Question

I can’t figure out what @UnionOfObjects offers that a simple valueForKey: or valueForKeyPath: can’t do.

Apple docs:

The @unionOfObjects operator returns an array containing the distinct
objects in the property specified by the key path to the right of the
operator. Unlike “@distinctUnionOfObjects,” duplicate objects are not
removed. The following example returns the payee property values for
the transactions in transactions:

NSArray *payees = [transactions valueForKeyPath:@"@unionOfObjects.payee"];

The resulting payees array contains the following strings: Green
Power, Green Power, Green Power, Car Loan, Car Loan, Car Loan, General
Cable, General Cable, General Cable, Mortgage, Mortgage, Mortgage,
Animal Hospital.

In the above example,

NSArray *payees = [transactions valueForKey:@"payee"];

would return the same array of values, but with less code. What am I missing?

Answer 1

All I can think of immediately is that it “returns an array containing …” (emphasis mine). So it’ll be convenient for:

NSSet *someSet = ...;

NSArray *result = [someSet valueForKey:@"@unionOfObjects.whatever"];

It’s therefore useful anywhere in Cocoa bindings where you want an NSSet (or other non-array collection) to push data into an NSArray shaped hole.