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POCoreData - How to use validateForDelete: to determine if a managed object should be deleted
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  1. POCoreData - How to use validateForDelete: to determine if a managed object should be deleted
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    copied!<p><strong>Goal:</strong> I want to check a managed object to make sure that it is okay for deletion</p> <p><strong>Expectation:</strong> <code>-[NSManagedObject validateForDelete:]</code> should return a <code>BOOL</code> based on the delete rules setup in the managed object model</p> <p><hr></p> <h1>NSManagedObject Class Reference</h1> <blockquote> <p>validateForDelete:</p> <p>Determines whether the receiver can be deleted in its current state.</p> <p><code>- (BOOL)validateForDelete:(NSError **)error</code></p> <p><strong>Parameters</strong></p> <p><em>error</em></p> <p>If the receiver cannot be deleted in its current state, upon return contains an instance of NSError that describes the problem.</p> <p><strong>Return Value</strong></p> <p><code>YES</code> if the receiver can be deleted in its current state, otherwise <code>NO</code>.</p> <p><strong>Discussion</strong></p> <p>An object cannot be deleted if it has a relationship has a “deny” delete rule and that relationship has a destination object.</p> <p><code>NSManagedObject</code>’s implementation sends the receiver’s entity description a message which performs basic checking based on the presence or absence of values.</p> </blockquote> <p>Reading this I assume that I should be able to do something like this:</p> <pre><code>BOOL canDelete = [myManagedObject validateForDelete:&amp;error]; if (canDelete) { [managedObjectContext deleteObject:myManagedObject]; } else { [self requestUserFeedback]; } </code></pre> <p>However, this method almost always returns <code>NO</code>.</p> <p><hr></p> <h1>Example Code</h1> <p><em>Managed Object Model</em> </p> <pre><code>Department - Attribute: name - Value: String - Relationship: employees - To Many - Destination: Employee - Delete Rule: Nullify - Inverse: department Employee - Attribute: name - Value: String - Relationship: department - To One - Destination: Department - Delete Rule: Nullify - Inverse: employees </code></pre> <p>Example Code:</p> <pre><code>NSManagedObject *department = [NSEntityDescription insertNewObjectForEntityForName:@"Department" inManagedObjectContext:self.managedObjectContext]; for (int i = 0; i &lt; 10; i++) { NSManagedObject *employee = [NSEntityDescription insertNewObjectForEntityForName:@"Employee" inManagedObjectContext:self.managedObjectContext]; [employee setValue:department forKey:@"department"]; if ([employee validateForDelete:NULL]) { NSLog(@"Can delete employee"); } else { NSLog(@"WARNING: Cannot delete employee"); } } NSError *error; BOOL canDelete = [department validateForDelete:&amp;error]; if (canDelete) { NSLog(@"Can delete department"); } else { NSLog(@"WARNING: Cannot delete department"); } </code></pre> <p>Output:</p> <blockquote> <p>Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> Can delete employee<br> WARNING: Cannot delete department </p> </blockquote> <p>To my surprise, <code>canDelete</code> is <code>NO</code>. Why would the department object be invalid for deletion when the employee objects are not invalid? The error returned contains the following (abbreviated)</p> <pre><code>Error Domain=NSCocoaErrorDomain Code=1600 UserInfo= &lt;&gt;{ NSValidationErrorObject=&lt; department object &gt; NSValidationErrorKey=employees NSValidationErrorValue=Relationship 'employees' on managed object } </code></pre> <p>So, <code>validateForDelete:</code> is not returning what I expect for to-many relationships, but does return the expected value for to-one relationships. But, go back to the documentation for a second:</p> <blockquote> <p>An object cannot be deleted if it has a relationship has <em>[sic]</em> a “deny” delete rule and that relationship has a destination object.</p> <p>NSManagedObject’s implementation sends the receiver’s entity description a message which performs basic checking based on the presence or absence of values.</p> </blockquote> <p>This calls out how the default implementation of validateForDelete: works and makes a concerted effort to draw attention to the "deny" delete rule. There is no mention of nullify or cascade delete rules or differences between to-one or to-many relationships.</p> <p><hr></p> <h1>Further Research</h1> <p>A Google search turns up <a href="http://lists.apple.com/archives/cocoa-dev/2009/Mar/msg00888.html" rel="nofollow">this hit on the Apple Mailing List archives from 2009</a>, which basically says: <code>-[NSManagedObject validateForDelete:]</code> is a hook for subclassers, you need to implement your own logic in a separate method to determine if an object is valid for deletion.</p> <p>What? Is this true? That's not what the documentation for this method would lead one to believe:</p> <blockquote> <p>Determines whether the receiver can be deleted in its current state.</p> </blockquote> <hr> <p><strong>How are we suppose to check if a managed object is in a state that is okay for deletion?</strong></p> <p>I've set up a small sample project on <a href="https://github.com/evandelaney/StackOverflow-20722804" rel="nofollow">GitHub</a> that illustrates my findings with both production code and unit tests.</p>
 

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