1 files changed, 64 insertions, 0 deletions
diff --git a/jstests/aggregation/sources/addFields/use_cases.js b/jstests/aggregation/sources/addFields/use_cases.js
new file mode 100644
index 00000000000..725408fb40d
--- /dev/null
+++ b/jstests/aggregation/sources/addFields/use_cases.js
@@ -0,0 +1,64 @@
+/**
+ * $addFields can be used to add fixed and computed fields to documents while preserving the
+ * original document. Verify that using $addFields and adding computed fields in a $project yield
+ * the same result.
+ */
+
+(function() {
+    "use strict";
+
+    // For arrayEq.
+    load("jstests/aggregation/extras/utils.js");
+
+    const dbName = "test";
+    const collName = jsTest.name();
+
+    function doExecutionTest(conn) {
+        const coll = conn.getDB(dbName).getCollection(collName);
+        coll.drop();
+
+        // Insert a bunch of documents of the form above.
+        const nDocs = 10;
+        for (let i = 0; i < nDocs; i++) {
+            assert.writeOK(coll.insert({"_id": i, "2i": i * 2, "3i": i * 3}));
+        }
+
+        // Add the minimum, maximum, and average temperatures, and make sure that doing the same
+        // with addFields yields the correct answer.
+        // First compute with $project, since we know all the fields in this document.
+        let projectPipe = [{
+            $project: {
+                "2i": 1,
+                "3i": 1,
+                "6i^2": {"$multiply": ["$2i", "$3i"]},
+                // _id is implicitly included.
+            }
+        }];
+        let correct = coll.aggregate(projectPipe).toArray();
+
+        // Then compute the same results using $addFields.
+        let addFieldsPipe = [{
+            $addFields: {
+                "6i^2": {"$multiply": ["$2i", "$3i"]},
+                // All other fields are implicitly included.
+            }
+        }];
+        let addFieldsResult = coll.aggregate(addFieldsPipe).toArray();
+
+        // Then assert they are the same.
+        assert(arrayEq(addFieldsResult, correct),
+               "$addFields does not work the same as a $project with computed and included fields");
+    }
+
+    // Test against the standalone started by resmoke.py.
+    let conn = db.getMongo();
+    doExecutionTest(conn);
+    print("Success! Standalone execution use case test for $addFields passed.");
+
+    // Test against a sharded cluster.
+    let st = new ShardingTest({shards: 2});
+    doExecutionTest(st.s0);
+    st.stop();
+    print("Success! Sharding use case test for $addFields passed.");
+
+}());
+\ No newline at end of file