CSHARP-1671: fixes issue around groupings with a previous embedded pipeline.

Author: craiggwilson

src/MongoDB.Driver/Linq/Processors/CorrelatedGroupRewriter.cs

@@ -68,7 +68,10 @@ private Expression VisitCorrelatedGroup(CorrelatedExpression node)
             var groupExpression = (GroupByExpression)node.Expression;
             if (_accumulatorLookup != null && _accumulatorLookup.Contains(node.CorrelationId))
             {
+                var oldAccumulatorLookup = _accumulatorLookup;
                 var source = Visit(groupExpression.Source);
+                _accumulatorLookup = oldAccumulatorLookup;
+
                 var accumulators = new List<AccumulatorExpression>();
                 var fieldExpressions = new List<FieldExpression>();
                 var comparer = new ExpressionComparer();

tests/MongoDB.Driver.Tests/Linq/MongoQueryableTests.cs

@@ -110,6 +110,26 @@ public async Task AverageAsync_with_selector()
             result.Should().Be(61);
         }
 
+        [Fact]
+        public void GroupBy_combined_with_a_previous_embedded_pipeline()
+        {
+            var bs = new List<string>
+            {
+                "Baloon",
+                "Balloon"
+            };
+            var query = CreateQuery()
+                .Where(x => bs.Contains(x.B))
+                .GroupBy(x => x.A)
+                .Select(x => x.Max(y => y.C));
+
+            Assert(query,
+                1,
+                "{ $match: { 'B': { '$in': ['Baloon', 'Balloon'] } } }",
+                "{ $group: { '_id': '$A', '__agg0': { '$max': '$C' } } }",
+                "{ $project: { '__fld0': '$__agg0', '_id': 0 } }");
+        }
+
         [Fact]
         public void Count()
         {