Support multiple different operators in the same relation.

[damianh]

(Originally posted to do discord, posting here as requested).

A use case for supporting multiple different operators in the same relation:

Given this model

type solution
  relations
    define application-viewer as self
    define owner as self

type application
  relations
    define owner as self
    define blocked as self 
    define viewer as self or application-viewer from owner

and the following tuples:

• Solution foo has a relation of owner with application bar. (tuple: "solution:foo", "owner", "application:bar")
• Solution foo has a relation of owner with application baz. (tuple: "solution:foo", "owner", "application:baz")
• An end user alice has a relation of application-view to solution foo. (tuple: "alice", "application-viewer", "solution:foo")

With this, alice as access to all applications in a solution.

• "is alice related to application:bar as viewer?" ✅
• "is alice related to application:baz as viewer?" ✅

So now I want to explicitly block alice from application:baz:

• Add tuple: "alice", "blocked", "application:baz"

Now the model needs to look like:

type solution
  relations
    define application-viewer as self
    define owner as self
type application
  relations
    define owner as self
    define blocked as self
    define viewer as self or application-viewer from owner
    define can_view as viewer but not blocked

However the but not blocked above is invalid syntax.

A work around was provided by @rhamzeh (thanks for quick response there !!) but would be nicer to have support in the DSL.

Thank you!

[aaguiarz]

Thanks @damianh, we are considering this change as part of our DSL improvements.

[aaguiarz]

This was implemented as part of this PR openfga/language#107