fix(caching): avoid storing metadata and results for CachingBehavior.IGNORE nodes

[Shoryamishra61]

Fixes #1704

Description

CachingBehavior.IGNORE is documented as "Results are never stored nor versioned" (adapter.py), but was previously included in the metadata storage branch in do_node_execute and the result storage branch in post_node_execute. Because pre_node_execute returns early for IGNORE nodes without creating a cache_key, storing metadata resulted in entries with cache_key = NULL. In SQLite, INSERT OR IGNORE does not treat NULL as duplicate, leading to orphaned metadata entries accumulating on every run for ignored nodes.

Changes

• In hamilton/caching/adapter.py:
  • do_node_execute: removed CachingBehavior.IGNORE from the metadata persistence block, restricting it to CachingBehavior.RECOMPUTE.
  • post_node_execute: removed CachingBehavior.IGNORE from the result store persistence block, keeping only DEFAULT and RECOMPUTE.
  • Kept node execution of IGNORE nodes and the "<ignore>" dependency version marker used by downstream nodes intact.
• In tests/caching/test_integration.py:
  • Updated test_ignore_behavior assertions to confirm that repeated executions with an ignored node do not increase metadata store size beyond the new <ignore> key created for downstream nodes.
  • Added test_ignore_behavior_from_start verifying that when a node is ignored from run 1, it writes 0 metadata entries and 0 result store entries, while downstream nodes can still cache under the <ignore> dependency key.

How I tested this

• Ran focused regression tests:
  pytest tests/caching/test_integration.py -k "test_ignore_behavior" -vv
• Ran full caching integration test suite:
  pytest tests/caching/test_integration.py -vv
• Ran full caching test suite:
  pytest tests/caching/ -vv
• Ran pre-commit hooks (ruff check, ruff format, license headers, etc.):
  pre-commit run --files hamilton/caching/adapter.py tests/caching/test_integration.py

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[Copilot]

Copilot review overview

🟡 Changes recommended

The moderate test assertion issue remains unresolved.

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Review effort: Lite
Findings: 1

Open (1)

• Assert result store contains only B's entry · New

What changed in this PR

Fixes CachingBehavior.IGNORE so ignored nodes do not persist cache metadata or results while downstream caching remains supported.

Changes:

• Restricts metadata persistence to RECOMPUTE.
• Restricts result persistence to DEFAULT and RECOMPUTE.
• Updates integration coverage for ignored nodes and downstream <ignore> keys.

Review finding: Moderate — Strengthen the assertion at tests/caching/test_integration.py:522 to verify the fresh result store contains exactly B’s single entry.

File	Summary
tests/​caching/​test_integration.py	Updates ignore-behavior expectations and adds regression coverage.
hamilton/​caching/​adapter.py	Prevents metadata and result persistence for ignored nodes.

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review comments, and regression coverage is included.

Review effort: Lite
Findings: None

Resolved since last review (1)

• Assert result store contains only B's entry

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved blocking issues were identified, and regression coverage is included.

Review effort: Lite
Findings: None