test.adb

type Model is interface;

type Model_Reference is access all Model'Class;

type Container is tagged record
   Contents : Model_Reference;
end record;

procedure Set_Model (Object : in out Container; Value : Model'Class) is
begin
   -- not working because initialization is missing:
   Object.Contents := new Model'Class;
   
   -- not working because aggregate expects (unknown) values of Model's fields
   Object.Contents := new Model'Class'(Value);
end Set_Model;