fix: make JoinDocuments correctly handle duplicate documents w null scores (#6261)

* fix error with null values

* release note

* simplify

haystack/nodes/other/join_docs.py

@@ -14,8 +14,8 @@ class JoinDocuments(JoinNode):
     A node to join documents outputted by multiple retriever nodes.
 
     The node allows multiple join modes:
-    * concatenate: combine the documents from multiple nodes. Any duplicate documents are discarded.
-                   The score is only determined by the last node that outputs the document.
+    * concatenate: combine the documents from multiple nodes.
+                   In case of duplicate documents, the one with the highest score is kept.
     * merge: merge scores of documents from multiple nodes. Optionally, each input score can be given a different
              `weight` & a `top_k` limit can be set. This mode can also be used for "reranking" retrieved documents.
     * reciprocal_rank_fusion: combines the documents based on their rank in multiple nodes.
@@ -130,7 +130,7 @@ class JoinDocuments(JoinNode):
                 for doc in result:
                     if doc.id == idx:
                         tmp.append(doc)
-            item_best_score = max(tmp, key=lambda x: x.score)
+            item_best_score = max(tmp, key=lambda x: x.score if x.score is not None else -inf)
             scores_map.update({idx: item_best_score.score})
         return scores_map
 

releasenotes/notes/fix-join-docs-null-score-746c392a87adffcc.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    When using `JoinDocuments` with `join_mode=concatenate` (default) and
+    passing duplicate documents, including some with a null score, this
+    node raised an exception.
+    Now this case is handled correctly and the documents are joined as expected.

test/nodes/test_join_documents.py

@@ -78,3 +78,38 @@ def test_joindocuments_concatenate_keep_only_highest_ranking_duplicate():
     result, _ = join_docs.run(inputs)
     assert len(result["documents"]) == 2
     assert result["documents"] == expected_outputs["documents"]
+
+
+@pytest.mark.unit
+def test_joindocuments_concatenate_duplicate_docs_null_score():
+    """
+    Test that the concatenate method correctly handles duplicate documents,
+    when one has a null score.
+    """
+    inputs = [
+        {
+            "documents": [
+                Document(content="text document 1", content_type="text", score=0.2),
+                Document(content="text document 2", content_type="text", score=0.3),
+                Document(content="text document 3", content_type="text", score=None),
+            ]
+        },
+        {
+            "documents": [
+                Document(content="text document 2", content_type="text", score=0.7),
+                Document(content="text document 1", content_type="text", score=None),
+            ]
+        },
+    ]
+    expected_outputs = {
+        "documents": [
+            Document(content="text document 2", content_type="text", score=0.7),
+            Document(content="text document 1", content_type="text", score=0.2),
+            Document(content="text document 3", content_type="text", score=None),
+        ]
+    }
+
+    join_docs = JoinDocuments(join_mode="concatenate")
+    result, _ = join_docs.run(inputs)
+    assert len(result["documents"]) == 3
+    assert result["documents"] == expected_outputs["documents"]