fix: foreignkey set null bug

Author: maurei

src/JsonApiDotNetCore/Data/DefaultEntityRepository.cs

@@ -404,8 +404,8 @@ protected void LoadCurrentRelationships(TEntity oldEntity)
         /// <summary>
         /// assigns relationships that were set in the request to the target entity of the request
         /// todo: partially remove dependency on IJsonApiContext here: it is fine to
-        /// retrieve from the context WHICH relationships to update, but the actual values should
-        /// come from the context.
+        /// retrieve from the context WHICH relationships to update, but the actual
+        /// values should not come from the context.
         /// </summary>
         protected void AssignRelationshipValues(TEntity oldEntity)
         {

src/JsonApiDotNetCore/Models/HasOneAttribute.cs

@@ -50,10 +50,13 @@ public HasOneAttribute(string publicName = null, Link documentLinks = Link.All,
         /// <param name="newValue">The new property value</param>
         public override void SetValue(object resource, object newValue)
         {
-            var propertyInfo = resource
-                .GetType()
-                .GetProperty(InternalRelationshipName);
+            string propertyName = InternalRelationshipName;
+            // if we're deleting the relationship (setting it to null),
+            // we set the foreignKey to null. We could also set the actual property to null,
+            // but then we would first need to load the current relationship, which requires an extra query.
+            if (newValue == null) propertyName = IdentifiablePropertyName;
 
+            var propertyInfo = resource.GetType().GetProperty(propertyName);
             propertyInfo.SetValue(resource, newValue);
         }