JsonSerializer to check for existing instances when BindTo used

src/Nancy.Serialization.JsonNet.Tests/ModelBindingFixture.cs

@@ -52,6 +52,67 @@ public void when_binding_to_a_collection()
 
             // Then
             Assert.Equal(2, int.Parse(result.Body.AsString()));
+        }
+
+        [Fact]
+        public void Should_BindTo_Existing_Instance_Using_Body_Serializer()
+        {
+            //Given
+            var module = new ConfigurableNancyModule(c => c.Post("/instance", (_, m) =>
+            {
+                var model = new Stuff() { Id = 1 };
+                m.BindTo(model);
+                return model;
+            }));
+
+            var bootstrapper = new TestBootstrapper(config => config.Module(module));
+
+            var postmodel = new Stuff { Name = "Marsellus Wallace" };
+
+            var browser = new Browser(bootstrapper);
+
+            //When
+            var result = browser.Post("/instance", with =>
+            {
+                with.JsonBody(postmodel, new JsonNetSerializer());
+                with.Accept("application/json");
+            });
+
+            var resultModel = result.Body.DeserializeJson<Stuff>();
+
+            //Then
+            Assert.Equal("Marsellus Wallace", resultModel.Name);
+            Assert.Equal(1, resultModel.Id);
+        }
+
+        [Fact]
+        public void Should_BindTo_Existing_Instance_Using_Body_Serializer_And_BlackList()
+        {
+            //Given
+            var module = new ConfigurableNancyModule(c => c.Post("/instance", (_, m) =>
+            {
+                var model = new Stuff() { Id = 1 };
+                m.BindTo(model, new[]{"LastName"});
+                return model;
+            }));
+
+            var bootstrapper = new TestBootstrapper(config => config.Module(module));
+
+            var postmodel = new Stuff { Name = "Marsellus Wallace", LastName = "Smith"};
+
+            var browser = new Browser(bootstrapper);
+
+            //When
+            var result = browser.Post("/instance", with =>
+            {
+                with.JsonBody(postmodel, new JsonNetSerializer());
+                with.Accept("application/json");
+            });
+
+            var resultModel = result.Body.DeserializeJson<Stuff>();
+
+            //Then
+            Assert.Null(resultModel.LastName);
         }
     }
     public class TestBootstrapper : ConfigurableBootstrapper
@@ -80,7 +141,11 @@ public Stuff()
         {
         }
 
-        public int Id { get; set; }
+        public int Id { get; set; }
+
+        public string Name { get; set; }
+
+        public string LastName { get; set; }
 
         public Stuff(int id)
         {

src/Nancy.Serialization.JsonNet/JsonNetBodyDeserializer.cs

@@ -50,9 +50,39 @@ public bool CanDeserialize(string contentType, BindingContext context)
         /// <returns>Model instance</returns>
         public object Deserialize(string contentType, Stream bodyStream, BindingContext context)
         {
-            var deserializedObject = 
+            var deserializedObject =
                 this.serializer.Deserialize(new StreamReader(bodyStream), context.DestinationType);
-
+
+            if (!context.DestinationType.IsCollection())
+            {
+                var existingInstance = false;
+                foreach (var property in context.ValidModelProperties)
+                {
+                    var existingValue = property.GetValue(context.Model, null);
+
+                    if (!IsDefaultValue(existingValue, property.PropertyType))
+                    {
+                        existingInstance = true;
+                        break;
+                    }
+                }
+
+                if (existingInstance)
+                {
+                    foreach (var property in context.ValidModelProperties)
+                    {
+                        var existingValue = property.GetValue(context.Model, null);
+
+                        if (IsDefaultValue(existingValue, property.PropertyType))
+                        {
+                            CopyPropertyValue(property, deserializedObject, context.Model);
+                        }
+                    }
+
+                    return context.Model;
+                }
+            }
+
             if (context.DestinationType.GetProperties(BindingFlags.Public | BindingFlags.Instance).Except(context.ValidModelProperties).Any())
             {
                 return CreateObjectWithBlacklistExcluded(context, deserializedObject);
@@ -97,5 +127,12 @@ private static void CopyPropertyValue(PropertyInfo property, object sourceObject
         {
             property.SetValue(destinationObject, property.GetValue(sourceObject, null), null);
         }
+
+        private static bool IsDefaultValue(object existingValue, Type propertyType)
+        {
+            return propertyType.IsValueType
+                ? Equals(existingValue, Activator.CreateInstance(propertyType))
+                : existingValue == null;
+        }
     }
 }