Transform object into integer

Let's say my object has a property which is a RLMArray of another object. I want to map each object of this array into a single int (which is its id) in the resulting JSON. How would I do that? Is that possible?

Something like:

```
[object1, object2, object3] => [1, 2, 3]
```

... where 1 is the `object1.identifier`, 2 is the `object2.identifier` etc.
