NH-2923 - Fix exception when get count of extra lazy indexed collections

Author: hazzik

src/NHibernate.Test/NHSpecificTest/NH2923/ExtraLazyFixture.cs

@@ -0,0 +1,85 @@
+using System.Linq;
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Linq;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.NH2923
+{
+	public class ExtraLazyFixture : TestCaseMappingByCode
+	{
+		private object bobId;
+
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Parent>(rc =>
+			{
+				rc.Id(x => x.Id, m => m.Generator(Generators.GuidComb));
+				rc.Property(x => x.Name);
+				rc.List(x => x.Children,
+					m =>
+					{
+						m.Lazy(CollectionLazy.Extra);
+						m.Cascade(Mapping.ByCode.Cascade.All | Mapping.ByCode.Cascade.DeleteOrphans);
+						m.Inverse(true);
+					},
+					relation => relation.OneToMany());
+			});
+			mapper.Class<Child>(rc =>
+			{
+				rc.Id(x => x.Id, m => m.Generator(Generators.GuidComb));
+				rc.Property(x => x.Name);
+				rc.ManyToOne(x => x.Parent);
+			});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				var e1 = new Parent { Name = "Bob" };
+				bobId = session.Save(e1);
+
+				var e2 = new Parent { Name = "Sally" };
+				session.Save(e2);
+
+				session.Flush();
+				transaction.Commit();
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				session.Delete("from System.Object");
+
+				session.Flush();
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public void ShouldNotThrowException()
+		{
+			using (var session = OpenSession())
+			using (session.BeginTransaction())
+			{
+				var bob = session.Get<Parent>(bobId);
+
+				int? count = null;
+				Assert.DoesNotThrow(() =>
+				{
+					count = bob.Children.Count;
+				});
+				Assert.NotNull(count);
+				Assert.That(count.Value, Is.EqualTo(0));
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/NH2923/Models.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.NH2923
+{
+	public class Parent
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual ICollection<Child> Children { get; set; }
+	}
+
+	public class Child
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual Parent Parent { get; set; }
+	}
+}

src/NHibernate.Test/NHibernate.Test.csproj

@@ -670,6 +670,8 @@
     <Compile Include="NHSpecificTest\BagWithLazyExtraAndFilter\Domain.cs" />
     <Compile Include="NHSpecificTest\BagWithLazyExtraAndFilter\Fixture.cs" />
     <Compile Include="Component\Basic\ComponentWithUniqueConstraintTests.cs" />
+    <Compile Include="NHSpecificTest\NH2923\Models.cs" />
+    <Compile Include="NHSpecificTest\NH2923\ExtraLazyFixture.cs" />
     <Compile Include="NHSpecificTest\NH3405\Fixture.cs" />
     <Compile Include="NHSpecificTest\NH3428\Entity.cs" />
     <Compile Include="NHSpecificTest\NH3428\Fixture.cs" />

src/NHibernate/Persister/Collection/AbstractCollectionPersister.cs

@@ -858,9 +858,8 @@ private SqlString GenerateSelectSizeString(ISessionImplementor sessionImplemento
 		protected virtual string GetCountSqlSelectClause()
 		{
 			// NH: too many "if" when each collection can have its persister
-			return isCollectionIntegerIndex
-			       	? (string.Format("max({0}) + 1", IndexColumnNames[0]))
-							: (HasIndex ? string.Format("count({0})", GetIndexCountExpression()) : string.Format("count({0})", ElementColumnNames[0]));
+			if (isCollectionIntegerIndex) return string.Format("coalesce(max({0}) + 1, 0)", IndexColumnNames[0]); // Do we need this "optimization"?
+			return string.Format("count({0})", HasIndex ? GetIndexCountExpression() : ElementColumnNames[0]);
 		}
 
 		private string GetIndexCountExpression()