fixes #4212 raise an error if selectable and flat/aliased are sent to with_polymorphic simultaenously

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Hi,
This is a proposed fix for Issue #4212

When the selectable argument is passed to with_polymorphic it is already aliased. Therefore an argument error should be raised if flat is also True. I have added an if statement to the with_polymorphic function, that raises an Argument Error if selectable is none/False AND if flat=True. I have also included a test.

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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [X ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Kind Regards,

Ramon

Closes: #5232
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5232
Pull-request-sha: eae20cdadac82ca1e0352000fe0211e20b590e8b

Change-Id: I49c3022296608ede43de7f508fa993fa97dda81a

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index 16aa98adb..28c311450 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -945,6 +945,13 @@ def with_polymorphic(
        only be specified if querying for one specific subtype only
     """
     primary_mapper = _class_to_mapper(base)
+
+    if selectable not in (None, False) and flat:
+        raise sa_exc.ArgumentError(
+            "the 'flat' and 'selectable' arguments cannot be passed "
+            "simultaneously to with_polymorphic()"
+        )
+
     if _existing_alias:
         assert _existing_alias.mapper is primary_mapper
         classes = util.to_set(classes)