SERVER-44321 treat $meta-only projections depending on context

1 files changed, 22 insertions, 0 deletions

diff --git a/jstests/aggregation/sources/project/id_meta_projection.js b/jstests/aggregation/sources/project/id_meta_projection.js
new file mode 100644
index 00000000000..1a7b653b9c1
--- /dev/null
+++ b/jstests/aggregation/sources/project/id_meta_projection.js
@@ -0,0 +1,22 @@
+(function() {
+"use strict";
+
+const coll = db.id_meta_projection;
+coll.drop();
+
+assert.commandWorked(coll.insert({_id: 0, a: 1, b: 1}));
+
+// Run the aggregate once where the $project can be pushed down.
+const projectPushedDownRes =
+    coll.aggregate([{$sort: {a: 1}}, {$project: {_id: 0, metaField: {$meta: "sortKey"}}}])
+        .toArray();
+
+// Run it again where it cannot be pushed down.
+const projectNotPushedDownRes = coll.aggregate([
+                                        {$sort: {a: 1}},
+                                        {$_internalInhibitOptimization: {}},
+                                        {$project: {_id: 0, metaField: {$meta: "sortKey"}}}
+                                    ])
+                                    .toArray();
+assert.eq(projectPushedDownRes, projectNotPushedDownRes);
+})();