SERVER-24879 Add $mergeObjects aggregation expression

15 files changed, 294 insertions, 26 deletions

diff --git a/src/mongo/db/pipeline/SConscript b/src/mongo/db/pipeline/SConscript
index 82c6fb925d2..0f9a473454c 100644
--- a/src/mongo/db/pipeline/SConscript
+++ b/src/mongo/db/pipeline/SConscript
@@ -190,6 +190,7 @@ env.Library(
         'accumulator_push.cpp',
         'accumulator_std_dev.cpp',
         'accumulator_sum.cpp',
+        'accumulator_merge_objects.cpp'
         ],
     LIBDEPS=[
         'document_value',
diff --git a/src/mongo/db/pipeline/accumulator.h b/src/mongo/db/pipeline/accumulator.h
index 28645e90a9b..a7a97620607 100644
--- a/src/mongo/db/pipeline/accumulator.h
+++ b/src/mongo/db/pipeline/accumulator.h
@@ -63,7 +63,7 @@ public:
     /** Marks the end of the evaluate() phase and return accumulated result.
      *  toBeMerged should be true when the outputs will be merged by process().
      */
-    virtual Value getValue(bool toBeMerged) const = 0;
+    virtual Value getValue(bool toBeMerged) = 0;
 
     /// The name of the op as used in a serialization of the pipeline.
     virtual const char* getOpName() const = 0;
@@ -106,7 +106,7 @@ public:
     explicit AccumulatorAddToSet(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -131,7 +131,7 @@ public:
     explicit AccumulatorFirst(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -149,7 +149,7 @@ public:
     explicit AccumulatorLast(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -166,7 +166,7 @@ public:
     explicit AccumulatorSum(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -198,7 +198,7 @@ public:
     AccumulatorMinMax(const boost::intrusive_ptr<ExpressionContext>& expCtx, Sense sense);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -237,7 +237,7 @@ public:
     explicit AccumulatorPush(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -254,7 +254,7 @@ public:
     explicit AccumulatorAvg(const boost::intrusive_ptr<ExpressionContext>& expCtx);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -280,7 +280,7 @@ public:
     AccumulatorStdDev(const boost::intrusive_ptr<ExpressionContext>& expCtx, bool isSamp);
 
     void processInternal(const Value& input, bool merging) final;
-    Value getValue(bool toBeMerged) const final;
+    Value getValue(bool toBeMerged) final;
     const char* getOpName() const final;
     void reset() final;
 
@@ -306,4 +306,20 @@ public:
     static boost::intrusive_ptr<Accumulator> create(
         const boost::intrusive_ptr<ExpressionContext>& expCtx);
 };
+
+class AccumulatorMergeObjects : public Accumulator {
+public:
+    AccumulatorMergeObjects(const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
+    void processInternal(const Value& input, bool merging) final;
+    Value getValue(bool toBeMerged) final;
+    const char* getOpName() const final;
+    void reset() final;
+
+    static boost::intrusive_ptr<Accumulator> create(
+        const boost::intrusive_ptr<ExpressionContext>& expCtx);
+
+private:
+    MutableDocument _output;
+};
 }
diff --git a/src/mongo/db/pipeline/accumulator_add_to_set.cpp b/src/mongo/db/pipeline/accumulator_add_to_set.cpp
index 00845b32fc8..4f6048529a1 100644
--- a/src/mongo/db/pipeline/accumulator_add_to_set.cpp
+++ b/src/mongo/db/pipeline/accumulator_add_to_set.cpp
@@ -70,7 +70,7 @@ void AccumulatorAddToSet::processInternal(const Value& input, bool merging) {
     }
 }
 
-Value AccumulatorAddToSet::getValue(bool toBeMerged) const {
+Value AccumulatorAddToSet::getValue(bool toBeMerged) {
     return Value(vector<Value>(_set.begin(), _set.end()));
 }
 
diff --git a/src/mongo/db/pipeline/accumulator_avg.cpp b/src/mongo/db/pipeline/accumulator_avg.cpp
index da0c08fce9c..6f0e8e04813 100644
--- a/src/mongo/db/pipeline/accumulator_avg.cpp
+++ b/src/mongo/db/pipeline/accumulator_avg.cpp
@@ -101,7 +101,7 @@ Decimal128 AccumulatorAvg::_getDecimalTotal() const {
     return _decimalTotal.add(_nonDecimalTotal.getDecimal());
 }
 
-Value AccumulatorAvg::getValue(bool toBeMerged) const {
+Value AccumulatorAvg::getValue(bool toBeMerged) {
     if (toBeMerged) {
         if (_isDecimal)
             return Value(Document{{subTotalName, _getDecimalTotal()}, {countName, _count}});
diff --git a/src/mongo/db/pipeline/accumulator_first.cpp b/src/mongo/db/pipeline/accumulator_first.cpp
index 3092bd0e9da..60c97f88124 100644
--- a/src/mongo/db/pipeline/accumulator_first.cpp
+++ b/src/mongo/db/pipeline/accumulator_first.cpp
@@ -53,7 +53,7 @@ void AccumulatorFirst::processInternal(const Value& input, bool merging) {
     }
 }
 
-Value AccumulatorFirst::getValue(bool toBeMerged) const {
+Value AccumulatorFirst::getValue(bool toBeMerged) {
     return _first;
 }
 
diff --git a/src/mongo/db/pipeline/accumulator_last.cpp b/src/mongo/db/pipeline/accumulator_last.cpp
index 5f465b1bf4e..c66fe5801dc 100644
--- a/src/mongo/db/pipeline/accumulator_last.cpp
+++ b/src/mongo/db/pipeline/accumulator_last.cpp
@@ -49,7 +49,7 @@ void AccumulatorLast::processInternal(const Value& input, bool merging) {
     _memUsageBytes = sizeof(*this) + _last.getApproximateSize() - sizeof(Value);
 }
 
-Value AccumulatorLast::getValue(bool toBeMerged) const {
+Value AccumulatorLast::getValue(bool toBeMerged) {
     return _last;
 }
 
diff --git a/src/mongo/db/pipeline/accumulator_merge_objects.cpp b/src/mongo/db/pipeline/accumulator_merge_objects.cpp
new file mode 100644
index 00000000000..1584d8ba2c4
--- /dev/null
+++ b/src/mongo/db/pipeline/accumulator_merge_objects.cpp
@@ -0,0 +1,93 @@
+/**
+ * Copyright (c) 2017 10gen Inc.
+ *
+ * This program is free software: you can redistribute it and/or  modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * 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
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * 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 GNU Affero General 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.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/pipeline/accumulator.h"
+
+#include "mongo/db/pipeline/accumulation_statement.h"
+#include "mongo/db/pipeline/expression.h"
+#include "mongo/db/pipeline/value.h"
+
+namespace mongo {
+
+using boost::intrusive_ptr;
+
+/* ------------------------- AccumulatorMergeObjects ----------------------------- */
+
+REGISTER_ACCUMULATOR(mergeObjects, AccumulatorMergeObjects::create);
+REGISTER_EXPRESSION(mergeObjects, ExpressionFromAccumulator<AccumulatorMergeObjects>::parse);
+
+const char* AccumulatorMergeObjects::getOpName() const {
+    return "$mergeObjects";
+}
+
+intrusive_ptr<Accumulator> AccumulatorMergeObjects::create(
+    const boost::intrusive_ptr<ExpressionContext>& expCtx) {
+    return new AccumulatorMergeObjects(expCtx);
+}
+
+AccumulatorMergeObjects::AccumulatorMergeObjects(
+    const boost::intrusive_ptr<ExpressionContext>& expCtx)
+    : Accumulator(expCtx) {
+    _memUsageBytes = sizeof(*this);
+}
+
+void AccumulatorMergeObjects::reset() {
+    _memUsageBytes = sizeof(*this);
+    _output.reset();
+}
+
+void AccumulatorMergeObjects::processInternal(const Value& input, bool merging) {
+    if (input.nullish()) {
+        return;
+    }
+
+    uassert(40400,
+            str::stream() << "$mergeObjects requires object inputs, but input " << input.toString()
+                          << " is of type "
+                          << typeName(input.getType()),
+            (input.getType() == BSONType::Object));
+
+    FieldIterator iter = input.getDocument().fieldIterator();
+    while (iter.more()) {
+        Document::FieldPair pair = iter.next();
+        // Ignore missing values only, null and undefined are still considered.
+        if (pair.second.missing())
+            continue;
+
+        _output.setField(pair.first, pair.second);
+    }
+    _memUsageBytes = sizeof(*this) + _output.getApproximateSize();
+}
+
+Value AccumulatorMergeObjects::getValue(bool toBeMerged) {
+    return _output.freezeToValue();
+}
+
+}  // namespace mongo
diff --git a/src/mongo/db/pipeline/accumulator_min_max.cpp b/src/mongo/db/pipeline/accumulator_min_max.cpp
index f5c4b254b88..ee086d2a2c2 100644
--- a/src/mongo/db/pipeline/accumulator_min_max.cpp
+++ b/src/mongo/db/pipeline/accumulator_min_max.cpp
@@ -61,7 +61,7 @@ void AccumulatorMinMax::processInternal(const Value& input, bool merging) {
     }
 }
 
-Value AccumulatorMinMax::getValue(bool toBeMerged) const {
+Value AccumulatorMinMax::getValue(bool toBeMerged) {
     if (_val.missing()) {
         return Value(BSONNULL);
     }
diff --git a/src/mongo/db/pipeline/accumulator_push.cpp b/src/mongo/db/pipeline/accumulator_push.cpp
index 9239d8b8a18..64b140a2c7c 100644
--- a/src/mongo/db/pipeline/accumulator_push.cpp
+++ b/src/mongo/db/pipeline/accumulator_push.cpp
@@ -67,7 +67,7 @@ void AccumulatorPush::processInternal(const Value& input, bool merging) {
     }
 }
 
-Value AccumulatorPush::getValue(bool toBeMerged) const {
+Value AccumulatorPush::getValue(bool toBeMerged) {
     return Value(vpValue);
 }
 
diff --git a/src/mongo/db/pipeline/accumulator_std_dev.cpp b/src/mongo/db/pipeline/accumulator_std_dev.cpp
index f45678dce27..159f95f16df 100644
--- a/src/mongo/db/pipeline/accumulator_std_dev.cpp
+++ b/src/mongo/db/pipeline/accumulator_std_dev.cpp
@@ -83,7 +83,7 @@ void AccumulatorStdDev::processInternal(const Value& input, bool merging) {
     }
 }
 
-Value AccumulatorStdDev::getValue(bool toBeMerged) const {
+Value AccumulatorStdDev::getValue(bool toBeMerged) {
     if (!toBeMerged) {
         const long long adjustedCount = (_isSamp ? _count - 1 : _count);
         if (adjustedCount <= 0)
diff --git a/src/mongo/db/pipeline/accumulator_sum.cpp b/src/mongo/db/pipeline/accumulator_sum.cpp
index bbbf596d4a8..524284e55ed 100644
--- a/src/mongo/db/pipeline/accumulator_sum.cpp
+++ b/src/mongo/db/pipeline/accumulator_sum.cpp
@@ -89,7 +89,7 @@ intrusive_ptr<Accumulator> AccumulatorSum::create(
     return new AccumulatorSum(expCtx);
 }
 
-Value AccumulatorSum::getValue(bool toBeMerged) const {
+Value AccumulatorSum::getValue(bool toBeMerged) {
     switch (totalType) {
         case NumberInt:
             if (nonDecimalTotal.fitsLong())
diff --git a/src/mongo/db/pipeline/accumulator_test.cpp b/src/mongo/db/pipeline/accumulator_test.cpp
index 3fcb1d70ad7..4ed1a6b486b 100644
--- a/src/mongo/db/pipeline/accumulator_test.cpp
+++ b/src/mongo/db/pipeline/accumulator_test.cpp
@@ -344,4 +344,62 @@ TEST(Accumulators, AddToSetRespectsCollation) {
                             Value(std::vector<Value>{Value("a"_sd)})}});
 }
 
+/* ------------------------- AccumulatorMergeObjects -------------------------- */
+
+namespace AccumulatorMergeObjects {
+
+TEST(AccumulatorMergeObjects, MergingZeroObjectsShouldReturnEmptyDocument) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+
+    assertExpectedResults("$mergeObjects", expCtx, {{{}, {Value(Document({}))}}});
+}
+
+TEST(AccumulatorMergeObjects, MergingWithSingleObjectShouldLeaveUnchanged) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+
+    assertExpectedResults("$mergeObjects", expCtx, {{{}, {Value(Document({}))}}});
+
+    auto doc = Value(Document({{"a", 1}, {"b", 1}}));
+    assertExpectedResults("$mergeObjects", expCtx, {{{doc}, doc}});
+}
+
+TEST(AccumulatorMergeObjects, MergingDisjointObjectsShouldIncludeAllFields) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+    auto first = Value(Document({{"a", 1}, {"b", 1}}));
+    auto second = Value(Document({{"c", 1}}));
+    assertExpectedResults("$mergeObjects",
+                          expCtx,
+                          {{{first, second}, Value(Document({{"a", 1}, {"b", 1}, {"c", 1}}))}});
+}
+
+TEST(AccumulatorMergeObjects, MergingIntersectingObjectsShouldOverrideInOrderReceived) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+    auto first = Value(Document({{"a", "oldValue"_sd}, {"b", 0}, {"c", 1}}));
+    auto second = Value(Document({{"a", "newValue"_sd}}));
+    assertExpectedResults(
+        "$mergeObjects",
+        expCtx,
+        {{{first, second}, Value(Document({{"a", "newValue"_sd}, {"b", 0}, {"c", 1}}))}});
+}
+
+TEST(AccumulatorMergeObjects, MergingIntersectingEmbeddedObjectsShouldOverrideInOrderReceived) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+    auto firstSubDoc = Document({{"a", 1}, {"b", 2}, {"c", 3}});
+    auto secondSubDoc = Document({{"a", 2}, {"b", 1}});
+    auto first = Value(Document({{"d", 1}, {"subDoc", firstSubDoc}}));
+    auto second = Value(Document({{"subDoc", secondSubDoc}}));
+    auto expected = Value(Document({{"d", 1}, {"subDoc", secondSubDoc}}));
+    assertExpectedResults("$mergeObjects", expCtx, {{{first, second}, expected}});
+}
+
+TEST(AccumulatorMergeObjects, MergingWithEmptyDocumentShouldIgnore) {
+    intrusive_ptr<ExpressionContext> expCtx(new ExpressionContextForTest());
+    auto first = Value(Document({{"a", 0}, {"b", 1}, {"c", 1}}));
+    auto second = Value(Document({}));
+    auto expected = Value(Document({{"a", 0}, {"b", 1}, {"c", 1}}));
+    assertExpectedResults("$mergeObjects", expCtx, {{{first, second}, expected}});
+}
+
+}  // namespace AccumulatorMergeObjects
+
 }  // namespace AccumulatorTests
diff --git a/src/mongo/db/pipeline/document.h b/src/mongo/db/pipeline/document.h
index 416dd24a4aa..e004f7ab982 100644
--- a/src/mongo/db/pipeline/document.h
+++ b/src/mongo/db/pipeline/document.h
@@ -523,6 +523,10 @@ public:
         return Document(storagePtr());
     }
 
+    size_t getApproximateSize() {
+        return peek().getApproximateSize();
+    }
+
 private:
     friend class MutableValue;  // for access to next constructor
     explicit MutableDocument(MutableValue mv) : _storageHolder(NULL), _storage(mv.getDocPtr()) {}
diff --git a/src/mongo/db/pipeline/expression_test.cpp b/src/mongo/db/pipeline/expression_test.cpp
index f08617396bb..4e35f76931d 100644
--- a/src/mongo/db/pipeline/expression_test.cpp
+++ b/src/mongo/db/pipeline/expression_test.cpp
@@ -54,24 +54,35 @@ using std::vector;
 using std::list;
 
 /**
+ * Creates an expression given by 'expressionName' and evaluates it using
+ * 'operands' as inputs, returning the result.
+ */
+static Value evaluateExpression(const string& expressionName,
+                                const vector<ImplicitValue>& operands) {
+    intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest());
+    VariablesIdGenerator idGenerator;
+    VariablesParseState vps(&idGenerator);
+    const BSONObj obj = BSON(expressionName << ImplicitValue::convertToValue(operands));
+    auto expression = Expression::parseExpression(expCtx, obj, vps);
+    Value result = expression->evaluate(Document());
+    return result;
+}
+
+/**
  * Takes the name of an expression as its first argument and a list of pairs of arguments and
  * expected results as its second argument, and asserts that for the given expression the arguments
  * evaluate to the expected results.
  */
-static void assertExpectedResults(string expression,
-                                  initializer_list<pair<vector<Value>, Value>> operations) {
+static void assertExpectedResults(
+    const string& expression,
+    initializer_list<pair<vector<ImplicitValue>, ImplicitValue>> operations) {
     for (auto&& op : operations) {
         try {
-            intrusive_ptr<ExpressionContextForTest> expCtx(new ExpressionContextForTest());
-            VariablesIdGenerator idGenerator;
-            VariablesParseState vps(&idGenerator);
-            const BSONObj obj = BSON(expression << Value(op.first));
-            auto expression = Expression::parseExpression(expCtx, obj, vps);
-            Value result = expression->evaluate(Document());
+            Value result = evaluateExpression(expression, op.first);
             ASSERT_VALUE_EQ(op.second, result);
             ASSERT_EQUALS(op.second.getType(), result.getType());
         } catch (...) {
-            log() << "failed with arguments: " << Value(op.first);
+            log() << "failed with arguments: " << ImplicitValue::convertToValue(op.first);
             throw;
         }
     }
@@ -3890,6 +3901,81 @@ TEST(BuiltinRemoveVariableTest, RemoveSerializesCorrectlyAfterOptimization) {
 
 }  // namespace BuiltinRemoveVariable
 
+/* ------------------------- ExpressionMergeObjects -------------------------- */
+
+namespace ExpressionMergeObjects {
+
+TEST(ExpressionMergeObjects, MergingWithSingleObjectShouldLeaveUnchanged) {
+    assertExpectedResults("$mergeObjects", {{{}, {Document({})}}});
+
+    auto doc = Document({{"a", 1}, {"b", 1}});
+    assertExpectedResults("$mergeObjects", {{{doc}, doc}});
+}
+
+TEST(ExpressionMergeObjects, MergingDisjointObjectsShouldIncludeAllFields) {
+    auto first = Document({{"a", 1}, {"b", 1}});
+    auto second = Document({{"c", 1}});
+    assertExpectedResults("$mergeObjects",
+                          {{{first, second}, Document({{"a", 1}, {"b", 1}, {"c", 1}})}});
+}
+
+TEST(ExpressionMergeObjects, MergingIntersectingObjectsShouldOverrideInOrderReceived) {
+    auto first = Document({{"a", "oldValue"_sd}, {"b", 0}, {"c", 1}});
+    auto second = Document({{"a", "newValue"_sd}});
+    assertExpectedResults(
+        "$mergeObjects", {{{first, second}, Document({{"a", "newValue"_sd}, {"b", 0}, {"c", 1}})}});
+}
+
+TEST(ExpressionMergeObjects, MergingIntersectingEmbeddedObjectsShouldOverrideInOrderReceived) {
+    auto firstSubDoc = Document({{"a", 1}, {"b", 2}, {"c", 3}});
+    auto secondSubDoc = Document({{"a", 2}, {"b", 1}});
+    auto first = Document({{"d", 1}, {"subDoc", firstSubDoc}});
+    auto second = Document({{"subDoc", secondSubDoc}});
+    auto expected = Document({{"d", 1}, {"subDoc", secondSubDoc}});
+    assertExpectedResults("$mergeObjects", {{{first, second}, expected}});
+}
+
+TEST(ExpressionMergeObjects, MergingWithEmptyDocumentShouldIgnore) {
+    auto first = Document({{"a", 0}, {"b", 1}, {"c", 1}});
+    auto second = Document({});
+    auto expected = Document({{"a", 0}, {"b", 1}, {"c", 1}});
+    assertExpectedResults("$mergeObjects", {{{first, second}, expected}});
+}
+
+TEST(ExpressionMergeObjects, MergingSingleArgumentArrayShouldUnwindAndMerge) {
+    std::vector<Document> first = {Document({{"a", 1}}), Document({{"a", 2}})};
+    auto expected = Document({{"a", 2}});
+    assertExpectedResults("$mergeObjects", {{{first}, expected}});
+}
+
+TEST(ExpressionMergeObjects, MergingArrayWithDocumentShouldThrowException) {
+    std::vector<Document> first = {Document({{"a", 1}}), Document({{"a", 2}})};
+    auto second = Document({{"b", 2}});
+    ASSERT_THROWS_CODE(evaluateExpression("$mergeObjects", {first, second}), UserException, 40400);
+}
+
+TEST(ExpressionMergeObjects, MergingArrayContainingInvalidTypesShouldThrowException) {
+    std::vector<Value> first = {Value(Document({{"validType", 1}})), Value("invalidType"_sd)};
+    ASSERT_THROWS_CODE(evaluateExpression("$mergeObjects", {first}), UserException, 40400);
+}
+
+TEST(ExpressionMergeObjects, MergingNonObjectsShouldThrowException) {
+    ASSERT_THROWS_CODE(
+        evaluateExpression("$mergeObjects", {"invalidArg"_sd}), UserException, 40400);
+
+    ASSERT_THROWS_CODE(
+        evaluateExpression("$mergeObjects", {"invalidArg"_sd, Document({{"validArg", 1}})}),
+        UserException,
+        40400);
+
+    ASSERT_THROWS_CODE(evaluateExpression("$mergeObjects", {1, Document({{"validArg", 1}})}),
+                       UserException,
+                       40400);
+}
+
+}  // namespace ExpressionMergeObjects
+
+
 namespace ToLower {
 
 class ExpectedResultBase {
diff --git a/src/mongo/db/pipeline/value.h b/src/mongo/db/pipeline/value.h
index 57aa2c6a789..4749194ae29 100644
--- a/src/mongo/db/pipeline/value.h
+++ b/src/mongo/db/pipeline/value.h
@@ -358,6 +358,16 @@ class ImplicitValue : public Value {
 public:
     template <typename T>
     ImplicitValue(T arg) : Value(std::move(arg)) {}
+
+    /**
+     * Converts a vector of Implicit values to a single Value object.
+     */
+    static Value convertToValue(const std::vector<ImplicitValue>& vec) {
+        std::vector<Value> values;
+        for_each(
+            vec.begin(), vec.end(), ([&](const ImplicitValue& val) { values.push_back(val); }));
+        return Value(values);
+    }
 };
 }