SERVER-85892 Fix incorrect output namespace in $merge spec (#19846) (#24833)

GitOrigin-RevId: 9f1eb0f38887e04f692e191be116c8035480df58

2 files changed, 114 insertions, 1 deletions

diff --git a/jstests/aggregation/documents_merge.js b/jstests/aggregation/documents_merge.js
new file mode 100644
index 00000000000..99e9a8f1a41
--- /dev/null
+++ b/jstests/aggregation/documents_merge.js
@@ -0,0 +1,113 @@
+/**
+ * This is the test for $documents stage along with $merge stage in an aggregation pipeline,
+ * including verifying the bug in SERVER-85892 is addressed when the spec 'whenMatched' is not
+ * empty.
+ *
+ * @tags: [
+ *   requires_fcv_80,
+ *   assumes_against_mongod_not_mongos,  # not yet supported until 7.2 with SERVER-65534
+ * ]
+ */
+
+load("jstests/aggregation/extras/merge_helpers.js");  // For withEachMergeMode and
+                                                      // dropWithoutImplicitRecreate.
+
+const outColl = db[`${jsTest.name()}_out`];
+const outCollName = outColl.getName();
+const expectedTotalDocs = 100;
+
+function assertDocsInsertedCorrectly(docs, pipeline) {
+    const msg = `Failed with pipeline: ${JSON.stringify(pipeline, null, 2)}`;
+
+    assert.eq(expectedTotalDocs, docs.length, msg);
+    for (let i = 0; i < expectedTotalDocs; i++) {
+        assert.eq(docs[i].x, i, msg);
+    }
+}
+
+const documentsStage = {
+    $documents: {$map: {input: {$range: [0, expectedTotalDocs]}, in : {x: "$$this"}}}
+};
+
+function testFn(pipeline, assertFn) {
+    // Creates an index as $merge requires a unique index with the 'on' identifier field. Then
+    // inserts a document allowed to be matched.
+    dropWithoutImplicitRecreate(outCollName);
+    assert.commandWorked(outColl.createIndex({x: 1}, {unique: true}));
+    assert.commandWorked(outColl.insert({x: 10}));
+
+    assert.doesNotThrow(() => db.aggregate(pipeline));
+    let res = outColl.find({}, {_id: 0}).sort({x: 1}).toArray();
+    assertDocsInsertedCorrectly(res, pipeline);
+    assertFn(res);
+}
+
+{  // Tests $merge with non-empty pipeline along with let in whenMatched spec.
+    const pipeline = [
+        documentsStage,
+        {
+            $merge: {
+                into: outCollName,
+                let : {num: 123},
+                whenMatched: [{$set: {number: "$$num"}}],
+                on: "x"
+            }
+        }
+    ];
+
+    testFn(pipeline, res => {
+        assert.eq(res.filter(elem => elem.number === 123).length, 1);
+    });
+}
+
+{  // Tests $merge with non-empty pipeline in whenMatched spec.
+    const pipeline = [
+        documentsStage,
+        {$merge: {into: outCollName, whenMatched: [{$set: {new: true}}], on: "x"}}
+    ];
+
+    testFn(pipeline, res => {
+        assert.eq(res.filter(elem => elem.new === true).length, 1);
+    });
+}
+
+// Tests each combination of merge modes.
+withEachMergeMode(({whenMatchedMode, whenNotMatchedMode}) => {
+    const expectErrorCode = whenMatchedMode === "fail"
+        ? ErrorCodes.DuplicateKey
+        : whenNotMatchedMode === "fail" ? ErrorCodes.MergeStageNoMatchingDocument : null;
+
+    // Creates an index as $merge requires a unique index with the 'on' identifier field. Then
+    // inserts a document allowed to be matched.
+    dropWithoutImplicitRecreate(outCollName);
+    assert.commandWorked(outColl.createIndex({x: 1}, {unique: true}));
+    assert.commandWorked(outColl.insert({x: 10, old: true}));
+
+    const pipeline = [
+        documentsStage,
+        {
+            $merge: {
+                into: outCollName,
+                whenMatched: whenMatchedMode,
+                whenNotMatched: whenNotMatchedMode,
+                on: "x",
+            }
+        }
+    ];
+
+    if (expectErrorCode) {
+        assert.throwsWithCode(() => db.aggregate(pipeline), expectErrorCode);
+        return;
+    }
+
+    assert.doesNotThrow(() => db.aggregate(pipeline));
+    let res = outColl.find({}, {_id: 0}).sort({x: 1}).toArray();
+    if (whenNotMatchedMode == "discard") {
+        assert.eq(outColl.count(), 1);
+    } else {
+        assertDocsInsertedCorrectly(res, pipeline);
+    }
+
+    // Asserts if the old document is replaced when 'whenMatchedMode' is "replace".
+    assert.eq(res.filter(elem => elem.old === true).length, whenMatchedMode == "replace" ? 0 : 1);
+});
diff --git a/src/mongo/db/pipeline/document_source_merge.cpp b/src/mongo/db/pipeline/document_source_merge.cpp
index 33be917de08..c46cfa8c641 100644
--- a/src/mongo/db/pipeline/document_source_merge.cpp
+++ b/src/mongo/db/pipeline/document_source_merge.cpp
@@ -624,7 +624,7 @@ Value DocumentSourceMerge::serialize(const SerializationOptions& opts) const {
             if (!_pipeline.has_value()) {
                 return boost::none;
             }
-            auto expCtxWithLetVariables = pExpCtx->copyWith(pExpCtx->ns);
+            auto expCtxWithLetVariables = pExpCtx->copyWith(getOutputNs());
             if (spec.getLet()) {
                 BSONObjBuilder cleanLetSpecBuilder;
                 for (auto&& [name, expr] : *_letVariables) {