a new summary object and associates it with the department, and <em>then checks that the inverse relation from the department to the summary object is also set</em>:</p> <pre><code>- (void) createSummary:(NSManagedObject*)employee { NSManagedObject* department = [employee valueForKey:@"department"]; NSManagedObject* summary = [NSEntityDescription insertNewObjectForEntityForName:@"DepartmentSummary" inManagedObjectContext:[self managedObjectContext]]; [summary setValue:department forKey:@"department"]; NSAssert([department valueForKey:@"summary"] == summary, @"Inverse relation not set"); } </code></pre> <p><em>This assertion fails.</em> Indeed, if we print the department and summary objects after the summary's department has been set, we get</p> <pre><code>entity: DepartmentSummary; id: ..DepartmentSummary/..AA14&gt; ; data: { department = "..Department/..AA13&gt;"; } </code></pre> <p>for the summary, as expected, but</p> <pre><code>entity: Department; id: ..Department/..AA13&gt; ; data: { employee = "..Employee/..AA12&gt;"; summary = nil; } </code></pre> <p>for the department (with a <code>nil</code> summary). If however we delay the call to <code>createSummary</code> so that it doesn't run until the next iteration of the runloop:</p> <pre><code>- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { [self performSelector:@selector(createSummary:) withObject:object afterDelay:0]; } </code></pre> <p>then everything works as expected. </p> <p>Delaying the <em>assertion</em> instead does <em>not</em> help: the inverse relation really does not get set <em>in the object graph</em>, though it does get set in the database (if you were to save the database, and restart the app, now all of a sudden the inverse relation appears).</p> <p>Is this a bug in Core Data? Is this documented behaviour which I have missed? Am I using Core Data in ways it was not intended? </p> <p>Note that the KVO handler gets called <em>while Core Data is (automatically) setting an(other) inverse</em>: we manually set the department's <code>employee</code> field, Core Data automatically sets the employee's <code>department</code> field, and that in turn triggers the KVO handler. Perhaps that is just too much for Core Data to handle :) Indeed, when we set</p> <pre><code>[employee setValue:department forKey:@"department"]; </code></pre> <p>instead, everything again works as expected.</p> <p>Any pointers would be appreciated. </p>
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    1. COWhat if you set the summary immediately, but delay your *assertion* until the next runloop?
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    2. COExcellent question. I'll edit the question to answer it -- basically, delaying the assertion does not help.
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    3. COhi, i noticed same, it was working before...
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