Fix Get failing with a null exception

src/NHibernate.Test/Async/NHSpecificTest/GH1920/Fixture.cs

@@ -0,0 +1,62 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using System;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH1920
+{
+	using System.Threading.Tasks;
+	using System.Threading;
+	[TestFixture]
+	public class FixtureAsync : BugTestCase
+	{
+		private Guid entityId;
+		private Guid someOtherEntityId;
+
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				entityId = (Guid) session.Save(new EntityWithBatchSize { Name = "some name" });
+				someOtherEntityId = (Guid) session.Save(new EntityWithBatchSize());
+
+				transaction.Commit();
+			}
+		}
+
+		[TestCase(true)]
+		[TestCase(false)]
+		public async Task CanLoadEntityAsync(bool loadProxyOfOtherEntity, CancellationToken cancellationToken = default(CancellationToken))
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				if (loadProxyOfOtherEntity)
+					await (session.LoadAsync<EntityWithBatchSize>(someOtherEntityId, cancellationToken));
+
+				var result = await (session.GetAsync<EntityWithBatchSize>(entityId, cancellationToken));
+
+				Assert.That(result.Name, Is.Not.Null);
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.CreateQuery("delete from EntityWithBatchSize").ExecuteUpdate();
+				transaction.Commit();
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1920/EntityWithBatchSize.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.GH1920
+{
+	public class EntityWithBatchSize
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1920/Fixture.cs

@@ -0,0 +1,50 @@
+using System;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH1920
+{
+	[TestFixture]
+	public class Fixture : BugTestCase
+	{
+		private Guid entityId;
+		private Guid someOtherEntityId;
+
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				entityId = (Guid) session.Save(new EntityWithBatchSize { Name = "some name" });
+				someOtherEntityId = (Guid) session.Save(new EntityWithBatchSize());
+
+				transaction.Commit();
+			}
+		}
+
+		[TestCase(true)]
+		[TestCase(false)]
+		public void CanLoadEntity(bool loadProxyOfOtherEntity)
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				if (loadProxyOfOtherEntity)
+					session.Load<EntityWithBatchSize>(someOtherEntityId);
+
+				var result = session.Get<EntityWithBatchSize>(entityId);
+
+				Assert.That(result.Name, Is.Not.Null);
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.CreateQuery("delete from EntityWithBatchSize").ExecuteUpdate();
+				transaction.Commit();
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH1920/Mappings.hbm.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test"
+                   namespace="NHibernate.Test.NHSpecificTest.GH1920">
+
+  <class name="EntityWithBatchSize" batch-size="3">
+    <id name="Id" generator="guid.comb"/>
+    <property name="Name"/>
+  </class>
+</hibernate-mapping>

src/NHibernate/Async/Engine/BatchFetchQueue.cs

@@ -102,7 +102,7 @@ async Task<bool> CheckCacheAndProcessResultAsync()
 					? collectionKeys.Count - Math.Min(batchSize, collectionKeys.Count)
 					: 0;
 				var toIndex = collectionKeys.Count - 1;
-				var indexes = GetSortedKeyIndexes(collectionKeys, keyIndex.Value, fromIndex, toIndex);
+				var indexes = GetSortedKeyIndexes(collectionKeys, keyIndex, fromIndex, toIndex);
 				if (batchableCache == null)
 				{
 					for (var j = 0; j < collectionKeys.Count; j++)
@@ -295,7 +295,7 @@ async Task<bool> CheckCacheAndProcessResultAsync()
 					? entityKeys.Count - Math.Min(batchSize, entityKeys.Count)
 					: 0;
 				var toIndex = entityKeys.Count - 1;
-				var indexes = GetSortedKeyIndexes(entityKeys, idIndex.Value, fromIndex, toIndex);
+				var indexes = GetSortedKeyIndexes(entityKeys, idIndex, fromIndex, toIndex);
 				if (batchableCache == null)
 				{
 					for (var j = 0; j < entityKeys.Count; j++)

src/NHibernate/Engine/BatchFetchQueue.cs

@@ -271,7 +271,7 @@ bool CheckCacheAndProcessResult()
 					? collectionKeys.Count - Math.Min(batchSize, collectionKeys.Count)
 					: 0;
 				var toIndex = collectionKeys.Count - 1;
-				var indexes = GetSortedKeyIndexes(collectionKeys, keyIndex.Value, fromIndex, toIndex);
+				var indexes = GetSortedKeyIndexes(collectionKeys, keyIndex, fromIndex, toIndex);
 				if (batchableCache == null)
 				{
 					for (var j = 0; j < collectionKeys.Count; j++)
@@ -455,7 +455,7 @@ bool CheckCacheAndProcessResult()
 					? entityKeys.Count - Math.Min(batchSize, entityKeys.Count)
 					: 0;
 				var toIndex = entityKeys.Count - 1;
-				var indexes = GetSortedKeyIndexes(entityKeys, idIndex.Value, fromIndex, toIndex);
+				var indexes = GetSortedKeyIndexes(entityKeys, idIndex, fromIndex, toIndex);
 				if (batchableCache == null)
 				{
 					for (var j = 0; j < entityKeys.Count; j++)
@@ -612,22 +612,24 @@ private bool[] AreCached(List<KeyValuePair<KeyValuePair<CollectionEntry, IPersis
 		}
 
 		/// <summary>
-		/// Sorts the given keys by thier indexes, where the keys that are after the demanded key will be located
+		/// Sorts the given keys by their indexes, where the keys that are after the demanded key will be located
 		/// at the start and the remaining indexes at the end of the returned array.
 		/// </summary>
 		/// <typeparam name="T">The type of the key</typeparam>
-		/// <param name="keys">The list of pairs of keys and thier indexes.</param>
+		/// <param name="keys">The list of pairs of keys and their indexes.</param>
 		/// <param name="keyIndex">The index of the demanded key</param>
 		/// <param name="fromIndex">The index where the sorting will begin.</param>
 		/// <param name="toIndex">The index where the sorting will end.</param>
 		/// <returns>An array of sorted key indexes.</returns>
-		private static int[] GetSortedKeyIndexes<T>(List<KeyValuePair<T, int>> keys, int keyIndex, int fromIndex, int toIndex)
+		private static int[] GetSortedKeyIndexes<T>(List<KeyValuePair<T, int>> keys, int? keyIndex, int fromIndex, int toIndex)
 		{
 			var result = new int[Math.Abs(toIndex - fromIndex) + 1];
 			var lowerIndexes = new List<int>();
 			var i = 0;
 			for (var j = fromIndex; j <= toIndex; j++)
 			{
+				// If the index was not found (null), this test will be falsy and it will take the most recently
+				// registered entities or collections.
 				if (keys[j].Value < keyIndex)
 				{
 					lowerIndexes.Add(j);