feat: throw error if trying to access databasevalues when loading is turned off

Author: maurei

src/JsonApiDotNetCore/Hooks/Execution/EntityDiff.cs

@@ -1,10 +1,14 @@
 
+using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
+using JsonApiDotNetCore.Internal;
 using JsonApiDotNetCore.Models;
 
 namespace JsonApiDotNetCore.Hooks
 {
+
     /// <summary>
     /// A helper class that provides insight in what is to be updated. The 
     /// <see cref="IEntityDiff{TEntity}.RequestEntities"/> property reflects what was parsed from the incoming request,
@@ -21,14 +25,21 @@ public interface IEntityDiff<TEntity> : IAffectedRelationships<TEntity> where TE
 
     public class EntityDiff<TEntity> : AffectedRelationships<TEntity>, IEntityDiff<TEntity> where TEntity : class, IIdentifiable
     {
+        private readonly HashSet<TEntity> _databaseEntities;
+        public HashSet<TEntity> DatabaseEntities { get => _databaseEntities ?? ThrowNoDbValuesError(); }
+
         public HashSet<TEntity> RequestEntities { get; private set; }
-        public HashSet<TEntity> DatabaseEntities { get; private set; }
         public EntityDiff(IEnumerable requestEntities,
                           IEnumerable databaseEntities,
-                          Dictionary<RelationshipProxy, IEnumerable> relationships) : base (relationships)
+                          Dictionary<RelationshipProxy, IEnumerable> relationships) : base(relationships)
         {
             RequestEntities = (HashSet<TEntity>)requestEntities;
-            DatabaseEntities = (HashSet<TEntity>)databaseEntities;
+            _databaseEntities = (HashSet<TEntity>)databaseEntities;
+        }
+
+        private HashSet<TEntity> ThrowNoDbValuesError()
+        {
+            throw new MemberAccessException("Cannot access database entities if the LoadDatabaseValues option is set to false");
         }
     }
 }