fix: preserve root_node in JoinNode's output (#4820)

* preserve root_node and add tests

* Added if statement to fix failing tests

---------

Co-authored-by: Silvano Cerza <3314350+silvanocerza@users.noreply.github.com>
Co-authored-by: Sebastian Husch Lee <sjrl423@gmail.com>

haystack/nodes/other/join.py

@@ -21,7 +21,10 @@ class JoinNode(BaseComponent):
         top_k_join: Optional[int] = None,
     ) -> Tuple[Dict, str]:
         if inputs:
-            return self.run_accumulated(inputs, top_k_join=top_k_join)
+            results = self.run_accumulated(inputs, top_k_join=top_k_join)
+            if "root_node" in inputs[0]:
+                results[0]["root_node"] = inputs[0]["root_node"]
+            return results
         warnings.warn("You are using a JoinNode with only one input. This is usually equivalent to a no-op.")
         return self.run_accumulated(
             inputs=[
@@ -55,7 +58,10 @@ class JoinNode(BaseComponent):
         top_k_join: Optional[int] = None,
     ) -> Tuple[Dict, str]:
         if inputs:
-            return self.run_batch_accumulated(inputs=inputs, top_k_join=top_k_join)
+            results = self.run_batch_accumulated(inputs=inputs, top_k_join=top_k_join)
+            if "root_node" in inputs[0]:
+                results[0]["root_node"] = inputs[0]["root_node"]
+            return results
         warnings.warn("You are using a JoinNode with only one input. This is usually equivalent to a no-op.")
         return self.run_batch_accumulated(
             inputs=[

test/nodes/test_join_answers.py

@@ -17,3 +17,15 @@ def test_joinanswers(join_mode):
     result, _ = join_answers.run(inputs, top_k_join=1)
     assert len(result["answers"]) == 1
     assert result["answers"][0].answer == "answer 2"
+
+
+@pytest.mark.unit
+def test_joinanswers_preserves_root_node():
+    # https://github.com/deepset-ai/haystack-private/issues/51
+    inputs = [
+        {"answers": [Answer(answer="answer 1", score=0.7)], "root_node": "Query"},
+        {"answers": [Answer(answer="answer 2", score=0.8)], "root_node": "Query"},
+    ]
+    join_docs = JoinAnswers()
+    result, _ = join_docs.run(inputs)
+    assert result["root_node"] == "Query"

test/nodes/test_join_documents.py

@@ -42,3 +42,15 @@ def test_joindocuments_score_none(join_mode, sort_by_score):
 
     result, _ = join_docs.run(inputs, top_k_join=1)
     assert len(result["documents"]) == 1
+
+
+@pytest.mark.unit
+def test_joindocuments_preserves_root_node():
+    # https://github.com/deepset-ai/haystack-private/issues/51
+    inputs = [
+        {"documents": [Document(content="text document 1", content_type="text", score=0.2)], "root_node": "File"},
+        {"documents": [Document(content="text document 2", content_type="text", score=None)], "root_node": "File"},
+    ]
+    join_docs = JoinDocuments()
+    result, _ = join_docs.run(inputs)
+    assert result["root_node"] == "File"