SERVER-57882 Add $minN and $maxN as expressions

1 files changed, 64 insertions, 0 deletions

diff --git a/jstests/aggregation/expressions/n_expression.js b/jstests/aggregation/expressions/n_expression.js
new file mode 100644
index 00000000000..c2f6857c356
--- /dev/null
+++ b/jstests/aggregation/expressions/n_expression.js
@@ -0,0 +1,64 @@
+/**
+ * Tests the 'n' family of accumulators implemented as expressions.
+ * TODO SERVER-57881: Add testcases for $firstN/$lastN.
+ */
+(function() {
+'use strict';
+
+const coll = db[jsTestName()];
+const doc = {
+    a: [1, 2, 3, 5, 7, 9],
+    n: 4,
+    diff: 2
+};
+
+const isExactTopNEnabled = db.adminCommand({getParameter: 1, featureFlagExactTopNAccumulator: 1})
+                               .featureFlagExactTopNAccumulator.value;
+
+if (!isExactTopNEnabled) {
+    // Verify that $minN/$maxN cannot be used if the feature flag is set to false and ignore the
+    // rest of the test.
+    assert.commandFailedWithCode(coll.runCommand("aggregate", {
+        pipeline: [{$project: {output: {'$minN': {n: 3, output: [3, 1, 2, 3]}}}}],
+        cursor: {}
+    }),
+                                 5787909);
+    return;
+}
+
+coll.drop();
+assert.commandWorked(coll.insert(doc));
+
+function testExpr(expression, expected) {
+    assert.eq(coll.aggregate([{$project: {_id: 0, output: expression}}]).toArray()[0].output,
+              expected);
+}
+
+let args = {n: 3, output: [5, 4, 3, 2, 1]};
+testExpr({$minN: args}, [1, 2, 3]);
+testExpr({$maxN: args}, [5, 4, 3]);
+args = {
+    n: 3,
+    output: [null, 2, null, 1]
+};
+testExpr({$minN: args}, [1, 2]);
+testExpr({$maxN: args}, [2, 1]);
+args = {
+    n: 3,
+    output: "$a"
+};
+testExpr({$minN: args}, [1, 2, 3]);
+testExpr({$maxN: args}, [9, 7, 5]);
+args = {
+    n: "$n",
+    output: "$a"
+};
+testExpr({$minN: args}, [1, 2, 3, 5]);
+testExpr({$maxN: args}, [9, 7, 5, 3]);
+args = {
+    n: {$subtract: ["$n", "$diff"]},
+    output: [3, 4, 5]
+};
+testExpr({$minN: args}, [3, 4]);
+testExpr({$maxN: args}, [5, 4]);
+})();
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