Hi Georg,
See inline

On Wed, Oct 30, 2019 at 12:26 PM Georg Fette <georg.fe...@uni-wuerzburg.de>

> Hello,
> I would like to typecheck AQL queries and have some problems doing that:
> The items in a CLUSTER are of type ITEM. If I access
> myCluster/items[at0001]/value, is there any possibility to type-check
> the validity of this path without having the concrete archetype
> definition at hand? Just using the reference model isn't enough for this
> task, because ITEMs do not have a value-field.
> How can (from an object oriented point of view) the values of the ITEMs
> be accessed without knowing if it is an ELEMENT ?

They cannot be accessed. This is one corner case of two level modelling
when it is applied on an RM class that uses a field of abstract type, such
as an ITEM.
The archetypes end up setting the concrete type so from a type
checking/validation perspective, the fact that a field of type ITEM should
have  instances of type CLUSTER or ELEMENT are declared by the archetype.
So you cannot check it without using the archetype, or validate it for that
matter. You'd actually hit this way up in the RM tree before you make it to
types used at the leaves: COMPOSITION has its content field defined as
There are no perfect design decisions in computer science/engineering, you
win some, you lose some. Tom did the right thing by using abstract types
when following a OO design for RM. The constraint mechanism (ADL) has to
deal with this correct decision, modelling tools then in turn have to deal
with this, then AQL validators and programmers who write code against
serialised data.

Why is there a distiguishment between ELEMENT, ITEM and CLUSTER at all ?
> If the fields "items" and "value" were already attached to the class
> ITEM it would be easier.
CLUSTER is a container of actual data, which allows you to represent
hierarchical structures. ELEMENT is actual data.
All the use cases and justifications for the composite design pattern apply
here, because this is a textbook use of that pattern.

All the best

> Georg
> --
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