refactor: Clarify error when right_path exists (#1555)

* refactor: Clarify error when right_path exists

Fixes #1552

This commit catches the assertion error when verifying the `right_path`
does not exist when a path has been identified on the left side of the
policy. This refactor was made to better clarify the cause of the error
for developers who may not be familiar with the technical limitations of
the current SQLAlchemy integration.

In my case, I had assumed that the use of foreign keys and paths on both
sides would be more performant, and thus the best choice for a policy,
when writing the authorization rules. The resulting error didn't include
any context about what was wrong with the policy I had written. This
commit aims to address this by providing a best effort guess to provide
a starting point for developers to debug the policy issue themselves.

Co-authored-by: gw <gwen@osohq.com>
Co-authored-by: Sam Scott <sam@osohq.com>

languages/python/sqlalchemy-oso/sqlalchemy_oso/partial.py

@@ -77,7 +77,7 @@
 from polar.partial import dot_path
 from polar.expression import Expression
 from polar.variable import Variable
-from polar.exceptions import UnsupportedError
+from polar.exceptions import UnsupportedError, PolarRuntimeError
 
 from sqlalchemy_oso.preprocess import preprocess
 
@@ -223,7 +223,11 @@ def translate_compare(expression: Expression, session: Session, model, get_model
     # Dot operation is on the left hand side
     if left_path[1:]:
         assert left_path[0] == Variable("_this")
-        assert not right_path
+        if right_path:
+            raise PolarRuntimeError(
+                "Invalid comparison in policy. This may be caused by comparing the "
+                + "foreign key field rather than the relationship property"
+            )
         path, field_name = left_path[1:-1], left_path[-1]
         return translate_dot(
             path,