From 4a7f6c68aef0260588549afd3c3f4c4ccd682829 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A9=20Ahlert?= <andre@aex.partners>
Date: Wed, 18 Mar 2026 11:09:42 -0300
Subject: [PATCH] fix: pass partition_key to child PointerModel in fork/spawn
 relationships

The _log_child_relationships function was hardcoding partition_key=None
when creating child PointerModels in children.jsonl. This caused the UI
to generate broken links for forked/spawned apps that use partition keys,
since the route includes the partition_key segment.

Closes #518
---
 burr/tracking/client.py                      |  6 +-
 tests/tracking/test_local_tracking_client.py | 67 ++++++++++++++++++++
 2 files changed, 71 insertions(+), 2 deletions(-)

diff --git a/burr/tracking/client.py b/burr/tracking/client.py
index 44919aed5..5375cdb4b 100644
--- a/burr/tracking/client.py
+++ b/burr/tracking/client.py
@@ -207,6 +207,7 @@ def _log_child_relationships(
         fork_parent_pointer_model: Optional[burr_types.ParentPointer],
         spawn_parent_pointer_model: Optional[burr_types.ParentPointer],
         app_id: str,
+        partition_key: Optional[str] = None,
     ):
         """Logs a child relationship. This is special as it does not log to the main log file. Rather
         it logs within the parent directory. Note this only exists to maintain (denormalized) bidirectional
@@ -227,7 +228,7 @@ def _log_child_relationships(
                         child=PointerModel(
                             app_id=app_id,
                             sequence_id=None,
-                            partition_key=None,  # TODO -- get partition key
+                            partition_key=partition_key,
                         ),
                         event_time=datetime.datetime.now(),
                         event_type="fork",
@@ -245,7 +246,7 @@ def _log_child_relationships(
                         child=PointerModel(
                             app_id=app_id,
                             sequence_id=None,
-                            partition_key=None,  # TODO -- get partition key
+                            partition_key=partition_key,
                         ),
                         event_time=datetime.datetime.now(),
                         event_type="spawn_start",
@@ -436,6 +437,7 @@ def post_application_create(
             parent_pointer,
             spawning_parent_pointer,
             app_id,
+            partition_key=partition_key,
         )
 
     def _append_write_line(self, model: pydantic.BaseModel):
diff --git a/tests/tracking/test_local_tracking_client.py b/tests/tracking/test_local_tracking_client.py
index 7a8196c0e..db071f96b 100644
--- a/tests/tracking/test_local_tracking_client.py
+++ b/tests/tracking/test_local_tracking_client.py
@@ -242,6 +242,73 @@ def test_persister_tracks_parent(tmpdir):
     assert metadata_parsed.parent_pointer.partition_key == "user123"
 
 
+def test_fork_children_have_correct_partition_key(tmpdir):
+    """Tests that children.jsonl in the parent app directory has the correct
+    partition_key for the forked child app. Regression test for #518."""
+    old_app_id = "parent_app"
+    new_app_id = "forked_app"
+    partition_key = "user123"
+    log_dir = os.path.join(tmpdir, "tracking")
+    project_name = "test_fork_children_partition_key"
+    tracking_client = LocalTrackingClient(project=project_name, storage_dir=log_dir)
+
+    # Create the parent app first
+    parent_app: Application = (
+        ApplicationBuilder()
+        .with_actions(counter, Result("count").with_name("result"))
+        .with_transitions(
+            ("counter", "counter", expr("counter < 3")),
+            ("counter", "result", default),
+        )
+        .with_state(counter=0, break_at=-1)
+        .with_entrypoint("counter")
+        .with_identifiers(app_id=old_app_id, partition_key=partition_key)
+        .with_tracker(tracking_client)
+        .build()
+    )
+    parent_app.run(halt_after=["result"])
+
+    # Fork from the parent
+    forked_app: Application = (
+        ApplicationBuilder()
+        .with_actions(counter, Result("count").with_name("result"))
+        .with_transitions(
+            ("counter", "counter", expr("counter < 5")),
+            ("counter", "result", default),
+        )
+        .initialize_from(
+            tracking_client,
+            resume_at_next_action=True,
+            default_state={"counter": 0, "break_at": -1},
+            default_entrypoint="counter",
+            fork_from_app_id=old_app_id,
+            fork_from_partition_key=partition_key,
+            fork_from_sequence_id=2,
+        )
+        .with_identifiers(app_id=new_app_id, partition_key=partition_key)
+        .with_tracker(tracking_client)
+        .build()
+    )
+    forked_app.run(halt_after=["result"])
+
+    # Check children.jsonl in the parent app directory
+    children_path = os.path.join(
+        log_dir, project_name, old_app_id, LocalTrackingClient.CHILDREN_FILENAME
+    )
+    assert os.path.exists(children_path), "children.jsonl should exist for the parent app"
+
+    with open(children_path) as f:
+        children = [ChildApplicationModel.model_validate(json.loads(line)) for line in f]
+
+    assert len(children) == 1
+    child = children[0]
+    assert child.child.app_id == new_app_id
+    assert child.child.partition_key == partition_key, (
+        f"Child partition_key should be '{partition_key}', got '{child.child.partition_key}'"
+    )
+    assert child.event_type == "fork"
+
+
 def test_multi_fork_tracking_client(tmpdir):
     """This is more of an end-to-end test. We shoudl probably break it out
     into smaller tests but the local tracking client being used as a persister is