SERVER-37791 Prevent exclusion of non-existent field from affecting field order.

Exclusion of an existing field can still affect the field
order produced by subsequent $project or $addFields stages.
Changing this is left as future work.

1 files changed, 33 insertions, 0 deletions

diff --git a/jstests/aggregation/bugs/exclusion_projection_does_not_affect_field_order.js b/jstests/aggregation/bugs/exclusion_projection_does_not_affect_field_order.js
new file mode 100644
index 00000000000..c781f1ede6d
--- /dev/null
+++ b/jstests/aggregation/bugs/exclusion_projection_does_not_affect_field_order.js
@@ -0,0 +1,33 @@
+// Test that excluding a non-existent field does not affect the field order produced by subsequent
+// changes.
+//
+// This is designed as a regression test for SERVER-37791.
+//
+// @tags: [requires_fcv_44]
+(function() {
+"use strict";
+
+const coll = db.exclusion_projection_does_not_affect_field_order;
+coll.drop();
+
+assert.commandWorked(coll.insert({_id: 1}));
+assert.commandWorked(coll.insert({_id: 2, c: 1}));
+assert.commandWorked(coll.insert({_id: 3, y: 1, z: 1}));
+
+// We expect $addFields to retain the position of pre-existing fields, and then append new fields in
+// the order that they are specified in the query. This rule should not be impacted by the presence
+// of a preceding exclusion projection.
+assert.eq(
+    [
+        {_id: 1, x: 3, y: 4, b: 5, c: 6, a: 7},
+        // Here "c" retains the position that it had prior to being excluded.
+        {_id: 2, c: 6, x: 3, y: 4, b: 5, a: 7},
+        {_id: 3, y: 4, z: 1, x: 3, b: 5, c: 6, a: 7}
+    ],
+    coll.aggregate([
+            {$project: {b: 0, c: 0}},
+            {$addFields: {x: 3, y: 4, b: 5, c: 6, a: 7}},
+            {$sort: {_id: 1}}
+        ])
+        .toArray());
+}());