How to create an NSFetchRequest which filters Core Data objects based on attributes AND relationships?
I have a Core Data model setup like so:
- Blockbuster Entity
- To-Many relationship to DVD entities.
- DVD Entity
- title attribute (string)
- To-One relationship to a parent Blockbuster entity
A single Blockbuster can have multiple DVD's inside of it (To-Many). Each DVD can only be part of a single Blockbuster (To-One).
There are multiple Blockbuster's, and some may contain DVD entities which share the same title as DVD entities from other Blockbuster's. For example, Blockbuster A might have a copy of "Transformers 2", and so does Blockbuster C and Blockbuster G. Let's pretend no Blockbuster has more than one copy of the same titled movie.
I'm interested in grabbing the Transformers 2 DVD from Blockbuster C. I don't want the copies from Blockbuster A or G, because my boss frequents there, and I can only get away with burning this piece of garbage in Blockbuster C.
My question is, how do I form an NSFetchRequest which grabs a DVD with the title "Transformers 2", which is a child of the Blockbuster "C" parent entity?
This type of request will grab all the "Transformer 2" DVD's, but not the one specifi开发者_运维知识库c to the Blockbuster C entity:
NSManagedObjectContext *moc = [self managedObjectContext];
NSString *aTitle = @"Transformers 2";
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"title == %@", aTitle];
[request setEntity:[NSEntityDescription entityForName:@"DVD" inManagedObjectContext:moc]];
[request setPredicate:predicate];
NSError *error = nil;
NSArray *results = [moc executeFetchRequest:request error:&error];
// error handling code
[request release];
Is there a way to pass, say, the Object ID of a related object inside the NSPredicate, filtering based on that?
You can traverse relationships in an NSPredicate
. For example, you could write something like
[NSPredicate predicateWithFormat:@"title == %@ AND blockbuster.name LIKE \"Blockbuster C\"", @"Transformers 2"]
Now, if you don't have a property to compare against and you need to check actual objects, then you could use something like
[NSPredicate predicateWithFormat:@"title == %@ AND blockbuster IN %@", @"Transformers 2", setOfBlockbusters]
The full syntax is documented here. But setOfBlockbusters
could be a set, an array, or a dictionary (if it's a dictionary, the values, not the keys, are used).
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