1 files changed, 94 insertions, 0 deletions
diff --git a/src/mongo/db/pipeline/group_from_first_document_transformation.h b/src/mongo/db/pipeline/group_from_first_document_transformation.h
new file mode 100644
index 00000000000..a7beafc4989
--- /dev/null
+++ b/src/mongo/db/pipeline/group_from_first_document_transformation.h
@@ -0,0 +1,94 @@
+/**
+ *    Copyright (C) 2022-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/db/pipeline/expression.h"
+#include "mongo/db/pipeline/transformer_interface.h"
+
+namespace mongo {
+
+/**
+ * GroupFromFirstTransformation consists of a list of (field name, expression pairs). It returns a
+ * document synthesized by assigning each field name in the output document to the result of
+ * evaluating the corresponding expression. If the expression evaluates to missing, we assign a
+ * value of BSONNULL. This is necessary to match the semantics of $first for missing fields.
+ */
+class GroupFromFirstDocumentTransformation final : public TransformerInterface {
+public:
+    GroupFromFirstDocumentTransformation(
+        const std::string& groupId,
+        StringData originalStageName,
+        std::vector<std::pair<std::string, boost::intrusive_ptr<Expression>>> accumulatorExprs)
+        : _accumulatorExprs(std::move(accumulatorExprs)),
+          _groupId(groupId),
+          _originalStageName(originalStageName) {}
+
+    TransformerType getType() const final {
+        return TransformerType::kGroupFromFirstDocument;
+    }
+
+    /**
+     * The path of the field that we are grouping on: i.e., the field in the input document that we
+     * will use to create the _id field of the ouptut document.
+     */
+    const std::string& groupId() const {
+        return _groupId;
+    }
+
+    StringData originalStageName() const {
+        return _originalStageName;
+    }
+
+    Document applyTransformation(const Document& input) final;
+
+    void optimize() final;
+
+    Document serializeTransformation(
+        boost::optional<ExplainOptions::Verbosity> explain) const final;
+
+    DepsTracker::State addDependencies(DepsTracker* deps) const final;
+
+    void addVariableRefs(std::set<Variables::Id>* refs) const final;
+
+    DocumentSource::GetModPathsReturn getModifiedPaths() const final;
+
+    static std::unique_ptr<GroupFromFirstDocumentTransformation> create(
+        const boost::intrusive_ptr<ExpressionContext>& expCtx,
+        const std::string& groupId,
+        StringData originalStageName,
+        std::vector<std::pair<std::string, boost::intrusive_ptr<Expression>>> accumulatorExprs);
+
+private:
+    std::vector<std::pair<std::string, boost::intrusive_ptr<Expression>>> _accumulatorExprs;
+    std::string _groupId;
+    StringData _originalStageName;
+};
+
+}  // namespace mongo