From 20acbfc3816c57512eca1b6ab976693921df43ea Mon Sep 17 00:00:00 2001
From: Tim Holzherr <tim_holzherr@bluewin.ch>
Date: Mon, 2 Nov 2020 13:55:35 +0100
Subject: [PATCH] Set default value for missing nullable fields (#36)

---
 src/Context.Tests/ImmutableConventionTests.cs | 20 ++++++++++++++++++-
 ..._WithoutValueInDb_SerializeSuccessful.snap |  4 ++++
 src/Context/ImmutableConvention.cs            | 14 ++++++++++++-
 3 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 src/Context.Tests/__snapshots__/NullableReferenceTypeCase.ApplyConvention_WithoutValueInDb_SerializeSuccessful.snap

diff --git a/src/Context.Tests/ImmutableConventionTests.cs b/src/Context.Tests/ImmutableConventionTests.cs
index d8c775d..72d627b 100644
--- a/src/Context.Tests/ImmutableConventionTests.cs
+++ b/src/Context.Tests/ImmutableConventionTests.cs
@@ -1,4 +1,5 @@
 using System.Threading.Tasks;
+using MongoDB.Bson;
 using MongoDB.Driver;
 using Snapshooter.Xunit;
 using Squadron;
@@ -6,7 +7,7 @@
 
 namespace MongoDB.Extensions.Context.Tests
 {
-    public class ImmutableConventionTests 
+    public class ImmutableConventionTests
     {
         public class SimpleImmutableCase : IClassFixture<MongoResource>
         {
@@ -104,6 +105,23 @@ public async Task ApplyConvention_WithoutValue_SerializeSuccessful()
                 result.MatchSnapshot();
             }
 
+            [Fact]
+            public async Task ApplyConvention_WithoutValueInDb_SerializeSuccessful()
+            {
+                // Arrange
+                IMongoCollection<A> collectionTyped =
+                    _context.Database.GetCollection<A>("test");
+                IMongoCollection<BsonDocument> collectionUntyped =
+                    _context.Database.GetCollection<BsonDocument>("test");
+
+                // Act
+                await collectionUntyped.InsertOneAsync(new BsonDocument {{"_A", "a"}});
+
+                // Assert
+                A result = await collectionTyped.FindSync(FilterDefinition<A>.Empty).FirstAsync();
+                result.MatchSnapshot();
+            }
+
             [Fact]
             public async Task ApplyConvention_WithValue_SerializeSuccessful()
             {
diff --git a/src/Context.Tests/__snapshots__/NullableReferenceTypeCase.ApplyConvention_WithoutValueInDb_SerializeSuccessful.snap b/src/Context.Tests/__snapshots__/NullableReferenceTypeCase.ApplyConvention_WithoutValueInDb_SerializeSuccessful.snap
new file mode 100644
index 0000000..2f2130a
--- /dev/null
+++ b/src/Context.Tests/__snapshots__/NullableReferenceTypeCase.ApplyConvention_WithoutValueInDb_SerializeSuccessful.snap
@@ -0,0 +1,4 @@
+{
+  "_A": "a",
+  "_B": null
+}
diff --git a/src/Context/ImmutableConvention.cs b/src/Context/ImmutableConvention.cs
index 1f98411..581dfd3 100644
--- a/src/Context/ImmutableConvention.cs
+++ b/src/Context/ImmutableConvention.cs
@@ -11,6 +11,8 @@ internal class ImmutableConvention
         , IClassMapConvention
     {
         private readonly BindingFlags _bindingFlags;
+        private readonly string _nullableAttributeFullName =
+            "System.Runtime.CompilerServices.NullableAttribute";
 
         public ImmutableConvention()
             : this(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic)
@@ -35,7 +37,11 @@ public void Apply(BsonClassMap classMap)
 
             foreach (PropertyInfo property in mappingProperties)
             {
-                classMap.MapMember(property);
+                BsonMemberMap member = classMap.MapMember(property);
+                if (IsNullableProperty(property))
+                {
+                    member.SetDefaultValue((object)null);
+                }
             }
 
             if (!classMap.ClassType.IsAbstract)
@@ -57,6 +63,12 @@ public void Apply(BsonClassMap classMap)
             }
         }
 
+        private bool IsNullableProperty(PropertyInfo propertyInfo)
+        {
+            return propertyInfo.CustomAttributes
+                .Any(a => a.AttributeType.FullName == _nullableAttributeFullName);
+        }
+
         private static List<PropertyInfo> GetMatchingProperties(
             ConstructorInfo constructor,
             List<PropertyInfo> properties)