1 files changed, 23 insertions, 3 deletions
diff --git a/src/mongo/db/pipeline/document_source_out.h b/src/mongo/db/pipeline/document_source_out.h
index 28241d82893..971f1652c47 100644
--- a/src/mongo/db/pipeline/document_source_out.h
+++ b/src/mongo/db/pipeline/document_source_out.h
@@ -38,6 +38,7 @@ namespace mongo {
 class DocumentSourceOut final : public DocumentSourceWriter<BSONObj> {
 public:
     static constexpr StringData kStageName = "$out"_sd;
+    static constexpr StringData kInternalStageName = "$internalOutToDifferentDB"_sd;
 
     /**
      * A "lite parsed" $out stage is similar to other stages involving foreign collections except in
@@ -51,6 +52,9 @@ public:
         static std::unique_ptr<LiteParsed> parse(const AggregationRequest& request,
                                                  const BSONElement& spec);
 
+        static std::unique_ptr<LiteParsed> parseToDifferentDB(const AggregationRequest& request,
+                                                              const BSONElement& spec);
+
         bool allowShardedForeignCollection(NamespaceString nss) const final {
             return _foreignNssSet.find(nss) == _foreignNssSet.end();
         }
@@ -63,6 +67,9 @@ public:
     ~DocumentSourceOut() override;
 
     const char* getSourceName() const final override {
+        if (_toDifferentDB) {
+            return kInternalStageName.rawData();
+        }
         return kStageName.rawData();
     }
 
@@ -80,21 +87,30 @@ public:
         boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final override;
 
     /**
-     * Creates a new $out stage from the given arguments.
+     * Creates a new $out or $internalOutToDifferentDB stage from the given arguments.
      */
     static boost::intrusive_ptr<DocumentSource> create(
         NamespaceString outputNs, const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
+    static boost::intrusive_ptr<DocumentSource> createAndAllowDifferentDB(
+        NamespaceString outputNs, const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
     /**
-     * Parses a $out stage from the user-supplied BSON.
+     * Parses a $out or $internalOutToDifferentDB stage from the user-supplied BSON.
      */
     static boost::intrusive_ptr<DocumentSource> createFromBson(
         BSONElement elem, const boost::intrusive_ptr<ExpressionContext>& pExpCtx);
+    static boost::intrusive_ptr<DocumentSource> createFromBsonToDifferentDB(
+        BSONElement elem, const boost::intrusive_ptr<ExpressionContext>& pExpCtx);
 
 private:
     DocumentSourceOut(NamespaceString outputNs,
                       const boost::intrusive_ptr<ExpressionContext>& expCtx)
-        : DocumentSourceWriter(kStageName.rawData(), std::move(outputNs), expCtx) {}
+        : DocumentSourceWriter(outputNs.db() == expCtx->ns.db() ? kStageName.rawData()
+                                                                : kInternalStageName.rawData(),
+                               std::move(outputNs),
+                               expCtx),
+          _toDifferentDB(getOutputNs().db() != expCtx->ns.db()) {}
 
     void initialize() override;
 
@@ -122,6 +138,10 @@ private:
 
     // The temporary namespace for the $out writes.
     NamespaceString _tempNs;
+
+    // Keep track of whether this document source is writing to a different DB for serialization
+    // purposes.
+    bool _toDifferentDB;
 };
 
 }  // namespace mongo