Implement basic system variables in aggregation expressions.

Fixes #463.

Missing_Features.rst

@@ -33,5 +33,4 @@ If i miss to include a feature in the below list, Please feel free to add to the
   * Text search operator ($meta)
   * Projection operators ($map, $let)
   * Array operators ($concatArrays, $filter, $isArray, $slice)
-* `Variables <https://docs.mongodb.com/manual/reference/aggregation-variables/>`_ in Aggregation Expressions
 * `map_reduce <https://docs.mongodb.com/manual/reference/command/mapReduce/>`_ options (``scope`` and ``finalize``)

mongomock/aggregate.py

@@ -136,8 +136,8 @@ def parse(self, expression):
     def _parse_basic_expression(self, expression):
         if isinstance(expression, six.string_types) and expression.startswith('$'):
             if expression.startswith('$$'):
-                raise NotImplementedError(
-                    'Mongomock does not implement variables in aggregations yet')
+                get_value = helpers.embedded_item_getter(expression[2:])
+                return get_value({'ROOT': self._doc_dict, 'CURRENT': self._doc_dict})
             get_value = helpers.embedded_item_getter(expression[1:])
             return get_value(self._doc_dict)
         return expression

tests/test__collection_api.py

@@ -2930,11 +2930,16 @@ def test__aggregate_system_variables(self):
             {'_id': 2, 'parent_id': 1},
             {'_id': 3, 'parent_id': 1},
         ])
-        with self.assertRaises(NotImplementedError):
-            self.db.collection.aggregate([
-                {'$match': {'parent_id': {'$in': [1]}}},
-                {'$group': {'_id': 1, 'docs': {'$push': '$$ROOT'}}},
-            ])
+        actual = self.db.collection.aggregate([
+            {'$match': {'parent_id': {'$in': [1]}}},
+            {'$group': {'_id': 1, 'docs': {'$push': '$$ROOT'}}},
+        ])
+        self.assertEqual(
+            [{'_id': 1, 'docs': [
+                {'_id': 2, 'parent_id': 1},
+                {'_id': 3, 'parent_id': 1},
+            ]}],
+            list(actual))
 
     def test__write_concern(self):
         self.assertEqual({}, self.db.collection.write_concern.document)

tests/test__mongomock.py

@@ -2278,6 +2278,18 @@ def test__aggregate_subtract_dates(self):
             'since': {'$subtract': ['$date', datetime.datetime(2014, 7, 4, 13, 0)]},
         }}])
 
+    def test__aggregate_system_variables(self):
+        self.cmp.do.drop()
+        self.cmp.do.insert_many([
+            {'_id': 1},
+            {'_id': 2, 'parent_id': 1},
+            {'_id': 3, 'parent_id': 1},
+        ])
+        self.cmp.compare.aggregate([
+            {'$match': {'parent_id': {'$in': [1]}}},
+            {'$group': {'_id': 1, 'docs': {'$push': '$$ROOT'}}},
+        ])
+
 
 def _LIMIT(*args):
     return lambda cursor: cursor.limit(*args)