Obtaining an object identifier in lsfusion

Question

I have a Document class and I have assigned a unique identifier to each object in that class. But I cannot understand how to get the document by its id?

CLASS Document;
id(Document d) = DATA INTEGER (Document);

Answer 1

You can do this using GROUP operator:

document(id) = GROUP MAX Document d BY id(d);

or alternatively:

document(id) = GROUP AGGR Document d BY id(d);

In the latter example a constraint on the uniqueness of the id will be created. This will result in an error message if you try to create two documents with the same identifier.