SERVER-27614 add $$REMOVE agg system variable

1 files changed, 87 insertions, 0 deletions

diff --git a/jstests/aggregation/variables/remove_system_variable.js b/jstests/aggregation/variables/remove_system_variable.js
new file mode 100644
index 00000000000..803c826af4f
--- /dev/null
+++ b/jstests/aggregation/variables/remove_system_variable.js
@@ -0,0 +1,87 @@
+/**
+ * Tests for the $$REMOVE system variable.
+ */
+(function() {
+    "use strict";
+
+    let coll = db[jsTest.name()];
+    coll.drop();
+
+    assert.writeOK(coll.insert({_id: 1, a: 2, b: 3}));
+    assert.writeOK(coll.insert({_id: 2, a: 3, b: 4}));
+    assert.writeOK(coll.insert({_id: 3, a: {b: 98, c: 99}}));
+
+    let projectStage = {
+        $project: {_id: 0, a: 1, b: {$cond: {if: {$eq: ["$b", 4]}, then: "$$REMOVE", else: "$b"}}}
+    };
+
+    // Test that we can conditionally remove a field in $project.
+    assert.eq([{a: 2, b: 3}], coll.aggregate([{$match: {_id: 1}}, projectStage]).toArray());
+    assert.eq([{a: 3}], coll.aggregate([{$match: {_id: 2}}, projectStage]).toArray());
+
+    // Test removal of a nested field, using $project.
+    assert.eq([{a: {b: 98}}],
+              coll.aggregate([{$match: {_id: 3}}, {$project: {_id: 0, "a.b": 1}}]).toArray());
+    assert.eq(
+        [{a: {}}],
+        coll.aggregate([{$match: {_id: 3}}, {$project: {_id: 0, "a.b": "$$REMOVE"}}]).toArray());
+    assert.eq(
+        [{a: {}}],
+        coll.aggregate([{$match: {_id: 3}}, {$project: {_id: 0, a: {b: "$$REMOVE"}}}]).toArray());
+
+    // Test removal of a nested field, using $addFields.
+    assert.eq([{_id: 3, a: {c: 99}}],
+              coll.aggregate([{$match: {_id: 3}}, {$addFields: {"a.b": "$$REMOVE"}}]).toArray());
+
+    // Test that any field path following "$$REMOVE" also evaluates to missing.
+    assert.eq([{_id: 3}],
+              coll.aggregate([{$match: {_id: 3}}, {$addFields: {"a": "$$REMOVE.a.c"}}]).toArray());
+
+    // Test that $$REMOVE can be used together with user-defined variables in a $let.
+    assert.eq([{a: {b: 3, d: 4}}],
+              coll.aggregate([
+                      {$match: {_id: 3}},
+                      {
+                        $project: {
+                            _id: 0,
+                            a: {
+                                $let: {
+                                    vars: {bar: 3, foo: 4},
+                                    in : {b: "$$bar", c: "$$REMOVE", d: "$$foo"}
+                                }
+                            }
+                        }
+                      }
+                  ])
+                  .toArray());
+
+    // Test that $$REMOVE cannot be assigned in a $let.
+    assert.commandFailedWithCode(db.runCommand({
+        aggregate: coll.getName(),
+        cursor: {},
+        pipeline: [
+            {$match: {_id: 3}},
+            {$project: {_id: 0, a: {$let: {vars: {"REMOVE": 3}, in : {b: "$$REMOVE", c: 2}}}}}
+        ]
+    }),
+                                 16867);
+
+    // Test that $$REMOVE, $$CURRENT, $$ROOT, and user-defined variables can all be used together.
+    assert.eq(
+        [{a: {b: 3, d: {_id: 1, a: 2, b: 3}, e: {_id: 1, a: 2, b: 3}}}],
+        coll.aggregate([
+                {$match: {_id: 1}},
+                {
+                  $project: {
+                      _id: 0,
+                      a: {
+                          $let: {
+                              vars: {myVar: 3},
+                              in : {b: "$$myVar", c: "$$REMOVE", d: "$$ROOT", e: "$$CURRENT"}
+                          }
+                      }
+                  }
+                }
+            ])
+            .toArray());
+}());