fix(MetaFieldRanker): apply missing_meta when no document has the field

[sclfcz]

Fixes #12952

What

MetaFieldRanker.run() returns the input documents unchanged when no document has the ranking field:

if len(docs_with_meta_field) == 0:
    logger.warning(...)
    return {"documents": deduplicated_documents[:top_k]}

That branch bypasses the configured missing_meta policy, so missing_meta="drop" stops filtering exactly in the case where everything should be dropped:

docs = [Document(id="a", content="a"), Document(id="b", content="b")]
MetaFieldRanker(meta_field="rating", missing_meta="drop").run(docs)["documents"]
# before: [a, b]   (expected [])
# after:  []

It is also inconsistent with the mixed case: with one rated document, the unrated ones are dropped as configured.

Change

Apply the configured policy in that branch too — drop returns an empty list, top/bottom keep returning the documents as before — and keep the existing warning, reworded to say that the policy is applied rather than that ranking is skipped. top_k is untouched for the top/bottom paths.

Testing

test/components/rankers/test_metafield.py::test_missing_meta_drop_when_no_document_has_the_field covers all three policies for the all-missing case; the mixed case is already covered by the existing tests, which are unchanged.

• with the change: 1 passed (-k missing_meta_drop_when_no_document)
• removing the drop branch: the same test fails
• test/components/rankers/test_metafield.py → 47 passed with the change (full file run in CI)

AI assistance

Written with an AI coding assistant (Claude-based agent): it located the branch, ran the before/after reproduction above and wrote the test. I reviewed the diff, the reproduction output and the reasoning, and I take responsibility for the change.

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[anakin87]

I did a minor adjustment.
Thank you

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/rankers
meta_field.py
Project Total

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