[news.eclipse.tools.emf] Re: Simple newbie question: Backreferences?

[Ed Merks <merks@xxxxxxxxxx>]

Michael,

Comments below.


Michael Lehmeier wrote:
> I need the following:
> I want a class A that has references to class B.
> B in turn has a reference to class A.
>
> I made a km3-file:
>
>  package somepackage {
>   class A {
>    attribute aName : String;
>    reference bspReferences [*] container : B oppositeOf refDestination;
>    reference backDestination [0-1] : B oppositeOf backReference;
>   }
>   class B {
>    attribute bName : String;
>    reference refDestination [0-1] : A oppositeOf bspReferences;
>    reference backReference container : A oppositeOf backDestination;
>   }
>  }
>  package PrimitiveTypes {
>   datatype String;
>  }
>
> Then I transformed it into an ecore file, fixed it, made a genmodel and
> then an editor.
>   
What was broken?
> When I used the editor to make a model for this I noticed that I can't
> reference from B back to A.
>   
Via which of the two references?
> I have to create a completely new A that is a child of the B.
>   
Generally we don't show container references in the properties view 
since they are implicit in the tree structure of the tree.  But you can 
modified the corresponding GenFeature's Property Type to make it 
Editable. I'm not familiar with the syntax so I'm not actually sure 
which which references are containments.  In Ecore we call the opposite 
of a containment reference a container references (it points up to the 
container) but given that a container references can't be multiplicity 
many (you can only have one container in a tree structure) I have the 
feeling that container means the opposite of what I would expect...
> This really isn't what I wanted.
> Is there no way to create a "true" reference, so that, for example, 
> one class instance is referenced by more than one other class instance?
>   
It's definitely possible.  Just have a look at the library model from 
the tutorial with links between books and writers.
> If this isn't possible I will have a big problem in the project that I
> am working on. :(
>
>   
It's definitely possible, but I'm slightly confused by what the syntax 
above maps to in Ecore so I have a bit of trouble giving better 
details.  Just keep in mind that the opposite of a containment reference 
must be multiplicity one because adding an object to a containment 
reference will always remove it from it's existing container and set it 
to the new one. 

Actually, now that I get to the end and think about it, if A contains B, 
the editor is not going to allow you to also make B contain A; that 
would create a circular tree, which of course isn't a tree a tall.  So 
it's unlikely you want both your references to be containment 
references.  So think about the serialization structure and whether the 
Bs will be nested under the As or vice versa; whichever way you choose, 
that's the one that should be a containment reference.