Fix remote_side for composite self-referential FK targeting a UNIQUE constraint (#484)

[NixBiks]

Changes

Fixes #484.

When a table has a self-referential foreign key that targets a UNIQUE constraint instead of the primary key, the generated remote_side on the many-to-one side named the primary key columns. Those columns take no part in the join, so mapper configuration failed:

sqlalchemy.exc.ArgumentError: Relationship Category.category could not determine any
unambiguous local/remote column pairs based on join condition and remote_side arguments.

The generated module still imports cleanly, so an import-only smoke test does not catch this; the error only appears on the first query or on configure_mappers().

The cause is in DeclarativeGenerator.generate_relationships, which built remote_side from constraint.referred_table.primary_key rather than from the columns the foreign key refers to. It now uses constraint.elements, so remote_side names the referred columns. Where the foreign key does target the primary key the two are the same, and the output does not change.

Checklist

• You've added tests (in tests/) which would fail without your patch — test_onetomany_selfref_composite_unique_target in tests/test_generator_declarative.py, covering a composite self-referential foreign key against a UNIQUE constraint. It fails on master with remote_side=[id] and passes with the fix. Full suite: 158 passed; pre-commit clean.
• You've added a new changelog entry (in CHANGES.rst).

[coveralls]

coverage: 97.798% (+0.004%) from 97.794% — NixBiks:fix/484-composite-self-fk-unique-target into agronholm:master

[sheinbergon]

10x @NixBiks

[agronholm]

Thanks!