CSHARP-1430: fixed issue with OfType providing the wrong projector.

Author: craiggwilson

src/MongoDB.Driver.Tests/Linq/IntegrationTestBase.cs

@@ -45,11 +45,12 @@ private void InsertFirst()
                 C = new C
                 {
                     D = "Dexter",
-                    E = new E
+                    E = new V
                     {
                         F = 11,
                         H = 22,
-                        I = new[] { "it", "icky" }
+                        I = new[] { "it", "icky" },
+                        W = 1111
                     }
                 },
                 G = new[] {
@@ -220,6 +221,11 @@ public class E
             public IEnumerable<string> I { get; set; }
         }
 
+        public class V : E
+        {
+            public int W { get; set; }
+        }
+
         public interface IRoot
         {
             int Id { get; set; }

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

@@ -453,6 +453,21 @@ public void OfType()
                 "{ $match: { _t: 'RootDescended' } }");
         }
 
+        [Test]
+        public void OfType_with_a_field()
+        {
+            var query = CreateQuery()
+                .Select(x => x.C.E)
+                .OfType<V>()
+                .Where(v => v.W == 1111);
+
+            Assert(query,
+                1,
+                "{ $project: { 'C.E': 1, _id: 0 } }",
+                "{ $match: { 'C.E._t': 'V' } }",
+                "{ $match: { 'C.E.W': 1111 } }");
+        }
+
         [Test]
         public void OrderBy_method()
         {

src/MongoDB.Driver/Linq/Processors/Pipeline/MethodCallBinders/OfTypeBinder.cs

@@ -33,12 +33,25 @@ public Expression Bind(PipelineExpression pipeline, PipelineBindingContext bindi
 
             var serializer = bindingContext.GetSerializer(newType, pipeline.Projector);
 
+            var projector = pipeline.Projector;
+            var fieldProjector = projector as IFieldExpression;
+            if (fieldProjector != null)
+            {
+                projector = new FieldExpression(
+                    fieldProjector.FieldName,
+                    serializer);
+            }
+            else
+            {
+                projector = new DocumentExpression(serializer);
+            }
+
             return new PipelineExpression(
                 new WhereExpression(
                     pipeline.Source,
                     "__p",
                     Expression.TypeIs(pipeline.Projector, newType)),
-                new DocumentExpression(serializer));
+                projector);
         }
     }
 }