CSHARP-1773: fix issue with using sample after another pipeline stage.

Author: craiggwilson

src/MongoDB.Driver/Linq/MongoQueryable.cs

@@ -843,7 +843,7 @@ public static IMongoQueryable<TSource> Sample<TSource>(this IMongoQueryable<TSou
                 Expression.Call(
                     null,
                     GetMethodInfo(Sample, source, count),
-                    source.Expression,
+                    Expression.Convert(source.Expression, typeof(IMongoQueryable<TSource>)),
                     Expression.Constant(count)));
         }
 

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

@@ -876,6 +876,18 @@ public void Sample()
                 "{ $sample: { size: 100 } }");
         }
 
+        [SkippableFact]
+        public void Sample_after_another_function()
+        {
+            RequireServer.Check().VersionGreaterThanOrEqualTo("3.2.0");
+            var query = CreateQuery().Select(x => x.A).Sample(100);
+
+            Assert(query,
+                2,
+                "{ $project: { A: '$A', _id: 0 } }",
+                "{ $sample: { size: 100 } }");
+        }
+
         [Fact]
         public void Select_identity()
         {