Ory is the largest open source community in the world for cloud software application security. We maintain advanced open source security software solving authentication, authorization, access control, application network security, and delegation. Ory implements a variety of industry and best-practice standards including OAuth 2.0 / OAuth 2.1, OpenID Connect, Zero Trust Networking, Google Zanzibar Policy Framework, FIDO2 U2F, WebAuthn, TOTP, and more.

Ory Community

Is there a way to say that there can only be a single subject with specific relationship to given a object?

No, because Keto is ultimately a graph database. We could think about a constraints system like in a DB, but that would then go on top.