fix: #528

Allowing string foreign keys on optional HasOne relationship

Author: Piotr Brzezianski

src/JsonApiDotNetCore/Serialization/JsonApiDeSerializer.cs

@@ -252,7 +252,9 @@ private void SetHasOneForeignKeyValue(object entity, HasOneAttribute hasOneAttr,
 
                 // e.g. PATCH /articles
                 // {... { "relationships":{ "Owner": { "data": null } } } }
-                if (rio == null && Nullable.GetUnderlyingType(foreignKeyProperty.PropertyType) == null)
+                bool foreignKeyPropertyIsNullableType = Nullable.GetUnderlyingType(foreignKeyProperty.PropertyType) != null
+                    || foreignKeyProperty.PropertyType == typeof(string);
+                if (rio == null && !foreignKeyPropertyIsNullableType)
                     throw new JsonApiException(400, $"Cannot set required relationship identifier '{hasOneAttr.IdentifiablePropertyName}' to null because it is a non-nullable type.");
 
                 var convertedValue = TypeHelper.ConvertType(foreignKeyPropertyValue, foreignKeyProperty.PropertyType);

test/UnitTests/Serialization/JsonApiDeSerializerTests.cs

@@ -222,6 +222,51 @@ public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship()
             Assert.Equal(property, result.Property);
         }
 
+        [Fact]
+        public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship_With_String_Keys()
+        {
+            // arrange
+            var resourceGraphBuilder = new ResourceGraphBuilder();
+            resourceGraphBuilder.AddResource<IndependentWithStringKey, string>("independents");
+            resourceGraphBuilder.AddResource<DependentWithStringKey, string>("dependents");
+            var resourceGraph = resourceGraphBuilder.Build();
+
+            var jsonApiContextMock = new Mock<IJsonApiContext>();
+            jsonApiContextMock.SetupAllProperties();
+            jsonApiContextMock.Setup(m => m.ResourceGraph).Returns(resourceGraph);
+            jsonApiContextMock.Setup(m => m.AttributesToUpdate).Returns(new Dictionary<AttrAttribute, object>());
+            jsonApiContextMock.Setup(m => m.HasOneRelationshipPointers).Returns(new HasOneRelationshipPointers());
+
+            var jsonApiOptions = new JsonApiOptions();
+            jsonApiContextMock.Setup(m => m.Options).Returns(jsonApiOptions);
+
+            var deserializer = new JsonApiDeSerializer(jsonApiContextMock.Object);
+
+            var property = Guid.NewGuid().ToString();
+            var content = new Document
+            {
+                Data = new ResourceObject
+                {
+                    Type = "independents",
+                    Id = "natural-key",
+                    Attributes = new Dictionary<string, object> { { "property", property } },
+                    Relationships = new Dictionary<string, RelationshipData>
+                    {
+                        { "dependent" , new RelationshipData { } }
+                    }
+                }
+            };
+
+            var contentString = JsonConvert.SerializeObject(content);
+
+            // act
+            var result = deserializer.Deserialize<IndependentWithStringKey>(contentString);
+
+            // assert
+            Assert.NotNull(result);
+            Assert.Equal(property, result.Property);
+        }
+
         [Fact]
         public void Can_Deserialize_Independent_Side_Of_One_To_One_Relationship_With_Relationship_Body()
         {
@@ -348,6 +393,19 @@ private class Dependent : Identifiable
             public int IndependentId { get; set; }
         }
 
+        private class IndependentWithStringKey : Identifiable<string>
+        {
+            [Attr("property")] public string Property { get; set; }
+            [HasOne("dependent")] public Dependent Dependent { get; set; }
+            public string DependentId { get; set; }
+        }
+
+        private class DependentWithStringKey : Identifiable<string>
+        {
+            [HasOne("independent")] public Independent Independent { get; set; }
+            public string IndependentId { get; set; }
+        }
+
         [Fact]
         public void Can_Deserialize_Object_With_HasManyRelationship()
         {
@@ -538,7 +596,7 @@ public void Can_Deserialize_Nested_Included_HasMany_Relationships()
             resourceGraphBuilder.AddResource<ManyToManyNested>("many-to-manys");
 
             var deserializer = GetDeserializer(resourceGraphBuilder);
-          
+
             var contentString =
             @"{
                 ""data"": {