Identifier of the role assignment.

def get_object_id(self, datum):
        """Identifier of the role assignment."""

        # Role assignment doesn't have identifier so one will be created
        # from the identifier of scope, user and role. This will guaranty the
        # unicity.
        scope_id = ""
        if "project" in datum.scope:
            scope_id = datum.scope["project"]["id"]
        elif "domain" in datum.scope:
            scope_id = datum.scope["domain"]["id"]

        assignee_id = ""
        if hasattr(datum, "user"):
            assignee_id = datum.user["id"]
        elif hasattr(datum, "group"):
            assignee_id = datum.group["id"]

        return "%s%s%s" % (assignee_id, datum.role["id"], scope_id)