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    <p>While @BarryWark's answer is correct when working with fetch requests, I want to write a warning to the folks who will try to apply this rule to a filtering of Core Data to-many relationships. </p> <p>Shortly: If when filtering to-many relationships you use a predicate and your array of objects for IN query is an array of objectIDs - then you should use <strong>self.objectID</strong> in your query string like</p> <pre><code>NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(self.objectID IN %@)", arrayOfObjectIDs]; </code></pre> <p>Because using just <code>(self IN %@)</code> in the case of filtering to-many relationships will result in incorrect results - it is just an NSArray that evaluates predicates and it knows nothing about Core Data's NSManagedObjectID stuff.</p> <p>I've crafted special test code showing this. Sorry for so many lines, but they worth it. There are two entities: User and Post and User has a to-many relationship named "posts".</p> <pre><code>User *user = [NSEntityDescription insertNewObjectForEntityForName:NSStringFromClass([User class]) inManagedObjectContext:managedObjectContext()]; Post *post = [NSEntityDescription insertNewObjectForEntityForName:NSStringFromClass([Post class]) inManagedObjectContext:managedObjectContext()]; [user addPostsObject:post]; [managedObjectContext() save:nil]; // 1. Both filtered relationship array and fetch result are correct! NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(self IN %@)", @[ post ]]; NSSet *filteredRelationship = [user.posts filteredSetUsingPredicate:predicate]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Post"]; NSArray *fetchResult = [managedObjectContext() executeFetchRequest:fetchRequest error:nil]; NSLog(@"\n\n\nPredicate: %@", predicate); NSLog(@"filteredRelationship: %@", filteredRelationship); NSLog(@"fetchResult: %@", fetchResult); // 2. Filtered relationship array is empty (wrong), fetch result is correct, ! predicate = [NSPredicate predicateWithFormat:@"(self IN %@)", @[ post.objectID ]]; filteredRelationship = [user.posts filteredSetUsingPredicate:predicate]; fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Post"]; fetchResult = [managedObjectContext() executeFetchRequest:fetchRequest error:nil]; NSLog(@"\n\n\nPredicate: %@", predicate); NSLog(@"filteredRelationship: %@", filteredRelationship); NSLog(@"fetchResult: %@", fetchResult); // 3. Filtered relationship array is empty (wrong), fetch result is correct predicate = [NSPredicate predicateWithFormat:@"(self.objectID IN %@)", @[ post ]]; filteredRelationship = [user.posts filteredSetUsingPredicate:predicate]; fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Post"]; fetchResult = [managedObjectContext() executeFetchRequest:fetchRequest error:nil]; NSLog(@"\n\n\nPredicate: %@", predicate); NSLog(@"filteredRelationship: %@", filteredRelationship); NSLog(@"fetchResult: %@", fetchResult); // 4. Filtered relationship array is correct, fetch result is correct predicate = [NSPredicate predicateWithFormat:@"(self.objectID IN %@)", @[ post.objectID ]]; filteredRelationship = [user.posts filteredSetUsingPredicate:predicate]; fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Post"]; fetchResult = [managedObjectContext() executeFetchRequest:fetchRequest error:nil]; NSLog(@"\n\n\nPredicate: %@", predicate); NSLog(@"filteredRelationship: %@", filteredRelationship); NSLog(@"fetchResult: %@", fetchResult); </code></pre> <hr> <p><strong>TLDR output</strong></p> <pre><code>&lt;redacted&gt; Predicate: SELF IN {&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: { content = nil; title = nil; user = "0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;"; })} &lt;redacted&gt; filteredRelationship: {(&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: { content = nil; title = nil; user = "0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;"; }) )} &lt;redacted&gt; fetchResult: ("&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: {\n content = nil;\n title = nil;\n user = \"0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;\";\n})") &lt;redacted&gt; Predicate: SELF IN {0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt;} &lt;redacted&gt; filteredRelationship: {()} &lt;redacted&gt; fetchResult: ("&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: {\n content = nil;\n title = nil;\n user = \"0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;\";\n})") &lt;redacted&gt; Predicate: objectID IN {&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: { content = nil; title = nil; user = "0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;";})} &lt;redacted&gt; filteredRelationship: {()} &lt;redacted&gt; fetchResult: ("&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: {\n content = nil;\n title = nil;\n user = \"0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;\";\n})") &lt;redacted&gt; Predicate: objectID IN {0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt;} &lt;redacted&gt; filteredRelationship: {(&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: { content = nil; title = nil; user = "0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;";}))} &lt;redacted&gt; fetchResult: ("&lt;Post: 0x2a04f10&gt; (entity: Post; id: 0x2a56c40 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/Post/p1&gt; ; data: {\n content = nil;\n title = nil;\n user = \"0x2af2a20 &lt;x-coredata://9D07BF41-2DC0-42C1-9DD8-6082A00E7BEB/User/p1&gt;\";\n})") </code></pre>
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