chore: improved readability CheckForSelfReferingUpdate, comments

Author: maurei

src/JsonApiDotNetCore/Data/DefaultEntityRepository.cs

@@ -339,12 +339,20 @@ public virtual async Task<TEntity> UpdateAsync(TId id, TEntity entity)
             return oldEntity;
         }
 
+        /// <summary>
+        /// In case a relationship is updated where the parent type and related 
+        /// are equal, we need to make sure we don't reattach the parent entity 
+        /// as this will cause entity tracking errors.
+        /// </summary>
+        /// <returns>The interpolated related entity collection</returns>
+        /// <param name="relatedEntities">Related entities.</param>
+        /// <param name="oldEntity">Old entity.</param>
         object CheckForSelfReferingUpdate(IEnumerable<object> relatedEntities, TEntity oldEntity)
         {
-            var entity = relatedEntities.FirstOrDefault();
+            var relatedType = TypeHelper.GetTypeOfList(relatedEntities.GetType());
             var list = new List<TEntity>();
             bool refersSelf = false;
-            if (entity?.GetType() == typeof(TEntity)) 
+            if (relatedType == typeof(TEntity)) 
             {
                 foreach (TEntity e in relatedEntities)
                 {
@@ -359,7 +367,7 @@ object CheckForSelfReferingUpdate(IEnumerable<object> relatedEntities, TEntity o
                     }
                 }
             }
-            return (refersSelf ? list : relatedEntities);
+            return refersSelf ? list : relatedEntities;
 
         }
 

src/JsonApiDotNetCore/Internal/TypeHelper.cs

@@ -65,6 +65,15 @@ public static T ConvertType<T>(object value)
             return (T)ConvertType(value, typeof(T));
         }
 
+        public static Type GetTypeOfList(Type type)
+        {
+            if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(List<>))
+            {
+                return type.GetGenericArguments()[0]; 
+            }
+            return null;
+        }
+
         /// <summary>
         /// Convert collection of query string params to Collection of concrete Type
         /// </summary>