fix(sqlserver): use not_in for $nin metadata filter operator

[DevNinja (0xDevNinja)]

Description

Re-opens #812, which was merged and then reverted in #852 because the test it added broke CI.

The original bug (still present on main after the revert): _handle_field_filter handles the $nin operator with

return queried_field.nin_([str(val) for val in filter_value])

SQLAlchemy has no nin_ method, so any $nin metadata filter raises AttributeError at query time. The supported API is not_in.

What caused the revert, and the fix

tests/unit_tests/test_filters.py builds the store with __new__ (to avoid needing a live DB), so it must set every attribute _get_embedding_store reads. Between when #812 was opened and when it merged, the use_binary_collation work (#801) landed and made _get_embedding_store read self._use_binary_collation. The test helper did not set it, so once #812 merged, main failed with:

AttributeError: SQLServerVectorStore object has no attribute _use_binary_collation

This PR is the original one-line source fix plus the test helper now setting _use_binary_collation = False. Rebased on current main, so it accounts for the collation change.

Testing

tests/unit_tests/test_filters.py covers $nin, $in and $like by compiling the filter expression to SQL, so no live database is required. Verified against current main: full sqlserver unit suite passes (17 passed, 1 skipped) and make lint (ruff + ruff format + mypy) is clean.

Sorry for the CI breakage on the first attempt -- I have double-checked this one against the post-collation main.

[Copilot]

Pull request overview

Fixes SQL Server metadata filtering for the $nin operator by using SQLAlchemy’s supported not_in operator, and reintroduces unit coverage that validates the compiled SQL without requiring a live database.

Changes:

• Replace the invalid nin_ call with not_in for $nin metadata filters in _handle_field_filter.
• Add unit tests that compile filter expressions to MSSQL SQL to validate $nin, $in, and $like.
• Update the test store factory to set _use_binary_collation so _get_embedding_store works when constructing via __new__.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
libs/sqlserver/langchain_sqlserver/vectorstores.py	Fix $nin implementation to use not_in instead of a nonexistent nin_ method.
libs/sqlserver/tests/unit_tests/test_filters.py	Add compile-only unit tests for $nin/$in/$like and ensure _use_binary_collation is set in the test helper.

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