SERVER-59133 Change 'output' parameter to 'input' in syntax for $firstN/$lastN/$minN/$maxN

3 files changed, 39 insertions, 34 deletions

diff --git a/src/mongo/db/pipeline/accumulator_multi.cpp b/src/mongo/db/pipeline/accumulator_multi.cpp
index 3430f527637..6cef4cd12c7 100644
--- a/src/mongo/db/pipeline/accumulator_multi.cpp
+++ b/src/mongo/db/pipeline/accumulator_multi.cpp
@@ -199,11 +199,11 @@ AccumulatorN::parseArgs(ExpressionContext* const expCtx,
                         const BSONObj& args,
                         VariablesParseState vps) {
     boost::intrusive_ptr<Expression> n;
-    boost::intrusive_ptr<Expression> output;
+    boost::intrusive_ptr<Expression> input;
     for (auto&& element : args) {
         auto fieldName = element.fieldNameStringData();
-        if (fieldName == kFieldNameOutput) {
-            output = Expression::parseOperand(expCtx, element, vps);
+        if (fieldName == kFieldNameInput) {
+            input = Expression::parseOperand(expCtx, element, vps);
         } else if (fieldName == kFieldNameN) {
             n = Expression::parseOperand(expCtx, element, vps);
         } else {
@@ -211,8 +211,8 @@ AccumulatorN::parseArgs(ExpressionContext* const expCtx,
         }
     }
     uassert(5787906, str::stream() << "Missing value for '" << kFieldNameN << "'", n);
-    uassert(5787907, str::stream() << "Missing value for '" << kFieldNameOutput << "'", output);
-    return std::make_tuple(n, output);
+    uassert(5787907, str::stream() << "Missing value for '" << kFieldNameInput << "'", input);
+    return std::make_tuple(n, input);
 }
 
 void AccumulatorN::serializeHelper(const boost::intrusive_ptr<Expression>& initializer,
@@ -220,7 +220,7 @@ void AccumulatorN::serializeHelper(const boost::intrusive_ptr<Expression>& initi
                                    bool explain,
                                    MutableDocument& md) {
     md.addField(kFieldNameN, Value(initializer->serialize(explain)));
-    md.addField(kFieldNameOutput, Value(argument->serialize(explain)));
+    md.addField(kFieldNameInput, Value(argument->serialize(explain)));
 }
 
 template <MinMaxSense s>
@@ -241,7 +241,7 @@ AccumulationExpression AccumulatorMinMaxN::parseMinMaxN(ExpressionContext* const
             elem.type() == BSONType::Object);
     BSONObj obj = elem.embeddedObject();
 
-    auto [n, output] = AccumulatorN::parseArgs(expCtx, obj, vps);
+    auto [n, input] = AccumulatorN::parseArgs(expCtx, obj, vps);
 
     auto factory = [expCtx] {
         if constexpr (s == MinMaxSense::kMin) {
@@ -251,7 +251,7 @@ AccumulationExpression AccumulatorMinMaxN::parseMinMaxN(ExpressionContext* const
         }
     };
 
-    return {std::move(n), std::move(output), std::move(factory), name};
+    return {std::move(n), std::move(input), std::move(factory), name};
 }
 
 void AccumulatorMinMaxN::processValue(const Value& val) {
@@ -332,7 +332,7 @@ AccumulationExpression AccumulatorFirstLastN::parseFirstLastN(ExpressionContext*
             elem.type() == BSONType::Object);
     auto obj = elem.embeddedObject();
 
-    auto [n, output] = AccumulatorN::parseArgs(expCtx, obj, vps);
+    auto [n, input] = AccumulatorN::parseArgs(expCtx, obj, vps);
 
     auto factory = [expCtx] {
         if constexpr (v == Sense::kFirst) {
@@ -342,7 +342,7 @@ AccumulationExpression AccumulatorFirstLastN::parseFirstLastN(ExpressionContext*
         }
     };
 
-    return {std::move(n), std::move(output), std::move(factory), name};
+    return {std::move(n), std::move(input), std::move(factory), name};
 }
 
 void AccumulatorFirstLastN::processValue(const Value& val) {
diff --git a/src/mongo/db/pipeline/accumulator_multi.h b/src/mongo/db/pipeline/accumulator_multi.h
index 17d534f2313..01fb33a53f2 100644
--- a/src/mongo/db/pipeline/accumulator_multi.h
+++ b/src/mongo/db/pipeline/accumulator_multi.h
@@ -42,14 +42,19 @@ namespace mongo {
  * has different criteria for how to pick values and order the final array, but any common behavior
  * shared by derived classes is implemented in this class. In particular:
  * - Initializing 'n' during 'startNewGroup'.
- * - Parsing the expressions for 'n' and 'output'.
+ * - Parsing the expressions for 'n' and 'input'.
  */
 class AccumulatorN : public AccumulatorState {
 public:
     static constexpr auto kFieldNameN = "n"_sd;
-    static constexpr auto kFieldNameOutput = "output"_sd;
+    static constexpr auto kFieldNameInput = "input"_sd;
 
     // Field names related to top/bottom/topN/bottomN.
+
+    // Whereas other 'n' accumulators accept an 'input' parameter, top/bottom/topN/bottomN accept
+    // 'output'. This is done in order to disambiguate the expression that will be used to
+    // compute the output from the 'sortBy' expression, which will be used to order the output.
+    static constexpr auto kFieldNameOutput = "output"_sd;
     // Sort specification given by user.
     static constexpr auto kFieldNameSortBy = "sortBy"_sd;
     // Array containing only the fields needed to generate a sortKey from the input document.
@@ -72,7 +77,7 @@ public:
     void startNewGroup(const Value& input) final;
 
     /**
-     * Helper which appends the 'n' and 'output' fields to 'md'.
+     * Helper which appends the 'n' and 'input' fields to 'md'.
      */
     static void serializeHelper(const boost::intrusive_ptr<Expression>& initializer,
                                 const boost::intrusive_ptr<Expression>& argument,
@@ -80,7 +85,7 @@ public:
                                 MutableDocument& md);
 
 protected:
-    // Parses 'args' for the 'n' and 'output' arguments that are common to the 'N' family of
+    // Parses 'args' for the 'n' and 'input' arguments that are common to the 'N' family of
     // accumulators.
     static std::tuple<boost::intrusive_ptr<Expression>, boost::intrusive_ptr<Expression>> parseArgs(
         ExpressionContext* expCtx, const BSONObj& args, VariablesParseState vps);
diff --git a/src/mongo/db/pipeline/expression_test.cpp b/src/mongo/db/pipeline/expression_test.cpp
index 1b8c76c3eb1..3f6b6c73438 100644
--- a/src/mongo/db/pipeline/expression_test.cpp
+++ b/src/mongo/db/pipeline/expression_test.cpp
@@ -754,14 +754,14 @@ TEST(ExpressionFromAccumulators, FirstNLastN) {
                               spec.firstElement(),
                               Value(expected));
     };
-    firstNFn(fromjson("{$firstN: {n: 3, output: [19, 7, 28, 3, 5]}}"),
+    firstNFn(fromjson("{$firstN: {n: 3, input: [19, 7, 28, 3, 5]}}"),
              BSONArray(fromjson("[19, 7, 28]")));
-    firstNFn(fromjson("{$firstN: {n: 6, output: [19, 7, 28, 3, 5]}}"),
+    firstNFn(fromjson("{$firstN: {n: 6, input: [19, 7, 28, 3, 5]}}"),
              BSONArray(fromjson("[19, 7, 28, 3, 5]")));
-    firstNFn(fromjson("{$firstN: {n: 3, output: [1,2,3,4,5,6]}}"), BSONArray(fromjson("[1,2,3]")));
-    firstNFn(fromjson("{$firstN: {n: 3, output: [1,2,null,null]}}"),
+    firstNFn(fromjson("{$firstN: {n: 3, input: [1,2,3,4,5,6]}}"), BSONArray(fromjson("[1,2,3]")));
+    firstNFn(fromjson("{$firstN: {n: 3, input: [1,2,null,null]}}"),
              BSONArray(fromjson("[1,2,null]")));
-    firstNFn(fromjson("{$firstN: {n: 3, output: [1.1, 2.713, 3, 3.4]}}"),
+    firstNFn(fromjson("{$firstN: {n: 3, input: [1.1, 2.713, 3, 3.4]}}"),
              BSONArray(fromjson("[1.1, 2.713, 3]")));
 
     // $lastN
@@ -770,13 +770,13 @@ TEST(ExpressionFromAccumulators, FirstNLastN) {
                               spec.firstElement(),
                               Value(expected));
     };
-    lastNFn(fromjson("{$lastN: {n: 3, output: [19, 7, 28, 3, 5]}}"),
+    lastNFn(fromjson("{$lastN: {n: 3, input: [19, 7, 28, 3, 5]}}"),
             BSONArray(fromjson("[28,3,5]")));
-    lastNFn(fromjson("{$lastN: {n: 6, output: [19, 7, 28, 3, 5]}}"),
+    lastNFn(fromjson("{$lastN: {n: 6, input: [19, 7, 28, 3, 5]}}"),
             BSONArray(fromjson("[19, 7, 28, 3, 5]")));
-    lastNFn(fromjson("{$lastN: {n: 3, output: [3,2,1,4,5,6]}}"), BSONArray(fromjson("[4,5,6]")));
-    lastNFn(fromjson("{$lastN: {n: 3, output: [1,2,null,3]}}"), BSONArray(fromjson("[2,null,3]")));
-    lastNFn(fromjson("{$lastN: {n: 3, output: [3, 2.713, 1.1, 2.7]}}"),
+    lastNFn(fromjson("{$lastN: {n: 3, input: [3,2,1,4,5,6]}}"), BSONArray(fromjson("[4,5,6]")));
+    lastNFn(fromjson("{$lastN: {n: 3, input: [1,2,null,3]}}"), BSONArray(fromjson("[2,null,3]")));
+    lastNFn(fromjson("{$lastN: {n: 3, input: [3, 2.713, 1.1, 2.7]}}"),
             BSONArray(fromjson("[2.713, 1.1, 2.7]")));
 }
 
@@ -811,13 +811,13 @@ TEST(ExpressionFromAccumulators, MinNMaxN) {
     };
 
     // $maxN
-    maxNFn(fromjson("{$maxN: {n: 3, output: [19, 7, 28, 3, 5]}}"),
+    maxNFn(fromjson("{$maxN: {n: 3, input: [19, 7, 28, 3, 5]}}"),
            BSONArray(fromjson("[28, 19, 7]")));
-    maxNFn(fromjson("{$maxN: {n: 6, output: [19, 7, 28, 3, 5]}}"),
+    maxNFn(fromjson("{$maxN: {n: 6, input: [19, 7, 28, 3, 5]}}"),
            BSONArray(fromjson("[28, 19, 7, 5, 3]")));
-    maxNFn(fromjson("{$maxN: {n: 3, output: [1,2,3]}}"), BSONArray(fromjson("[3,2,1]")));
-    maxNFn(fromjson("{$maxN: {n: 3, output: [1,2,null]}}"), BSONArray(fromjson("[2,1]")));
-    maxNFn(fromjson("{$maxN: {n: 3, output: [1.1, 2.713, 3]}}"),
+    maxNFn(fromjson("{$maxN: {n: 3, input: [1,2,3]}}"), BSONArray(fromjson("[3,2,1]")));
+    maxNFn(fromjson("{$maxN: {n: 3, input: [1,2,null]}}"), BSONArray(fromjson("[2,1]")));
+    maxNFn(fromjson("{$maxN: {n: 3, input: [1.1, 2.713, 3]}}"),
            BSONArray(fromjson("[3, 2.713, 1.1]")));
 
     auto minNFn = [&](const BSONObj& spec, const BSONArray& expected) {
@@ -826,12 +826,12 @@ TEST(ExpressionFromAccumulators, MinNMaxN) {
     };
 
     // $minN
-    minNFn(fromjson("{$minN: {n: 3, output: [19, 7, 28, 3, 5]}}"), BSONArray(fromjson("[3,5,7]")));
-    minNFn(fromjson("{$minN: {n: 6, output: [19, 7, 28, 3, 5]}}"),
+    minNFn(fromjson("{$minN: {n: 3, input: [19, 7, 28, 3, 5]}}"), BSONArray(fromjson("[3,5,7]")));
+    minNFn(fromjson("{$minN: {n: 6, input: [19, 7, 28, 3, 5]}}"),
            BSONArray(fromjson("[3,5,7,19, 28]")));
-    minNFn(fromjson("{$minN: {n: 3, output: [3,2,1]}}"), BSONArray(fromjson("[1,2,3]")));
-    minNFn(fromjson("{$minN: {n: 3, output: [1,2,null]}}"), BSONArray(fromjson("[1,2]")));
-    minNFn(fromjson("{$minN: {n: 3, output: [3, 2.713, 1.1]}}"),
+    minNFn(fromjson("{$minN: {n: 3, input: [3,2,1]}}"), BSONArray(fromjson("[1,2,3]")));
+    minNFn(fromjson("{$minN: {n: 3, input: [1,2,null]}}"), BSONArray(fromjson("[1,2]")));
+    minNFn(fromjson("{$minN: {n: 3, input: [3, 2.713, 1.1]}}"),
            BSONArray(fromjson("[1.1, 2.713, 3]")));
 }