feat: Add options for what to do with missing metadata fields in MetaFieldRanker (#7700)

* Add `missing_meta` param to `MetaFieldRanker`, plus checks for validation.

* Implement `missing_meta` functionality in `run()`.

* Finish first draft of revised `MetaFieldRanker` functionality.

* Add tests for `MetaFieldRanker` `missing_meta` functionality.

* Add `missing_meta` param to `MetaFieldRanker`, plus checks for validation.

* Implement `missing_meta` functionality in `run()`.

* Finish first draft of revised `MetaFieldRanker` functionality.

* Add tests for `MetaFieldRanker` `missing_meta` functionality.

* Add release notes for new `missing_meta` param of `MetaFieldRanker`

* Move part of docs_missing_meta_field warning string outside of `if...elif...else`.

haystack/components/rankers/meta_field.py

@@ -43,6 +43,7 @@ class MetaFieldRanker:
         top_k: Optional[int] = None,
         ranking_mode: Literal["reciprocal_rank_fusion", "linear_score"] = "reciprocal_rank_fusion",
         sort_order: Literal["ascending", "descending"] = "descending",
+        missing_meta: Literal["drop", "top", "bottom"] = "bottom",
         meta_value_type: Optional[Literal["float", "int", "date"]] = None,
     ):
         """
@@ -65,6 +66,14 @@ class MetaFieldRanker:
         :param sort_order:
             Whether to sort the meta field by ascending or descending order.
             Possible values are `descending` (default) and `ascending`.
+        :param missing_meta:
+            What to do with documents that are missing the sorting metadata field.
+            Possible values are:
+            - 'drop' will drop the documents entirely.
+            - 'top' will place the documents at the top of the metadata-sorted list
+                (regardless of 'ascending' or 'descending').
+            - 'bottom' will place the documents at the bottom of metadata-sorted list
+                (regardless of 'ascending' or 'descending').
         :param meta_value_type:
             Parse the meta value into the data type specified before sorting.
             This will only work if all meta values stored under `meta_field` in the provided documents are strings.
@@ -82,11 +91,13 @@ class MetaFieldRanker:
         self.top_k = top_k
         self.ranking_mode = ranking_mode
         self.sort_order = sort_order
+        self.missing_meta = missing_meta
         self._validate_params(
             weight=self.weight,
             top_k=self.top_k,
             ranking_mode=self.ranking_mode,
             sort_order=self.sort_order,
+            missing_meta=self.missing_meta,
             meta_value_type=meta_value_type,
         )
         self.meta_value_type = meta_value_type
@@ -97,6 +108,7 @@ class MetaFieldRanker:
         top_k: Optional[int],
         ranking_mode: Literal["reciprocal_rank_fusion", "linear_score"],
         sort_order: Literal["ascending", "descending"],
+        missing_meta: Literal["drop", "top", "bottom"],
         meta_value_type: Optional[Literal["float", "int", "date"]],
     ):
         if top_k is not None and top_k <= 0:
@@ -125,6 +137,14 @@ class MetaFieldRanker:
                 "MetaFieldRanker." % sort_order
             )
 
+        if missing_meta not in ["drop", "top", "bottom"]:
+            raise ValueError(
+                "The value of parameter <missing_meta> must be 'drop', 'top', or 'bottom', "
+                "but is currently set to '%s'.\n"
+                "Change the <missing_meta> value to 'drop', 'top', or 'bottom' when initializing the "
+                "MetaFieldRanker." % missing_meta
+            )
+
         if meta_value_type not in ["float", "int", "date", None]:
             raise ValueError(
                 "The value of parameter <meta_value_type> must be 'float', 'int', 'date' or None but is "
@@ -141,6 +161,7 @@ class MetaFieldRanker:
         weight: Optional[float] = None,
         ranking_mode: Optional[Literal["reciprocal_rank_fusion", "linear_score"]] = None,
         sort_order: Optional[Literal["ascending", "descending"]] = None,
+        missing_meta: Optional[Literal["drop", "top", "bottom"]] = None,
         meta_value_type: Optional[Literal["float", "int", "date"]] = None,
     ):
         """
@@ -171,6 +192,15 @@ class MetaFieldRanker:
             Whether to sort the meta field by ascending or descending order.
             Possible values are `descending` (default) and `ascending`.
             If not provided, the sort_order provided at initialization time is used.
+        :param missing_meta:
+            What to do with documents that are missing the sorting metadata field.
+            Possible values are:
+            - 'drop' will drop the documents entirely.
+            - 'top' will place the documents at the top of the metadata-sorted list
+                (regardless of 'ascending' or 'descending').
+            - 'bottom' will place the documents at the bottom of metadata-sorted list
+                (regardless of 'ascending' or 'descending').
+            If not provided, the missing_meta provided at initialization time is used.
         :param meta_value_type:
             Parse the meta value into the data type specified before sorting.
             This will only work if all meta values stored under `meta_field` in the provided documents are strings.
@@ -199,12 +229,14 @@ class MetaFieldRanker:
         weight = weight if weight is not None else self.weight
         ranking_mode = ranking_mode or self.ranking_mode
         sort_order = sort_order or self.sort_order
+        missing_meta = missing_meta or self.missing_meta
         meta_value_type = meta_value_type or self.meta_value_type
         self._validate_params(
             weight=weight,
             top_k=top_k,
             ranking_mode=ranking_mode,
             sort_order=sort_order,
+            missing_meta=missing_meta,
             meta_value_type=meta_value_type,
         )
 
@@ -227,13 +259,27 @@ class MetaFieldRanker:
             return {"documents": documents[:top_k]}
 
         if len(docs_missing_meta_field) > 0:
-            logger.warning(
+            warning_start = (
-                "The parameter <meta_field> is currently set to '{meta_field}' but the Documents with IDs {document_ids} don't have this meta key.\n"
+                f"The parameter <meta_field> is currently set to '{self.meta_field}' but the Documents "
-                "These Documents will be placed at the end of the sorting order.",
+                f"with IDs {','.join([doc.id for doc in docs_missing_meta_field])} don't have this meta key.\n"
-                meta_field=self.meta_field,
-                document_ids=",".join([doc.id for doc in docs_missing_meta_field]),
             )
 
+            if missing_meta == "bottom":
+                logger.warning(
+                    "{warning_start}Because the parameter <missing_meta> is set to 'bottom', these Documents will be placed at the end of the sorting order.",
+                    warning_start=warning_start,
+                )
+            elif missing_meta == "top":
+                logger.warning(
+                    "{warning_start}Because the parameter <missing_meta> is set to 'top', these Documents will be placed at the top of the sorting order.",
+                    warning_start=warning_start,
+                )
+            else:
+                logger.warning(
+                    "{warning_start}Because the parameter <missing_meta> is set to 'drop', these Documents will be removed from the list of retrieved Documents.",
+                    warning_start=warning_start,
+                )
+
         # If meta_value_type is provided try to parse the meta values
         parsed_meta = self._parse_meta(docs_with_meta_field=docs_with_meta_field, meta_value_type=meta_value_type)
         tuple_parsed_meta_and_docs = list(zip(parsed_meta, docs_with_meta_field))
@@ -252,10 +298,18 @@ class MetaFieldRanker:
             )
             return {"documents": documents[:top_k]}
 
-        # Add the docs missing the meta_field back on the end
+        # Merge rankings and handle missing meta fields as specified in the missing_meta parameter
         sorted_by_meta = [doc for meta, doc in tuple_sorted_by_meta]
-        sorted_documents = sorted_by_meta + docs_missing_meta_field
+        if missing_meta == "bottom":
-        sorted_documents = self._merge_rankings(documents, sorted_documents, weight, ranking_mode)
+            sorted_documents = sorted_by_meta + docs_missing_meta_field
+            sorted_documents = self._merge_rankings(documents, sorted_documents, weight, ranking_mode)
+        elif missing_meta == "top":
+            sorted_documents = docs_missing_meta_field + sorted_by_meta
+            sorted_documents = self._merge_rankings(documents, sorted_documents, weight, ranking_mode)
+        else:
+            sorted_documents = sorted_by_meta
+            sorted_documents = self._merge_rankings(docs_with_meta_field, sorted_documents, weight, ranking_mode)
+
         return {"documents": sorted_documents[:top_k]}
 
     def _parse_meta(

releasenotes/notes/metaranker-missing-meta-options-1a969008d632a523.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Add a new `missing_meta` param to `MetaFieldRanker`, which determines what to do with
+    documents that lack the ranked meta field. Supported values are `"bottom"` (which
+    puts documents with missing meta at the bottom of the sorted list), `"top"` (which puts them
+    at the top), and `"drop"` (which removes them from the results entirely).

test/components/rankers/test_metafield.py

@@ -175,6 +175,10 @@ class TestMetaFieldRanker:
         with pytest.raises(ValueError):
             MetaFieldRanker(meta_field="rating", sort_order="wrong_order")
 
+    def test_raises_value_error_if_wrong_missing_meta(self):
+        with pytest.raises(ValueError):
+            MetaFieldRanker(meta_field="rating", missing_meta="wrong_missing_meta")
+
     def test_raises_value_error_if_wrong_meta_value_type(self):
         with pytest.raises(ValueError):
             MetaFieldRanker(meta_field="rating", meta_value_type="wrong_type")
@@ -239,3 +243,39 @@ class TestMetaFieldRanker:
         output = ranker.run(documents=docs_before, ranking_mode="reciprocal_rank_fusion")
         docs_after = output["documents"]
         assert docs_after[0].score == pytest.approx(0.016261, abs=1e-5)
+
+    def test_missing_meta_bottom(self):
+        ranker = MetaFieldRanker(meta_field="rating", ranking_mode="linear_score", weight=0.5, missing_meta="bottom")
+        docs_before = [
+            Document(id="1", content="abc", meta={"rating": 1.3}, score=0.6),
+            Document(id="2", content="abc", meta={}, score=0.4),
+            Document(id="3", content="abc", meta={"rating": 2.1}, score=0.39),
+        ]
+        output = ranker.run(documents=docs_before)
+        docs_after = output["documents"]
+        assert len(docs_after) == 3
+        assert docs_after[2].id == "2"
+
+    def test_missing_meta_top(self):
+        ranker = MetaFieldRanker(meta_field="rating", ranking_mode="linear_score", weight=0.5, missing_meta="top")
+        docs_before = [
+            Document(id="1", content="abc", meta={"rating": 1.3}, score=0.6),
+            Document(id="2", content="abc", meta={}, score=0.59),
+            Document(id="3", content="abc", meta={"rating": 2.1}, score=0.4),
+        ]
+        output = ranker.run(documents=docs_before)
+        docs_after = output["documents"]
+        assert len(docs_after) == 3
+        assert docs_after[0].id == "2"
+
+    def test_missing_meta_drop(self):
+        ranker = MetaFieldRanker(meta_field="rating", ranking_mode="linear_score", weight=0.5, missing_meta="drop")
+        docs_before = [
+            Document(id="1", content="abc", meta={"rating": 1.3}, score=0.6),
+            Document(id="2", content="abc", meta={}, score=0.59),
+            Document(id="3", content="abc", meta={"rating": 2.1}, score=0.4),
+        ]
+        output = ranker.run(documents=docs_before)
+        docs_after = output["documents"]
+        assert len(docs_after) == 2
+        assert "2" not in [doc.id for doc in docs_after]