fix: AnswerBuilder returned referenced documents in scrambled order

The referenced document indices were kept in a set and iterated directly, so the
returned documents came out in the set's hash-table order rather than ascending
source-index order (e.g. citations [3] [10] [50] yielded documents ordered 10, 3,
50). This order was deterministic but did not match the source order. Iterate the
indices sorted so documents follow ascending source index.

Author: otiscuilei

haystack/components/builders/answer_builder.py

@@ -222,7 +222,7 @@ def run(
                     else set(range(len(documents)))
                 )
 
-                for idx in doc_idxs:
+                for idx in sorted(doc_idxs):
                     try:
                         doc = documents[idx]
                     except IndexError:

releasenotes/notes/fix-answer-builder-doc-order-f63c572cd6560bf4.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed ``AnswerBuilder`` returning referenced documents in a scrambled order
+    instead of ascending source-index order. The referenced document indices
+    were collected in a ``set`` and iterated directly, so documents were emitted
+    in the set's internal hash-table order (e.g. citations [3] [10] [50]
+    yielded documents ordered 10, 3, 50). This order was deterministic but did
+    not match the intuitive source order. The referenced documents are now
+    returned sorted by their source index.

test/components/builders/test_answer_builder.py

@@ -146,6 +146,16 @@ def test_run_with_documents_with_reference_pattern(self):
         assert answers[0].documents[0].meta["referenced"] is True
         assert answers[0].documents[0].meta["source_index"] == 2
 
+    def test_run_returns_referenced_documents_in_source_order(self):
+        component = AnswerBuilder(reference_pattern="\\[(\\d+)\\]", return_only_referenced_documents=True)
+        output = component.run(
+            query="test query",
+            replies=["First [3], then [10], and finally [50]."],
+            documents=[Document(content=f"doc{i}") for i in range(1, 51)],
+        )
+        source_indices = [doc.meta["source_index"] for doc in output["answers"][0].documents]
+        assert source_indices == [3, 10, 50]
+
     def test_run_with_documents_with_reference_pattern_return_all_documents(self):
         component = AnswerBuilder(reference_pattern="\\[(\\d+)\\]", return_only_referenced_documents=False)
         output = component.run(