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

haystack/components/rankers/meta_field.py

@@ -258,16 +258,17 @@ def run(
         docs_with_meta_field = [doc for doc in deduplicated_documents if doc.meta.get(self.meta_field) is not None]
         docs_missing_meta_field = [doc for doc in deduplicated_documents if doc.meta.get(self.meta_field) is None]
 
-        # If all docs are missing self.meta_field return original documents
         if len(docs_with_meta_field) == 0:
             logger.warning(
                 "The parameter <meta_field> is currently set to '{meta_field}', but none of the provided "
                 "Documents with IDs {document_ids} have this meta key.\n"
                 "Set <meta_field> to the name of a field that is present within the provided Documents.\n"
-                "Returning the <top_k> of the original Documents since there are no values to rank.",
+                "Applying the configured <missing_meta> policy instead of ranking.",
                 meta_field=self.meta_field,
                 document_ids=",".join([doc.id for doc in deduplicated_documents]),
             )
+            if missing_meta == "drop":
+                return {"documents": []}
             return {"documents": deduplicated_documents[:top_k]}
 
         if len(docs_missing_meta_field) > 0:

releasenotes/notes/fix-metafield-ranker-missing-meta-99197f6d6faaf8e3.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix ``MetaFieldRanker`` to return no documents when ``missing_meta="drop"``
+    and all documents lack the ranking field or have a ``None`` value.

test/components/rankers/test_metafield.py

@@ -338,3 +338,22 @@ def test_none_meta_value_is_handled_as_missing_meta(self, missing_meta, expected
         ]
         output = ranker.run(documents=docs_before)
         assert [doc.id for doc in output["documents"]] == expected_ids
+
+    @pytest.mark.parametrize(
+        ("meta", "init_missing_meta", "run_missing_meta", "expected_ids"),
+        [
+            ({}, "drop", None, []),
+            ({}, "top", None, ["a", "b"]),
+            ({}, "bottom", None, ["a", "b"]),
+            ({}, "bottom", "drop", []),
+            ({}, "drop", "top", ["a", "b"]),
+            ({"rating": None}, "drop", None, []),
+        ],
+    )
+    def test_missing_meta_when_all_values_are_missing(self, meta, init_missing_meta, run_missing_meta, expected_ids):
+        ranker = MetaFieldRanker(meta_field="rating", missing_meta=init_missing_meta)
+        docs = [Document(id="a", content="a", meta=meta), Document(id="b", content="b", meta=meta)]
+
+        output = ranker.run(documents=docs, missing_meta=run_missing_meta)
+
+        assert [doc.id for doc in output["documents"]] == expected_ids