feat: allow DocumentJoiner to accept top_k parameter in run method (#7709)

* feat: allow DocumentJoiner to accept top_k parameter in run method

* Added release note for DocumentJoiner top_k fix

haystack/components/joiners/document_joiner.py

@@ -74,12 +74,14 @@ class DocumentJoiner:
         self.sort_by_score = sort_by_score
 
     @component.output_types(documents=List[Document])
-    def run(self, documents: Variadic[List[Document]]):
+    def run(self, documents: Variadic[List[Document]], top_k: Optional[int] = None):
         """
         Joins multiple lists of Documents into a single list depending on the `join_mode` parameter.
 
         :param documents:
             List of list of Documents to be merged.
+        :param top_k:
+            The maximum number of Documents to return. Overrides the instance's `top_k` if provided.
 
         :returns:
             A dictionary with the following keys:
@@ -103,8 +105,11 @@ class DocumentJoiner:
                     "score, so those with score=None were sorted as if they had a score of -infinity."
                 )
 
-        if self.top_k:
+        if top_k:
+            output_documents = output_documents[:top_k]
+        elif self.top_k:
             output_documents = output_documents[: self.top_k]
+
         return {"documents": output_documents}
 
     def _concatenate(self, document_lists):

releasenotes/notes/fix-documentjoiner-topk-173141a894e5c093.yaml

@@ -0,0 +1,5 @@
+---
+
+enhancements:
+  - |
+     The `DocumentJoiner` component's `run` method now accepts a `top_k` parameter, allowing users to specify the maximum number of documents to return at query time. This fixes issue #7702.

test/components/joiners/test_document_joiner.py

@@ -115,6 +115,14 @@ class TestDocumentJoiner:
         ]
         assert all(doc.id in expected_document_ids for doc in output["documents"])
 
+    def test_run_with_top_k_in_run_method(self):
+        joiner = DocumentJoiner()
+        documents_1 = [Document(content="a"), Document(content="b"), Document(content="c")]
+        documents_2 = [Document(content="d"), Document(content="e"), Document(content="f")]
+        top_k = 4
+        output = joiner.run([documents_1, documents_2], top_k=top_k)
+        assert len(output["documents"]) == top_k
+
     def test_sort_by_score_without_scores(self, caplog):
         joiner = DocumentJoiner()
         with caplog.at_level(logging.INFO):