fix: Make output_type optional in MetadataRouter.from_dict for YAML loading (#9724)

* Make output type optional in yaml

* Add reno

Author: sjrl

haystack/components/routers/metadata_router.py

@@ -157,5 +157,7 @@ def from_dict(cls, data: dict[str, Any]) -> "MetadataRouter":
             The deserialized component instance.
         """
         init_params = data.get("init_parameters", {})
-        init_params["output_type"] = deserialize_type(init_params["output_type"])
+        if "output_type" in init_params:
+            # Deserialize the output_type to its original type
+            init_params["output_type"] = deserialize_type(init_params["output_type"])
         return default_from_dict(cls, data)

releasenotes/notes/metadata-router-output-type-optional-16a4d4d583202fde.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixed the `from_dict` method of `MetadataRouter` so the `output_type` parameter introduced in Haystack 2.17 is now optional when loading from YAML. This ensures compatibility with older Haystack pipelines.

test/components/routers/test_metadata_router.py

@@ -174,6 +174,27 @@ def test_from_dict_with_parameters(self):
         }
         assert router.output_type == list[Union[ByteStream, Document]]
 
+    def test_from_dict_no_output_type(self):
+        router_dict = {
+            "type": "haystack.components.routers.metadata_router.MetadataRouter",
+            "init_parameters": {
+                "rules": {
+                    "edge_1": {
+                        "operator": "AND",
+                        "conditions": [{"field": "meta.created_at", "operator": ">=", "value": "2025-02-01"}],
+                    }
+                }
+            },
+        }
+        router = MetadataRouter.from_dict(router_dict)
+        assert router.rules == {
+            "edge_1": {
+                "operator": "AND",
+                "conditions": [{"field": "meta.created_at", "operator": ">=", "value": "2025-02-01"}],
+            }
+        }
+        assert router.output_type == list[Document]
+
     def test_metadata_router_in_pipeline(self):
         document_store = InMemoryDocumentStore()
         p = Pipeline()