Extend existing relation by modules

[thomas-rudlof-bl]

In our system, we have a resource that shall be capable to inherit its access from other resources "using" this resource as sub-resource. These other resources are provided and managed by independent applications and use their own types and relations for modelling the access to their resources.

Our service managing the shared resources should be able to use a single access model for all its resources.

Each application using these resources should be able to extend this model with its own modules.

So it would be nice to be able to do something like this:

Define a core module with the shared resource type:

module core

type user
  
type shared-sub-resource
  relations
    define can_access: [user] shared-subresource # in core module, only direct acl is supported

And for each application, define a separate module for the application specific resources and their access model, like e.g.:

module application1

type resource1
  relations
    define can_access: [user] resource1 # this could be, of course, far more complex and application specific
 
extend type shared-sub-resource
  relations
    define resource1_parent: [resource1]
    extend can_access:  can_access from resource1_parent # this should form a union with the already defined
                                                         # relation in the core module and all other extension of this
                                                         # relation in other modules