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Postsdbo
POCoreData relationships not merging from one MOC to another
 

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  1. POCoreData relationships not merging from one MOC to another
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    <p>I've been banging my head against the wall with this bug. Basically relationships aren't being merged from one context to another correctly. </p> <p>Let's say I have two entity types in my model, Student and Teacher. A student can only have one teacher and a teacher can obviously have many students. I set this up correctly in my xcdatamodel class.</p> <p>Because I'm syncing these entities from the server I have 2 Managed Object Contexts - one called <code>uiContext</code> which has it its concurrency type set to <code>NSMainQueueConcurrencyType</code> and another called <code>backgroundContext</code> with its concurrency type set to <code>NSPrivateQueueConcurrencyType</code>. </p> <p>I've also set up a nested structure using <code>[self.uiContext setParentContext:self.backgroundContext];</code></p> <p>So this is what I'm doing:</p> <p>1) Fetch Teachers from the server</p> <p>2) Create managed objects for them in <code>backgroundContext</code></p> <p>3) Call <code>save</code> on <code>backgroundContext</code></p> <p>4) I listen for the <code>NSManagedObjectContextDidSaveNotification</code>, and execute the following code: </p> <blockquote> <pre><code>[self.uiContext performBlock:^(void) { [self.uiContext mergeChangesFromContextDidSaveNotification:notif]; }]; </code></pre> </blockquote> <p>So far this works just fine. My uiContext gets the Teacher managed objects and my UI updates accordingly.</p> <p>It breaks down though when I do the following:</p> <p>5) Fetch the student data from the server</p> <p>6) Create managed objects for them in the <code>backgroundContext</code> and <strong>set the relationship from student to the appropriate teacher.</strong> </p> <p>7) Call <code>save</code> on <code>backgroundContext</code></p> <p>8) Since I'm listening for the <code>NSManagedObjectContextDidSaveNotification</code> notification this save should get merged into the <code>uiContext</code></p> <p>However, here's where I start experiencing issues. Even though the student objects are merged into the <code>uiContext</code> correctly, the relationship from teacher to student is not set. The relationship from student to teacher is correctly set but not the other way around. Specifically, <code>[someTeacherObject.students count] == 0</code> but <code>someStudentObject.teacher != nil</code></p> <p>The relationships are set correctly in both the <code>backgroundContext</code> and the persistent store (verified). Just not in the <code>uiContext</code>. </p> <p>Any ideas what could be going on? Any idea how to better debug this?</p>
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    1. COIn your data model, is the student entity's `teacher` relationship the inverse of the teacher entity's `students` relationship?
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      1. USTom Harrington
    2. COYes that is set correctly. And I believe it must be because the data is setup correctly in the backgroundContext and in the persistant store.
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      1. USaloo
    3. COIf that's the case then it should be impossible to have `[someTeacherObject.students count] == 0` and `someStudentObject.teacher != nil`-- unless `someStudentObject.teacher` is actually pointing to a different teacher object than the `someTeacherObject` you're checking. As inverse relationships, setting the one to non-nil would force the other to have a count > 0. Are you sure you're comparing the same instances?
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