FIX: emit FETCH clause for qualify queries sliced with offset=0 (#525)

[Muhammad Faseeh (Faseeh06)]

Fixes #525

A Window-function .filter() (e.g. .annotate(row_number=Window(...)).filter(row_number=1)) routes SQLCompiler.as_sql() through the get_qualify_sql() branch, which wraps the query in a subquery and entirely bypasses the plain-SELECT TOP %d insertion further down in the method. The only place a limit/offset clause got appended afterward was the final if do_offset: block — gated solely on low_mark != 0.

So a qualify query sliced with offset=0 (qs[:50]) compiles with do_limit true but do_offset false: no limit clause is emitted anywhere, and the query returns every matching row instead of the requested slice. Non-zero offsets (qs[1:50]) already worked correctly, since do_offset was true for them — that's exactly the asymmetry described in the issue.

Changes

• mssql/compiler.py: emit the FETCH-only form of the offset/limit clause (self.connection.ops.limit_offset_sql(...), which already produces OFFSET 0 ROWS FETCH FIRST N ROWS ONLY for low_mark=0) for the qualify and do_limit and not do_offset case.
• Adds testapp/tests/test_compiler_qualify_limit.py with regression coverage for: the previously-broken offset=0 slice, the already-working non-zero-offset slice (guard against regressing it), and the unsliced case (confirming no limit clause is added when none was requested).

Test plan

• Reproduced the exact repro from the issue against current dev, confirmed the FETCH clause was missing before and is emitted correctly after
• New tests build the SQL text directly via str(queryset.query), which only invokes SQLCompiler.as_sql() and never opens a cursor — the one call that would otherwise need a live connection, sql_server_version, is pre-seeded through DatabaseWrapper's own version cache, so python manage.py test testapp.tests.test_compiler_qualify_limit passes with no database configured (Skipping setup of unused database(s): default, other, sqlite.)
• Confirmed the new tests fail on dev without this change and pass with it
• flake8 clean on both changed files

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Pull request overview

This PR fixes a SQL compilation bug in the SQL Server backend where qualified (window-function-filtered) querysets sliced with offset=0 (e.g. qs[:50]) could lose their row limit and return an unbounded result set. The change updates mssql/compiler.py to emit the OFFSET 0 ROWS FETCH FIRST N ROWS ONLY clause for the qualify + limit-only case, and adds regression tests in testapp/ to prevent recurrence.

Changes:

• Fix SQLCompiler.as_sql() to append limit_offset_sql() for qualify queries when do_limit is true and do_offset is false (i.e. low_mark == 0).
• Add regression tests covering [:N], [1:N], and the unsliced qualify query SQL output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mssql/compiler.py	Ensures qualify queries sliced with offset=0 still emit an outer limit via OFFSET 0 ... FETCH ....
testapp/tests/test_compiler_qualify_limit.py	Adds regression coverage validating SQL generation for qualify queries with/without slicing.

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[Gaurav Sharma (bewithgaurav)]

/azp run

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[Muhammad Faseeh (Faseeh06)]

@microsoft-github-policy-service agree

[github-actions]

📊 Code Coverage Report

🔥 Diff Coverage 100%

🎯 Overall Coverage 83.29%

📈 Total Lines Covered: 2533 out of 3041 📁 Project: mssql-django

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📋 Files Needing Attention

📉 Files with overall lowest coverage (click to expand)

- mssql/client.py: 19.5%  (41 lines)
- mssql/__init__.py: 50.0%  (2 lines)
- mssql/creation.py: 56.8%  (74 lines)
- mssql/operations.py: 78.8%  (396 lines)
- mssql/compiler.py: 82.3%  (646 lines)
- mssql/functions.py: 84.1%  (428 lines)
- mssql/base.py: 84.8%  (514 lines)
- mssql/schema.py: 88.3%  (719 lines)
- mssql/introspection.py: 90.2%  (133 lines)
- mssql/features.py: 98.9%  (88 lines)

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