CoreData: How to fetch a specific object using a predicate

The situation:

I fetch a complete table from my sqllite core data database and show it in a TableView like this:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyTable"
                                          inManagedObjectContext:managedObjectContext];

The Challenge:

How do I get the EntryID and fetch the specific entry from the database (e.g. if i click on an entry)? I think this goes in the right d开发者_如何学运维irection?

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"(id = %@)", myEntryID];

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If you have an entry object called entry, it would be a predicate like this:

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"(SELF = %@)", entry];

Which is roughly equivalent to

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"(objectID = %@)", entry.objectID];

For a NSManagedObjectID, you get something like:

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"(objectID = %@)", myEntryID];

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This is proper predicate:

[NSPredicate predicateWithFormat:@"SELF = %@", objectID];

Where objectID is NSManagedObjectID instance.

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You might want to look at -[NSManagedObject objectID] and -[NSManagedObjectContext objectWithID:].

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When you build the table you do so either with an array returned by a fetch or by the implicit array inside a NSFetchResultsController. Therefore, the object you want has the same index in the array/controller as it does in the table.

So, it becomes simply a matter of calling:

myObject=[fetchedArray objectAtIndex:tableRowIndex];

or

myObject=[myFetchedResultsController objectAtIndexPath:tableRowIndex];

This is the real genius of the UITable. It always reflects the data model precisely. You never have to translate back and forth between the table indexes and your data model indexes.