fix: Repo.exists? generates valid SQL with SELECT 1

[ricardo-valero]

Summary

• Fix Repo.exists? generating invalid SQL with empty SELECT clause
• Add tests for Repo.exists?/2 functionality

Problem

Repo.exists? was generating invalid SQL:

SELECT  FROM "users" AS s0 WHERE (s0."age" > 25) LIMIT 1

Instead of:

SELECT 1 FROM "users" AS s0 WHERE (s0."age" > 25) LIMIT 1

This caused: SQLite failure: near "FROM": syntax error

Solution

Added a clause to handle empty field lists in select_fields/3:

defp select_fields(%{fields: []}, _sources, _query), do: "1"

This matches the behaviour of the ecto_sqlite3 adapter.

Testing

• Added comprehensive tests in test/exists_query_test.exs
• All existing tests still pass

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of empty field selections in SELECT queries to ensure valid output.

• Tests

  • Added comprehensive test coverage for exists? functionality, including edge cases with empty selections and complex query scenarios.

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[coderabbitai]

Walkthrough

The changes add a new private clause in the LibSQL adapter's select_fields function to handle empty field selections by returning "1" instead of constructing fields, and introduce a comprehensive test module validating Repo.exists?/2 behaviour across various query scenarios including empty selects and transactions.

Changes

Cohort / File(s)	Summary
LibSQL Adapter lib/ecto/adapters/libsql/connection.ex	Adds a new private clause to select_fields that returns "1" when handling empty field selections, providing a valid default for SELECT generation.
Test Suite test/exists_query_test.exs	New test module with comprehensive ExUnit tests for Repo.exists?/2 functionality across empty selects, simple/complex where clauses, transactions, and includes test setup with TestRepo and User schema.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Poem

🐰 In fields so empty, "1" shall bloom,
Our tests hop through the query room,
LibSQL's path now smooth and bright,
Exists? Exists! We checked it right! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding support for generating valid SQL with SELECT 1 when Repo.exists? has empty field selections.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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