SERVER-66310 Make ExpressionSetUnion::isCommutative() collation aware

2 files changed, 17 insertions, 1 deletions

diff --git a/jstests/aggregation/expressions/collation_expressions.js b/jstests/aggregation/expressions/collation_expressions.js
index 1b8b98eda2b..8ef91220be5 100644
--- a/jstests/aggregation/expressions/collation_expressions.js
+++ b/jstests/aggregation/expressions/collation_expressions.js
@@ -84,6 +84,19 @@ results = coll.aggregate([{$project: {out: {$setUnion: [["a", "B", "c"], ["d", "
 assert.eq(1, results.length);
 assert.eq(4, results[0].out.length);
 
+// Test that $setUnion is not commutative when a collation is set. A non-const value is required for
+// reordering to occur.
+assert(coll.drop);
+coll.drop();
+assert.commandWorked(coll.insert({_id: 1, upper: "A", lower: "a"}));
+var results1 = coll.aggregate([{$project: {out: {$setUnion: [["$upper"], ["a"]]}}}],
+                              {collation: caseInsensitive})
+                   .toArray();
+var results2 = coll.aggregate([{$project: {out: {$setUnion: [["A"], ["$lower"]]}}}],
+                              {collation: caseInsensitive})
+                   .toArray();
+assert.eq(results1, results2);
+
 // Test that $setDifference respects the collation.
 testExpressionWithCollation(coll, {$setDifference: [["a", "B"], ["b", "A"]]}, [], caseInsensitive);
 
diff --git a/src/mongo/db/pipeline/expression.h b/src/mongo/db/pipeline/expression.h
index 95ba8820523..3845a6e23e8 100644
--- a/src/mongo/db/pipeline/expression.h
+++ b/src/mongo/db/pipeline/expression.h
@@ -2157,7 +2157,10 @@ public:
     }
 
     bool isCommutative() const final {
-        return true;
+        // Only commutative when performing binary string comparison. The first value entered when
+        // multiple collation-equal but binary-unequal values are added will dictate what is stored
+        // in the set.
+        return getExpressionContext()->getCollator() == nullptr;
     }
 
     void acceptVisitor(ExpressionVisitor* visitor) final {