From 887920744a9cb7601a6c1f1929a283dba76c3cb5 Mon Sep 17 00:00:00 2001
From: Jim Bosch <jbosch@astro.princeton.edu>
Date: Fri, 28 Jul 2023 14:18:13 -0400
Subject: [PATCH] Add commutator for FindFirstDataset.

I don't know why I previously assumed this wouldn't commute with
anything; it commutes in essentially exactly the same way as a
Selection (i.e. WHERE clause).
---
 doc/changes/DM-40184.bugfix.md                |  3 +++
 .../registry/queries/find_first_dataset.py    | 23 ++++++++++++++++++-
 .../daf/butler/registry/tests/_registry.py    |  1 -
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 doc/changes/DM-40184.bugfix.md

diff --git a/doc/changes/DM-40184.bugfix.md b/doc/changes/DM-40184.bugfix.md
new file mode 100644
index 0000000000..e814996cea
--- /dev/null
+++ b/doc/changes/DM-40184.bugfix.md
@@ -0,0 +1,3 @@
+Fix a rare bug in follow-up dataset queries involving relation commutators.
+
+This occurred when building QuantumGraphs where a "warp" dataset type was an overall input to the pipeline and present in more than one input RUN collection.
diff --git a/python/lsst/daf/butler/registry/queries/find_first_dataset.py b/python/lsst/daf/butler/registry/queries/find_first_dataset.py
index b73cef6afd..78f40a2ded 100644
--- a/python/lsst/daf/butler/registry/queries/find_first_dataset.py
+++ b/python/lsst/daf/butler/registry/queries/find_first_dataset.py
@@ -26,7 +26,7 @@
 from collections.abc import Sequence, Set
 from typing import final
 
-from lsst.daf.relation import ColumnTag, Relation, RowFilter
+from lsst.daf.relation import ColumnTag, Relation, RowFilter, UnaryCommutator, UnaryOperationRelation
 from lsst.utils.classes import cached_getter
 
 from ...core import DatasetColumnTag, DimensionKeyColumnTag
@@ -73,3 +73,24 @@ def __str__(self) -> str:
     def applied_min_rows(self, target: Relation) -> int:
         # Docstring inherited.
         return 1 if target.min_rows else 0
+
+    def commute(self, current: UnaryOperationRelation) -> UnaryCommutator:
+        # Docstring inherited.
+        if not self.columns_required <= current.target.columns:
+            return UnaryCommutator(
+                first=None,
+                second=current.operation,
+                done=False,
+                messages=(
+                    f"{current.target} is missing columns "
+                    f"{set(self.columns_required - current.target.columns)}",
+                ),
+            )
+        if current.operation.is_count_dependent:
+            return UnaryCommutator(
+                first=None,
+                second=current.operation,
+                done=False,
+                messages=(f"{current.operation} is count-dependent",),
+            )
+        return UnaryCommutator(self, current.operation)
diff --git a/python/lsst/daf/butler/registry/tests/_registry.py b/python/lsst/daf/butler/registry/tests/_registry.py
index cafce6440d..dd1363fd0e 100644
--- a/python/lsst/daf/butler/registry/tests/_registry.py
+++ b/python/lsst/daf/butler/registry/tests/_registry.py
@@ -3428,7 +3428,6 @@ def pop_transfer(tree: Relation) -> Relation:
             sql.Engine,
         )
 
-    @unittest.expectedFailure
     def test_query_find_datasets_drop_postprocessing(self) -> None:
         """Test that DataCoordinateQueryResults.findDatasets avoids commutator
         problems with the FindFirstDataset relation operation.