comparying 2 array's with NSPredicate (cocoa)

Question

i have an NSObject with 2 property's

@interface Entity : NSObject  {

     NSNumber *nid;
     NSString *title;
}

i have 2 array's with Entity's objects in it and I want to compare those two on the nid with a predicate

array1: ({nid=1,title="test"},{nid=2,title="test2"})
array2: ({nid=2,title="test2"},{nid=3,title="test3"})

the 2 arrays have both a nid with value 2 so my output should but

array3: ({nid=2,title="test2"})

so i can produce an array with only matching nid's

Answer 1

The following code seems to work for me (it obviously leaks MyEntity objects but that was not the sample target):

NSArray* array1 = [NSArray arrayWithObjects:[[MyEntity alloc] initWithID:[NSNumber numberWithInt:1] title:@"1"], 
                   [[MyEntity alloc] initWithID:[NSNumber numberWithInt:2] title:@"2"], nil];

NSArray* array2 = [NSArray arrayWithObjects:[[MyEntity alloc] initWithID:[NSNumber numberWithInt:2] title:@"2"], 
                   [[MyEntity alloc] initWithID:[NSNumber numberWithInt:3] title:@"3"], nil];

NSArray* idsArray = [array1 valueForKey:@"nid"];
NSArray* filteredArray = [array2 filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:@"nid IN %@", idsArray]];

filteredArray contains entities whose ids present in both array.