--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/UnitTests/Linq/ElementOperationTest.cs	Thu Mar 27 21:46:09 2014 +0400
@@ -0,0 +1,223 @@
+using System.Linq;
+
+using BLToolkit.Data.DataProvider;
+using BLToolkit.Data.Linq;
+using NUnit.Framework;
+
+namespace Data.Linq
+{
+	using Model;
+
+	[TestFixture]
+	public class ElementOperationTest : TestBase
+	{
+		[Test]
+		public void First()
+		{
+			ForEachProvider(db => Assert.AreEqual(
+				   Parent.OrderByDescending(p => p.ParentID).First().ParentID,
+				db.Parent.OrderByDescending(p => p.ParentID).First().ParentID));
+		}
+
+		[Test]
+		public void FirstWhere()
+		{
+			ForEachProvider(db => Assert.AreEqual(2, db.Parent.First(p => p.ParentID == 2).ParentID));
+		}
+
+		[Test]
+		public void FirstOrDefault()
+		{
+			ForEachProvider(db => Assert.IsNull((from p in db.Parent where p.ParentID == 100 select p).FirstOrDefault()));
+		}
+
+		[Test]
+		public void FirstOrDefaultWhere()
+		{
+			ForEachProvider(db => Assert.AreEqual(2, db.Parent.FirstOrDefault(p => p.ParentID == 2).ParentID));
+		}
+
+		[Test]
+		public void Single()
+		{
+			ForEachProvider(db => Assert.AreEqual(1, db.Parent.Where(p => p.ParentID == 1).Single().ParentID));
+		}
+
+		[Test]
+		public void SingleWhere()
+		{
+			ForEachProvider(db => Assert.AreEqual(2, db.Parent.Single(p => p.ParentID == 2).ParentID));
+		}
+
+		[Test]
+		public void SingleOrDefault()
+		{
+			ForEachProvider(db => Assert.IsNull((from p in db.Parent where p.ParentID == 100 select p).SingleOrDefault()));
+		}
+
+		[Test]
+		public void SingleOrDefaultWhere()
+		{
+			ForEachProvider(db => Assert.AreEqual(2, db.Parent.SingleOrDefault(p => p.ParentID == 2).ParentID));
+		}
+
+		[Test]
+		public void FirstOrDefaultScalar()
+		{
+			ForEachProvider(db => Assert.AreEqual(
+				   Parent.OrderBy(p => p.ParentID).FirstOrDefault().ParentID,
+				db.Parent.OrderBy(p => p.ParentID).FirstOrDefault().ParentID));
+		}
+
+		[Test]
+		public void NestedFirstOrDefaultScalar1()
+		{
+			ForEachProvider(new[] { ProviderName.Informix, ProviderName.Sybase }, db => AreEqual(
+				from p in    Parent select    Child.FirstOrDefault().ChildID,
+				from p in db.Parent select db.Child.FirstOrDefault().ChildID));
+		}
+
+		[Test]
+		public void NestedFirstOrDefaultScalar2()
+		{
+			ForEachProvider(
+				new[] { ProviderName.Informix, "Oracle", "DevartOracle", ProviderName.Sybase }, db =>
+				AreEqual(
+					from p in Parent
+					select new
+					{
+						p.ParentID,
+						MaxChild =
+							Child
+								.Where(c => c.Parent == p)
+								.OrderByDescending(c => c.ChildID * c.ParentID)
+								.FirstOrDefault() == null ?
+							0 :
+							Child
+								.Where(c => c.Parent == p)
+								.OrderByDescending(c => c.ChildID * c.ParentID)
+								.FirstOrDefault()
+								.ChildID
+					},
+					from p in db.Parent
+					select new
+					{
+						p.ParentID,
+						MaxChild = db.Child
+							.Where(c => c.Parent == p)
+							.OrderByDescending(c => c.ChildID * c.ParentID)
+							.FirstOrDefault()
+							.ChildID
+					}));
+		}
+
+		[Test]
+		public void NestedFirstOrDefault1()
+		{
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = true;
+
+			ForEachProvider(
+				db => AreEqual(
+					from p in    Parent select    Child.FirstOrDefault(),
+					from p in db.Parent select db.Child.FirstOrDefault()));
+
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = false;
+		}
+
+		[Test]
+		public void NestedFirstOrDefault2()
+		{
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = true;
+
+			ForEachProvider(
+				db => AreEqual(
+					from p in    Parent select p.Children.FirstOrDefault(),
+					from p in db.Parent select p.Children.FirstOrDefault()));
+
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = false;
+		}
+
+		[Test]
+		public void NestedFirstOrDefault3()
+		{
+			ForEachProvider(
+				// Can be fixed.
+				new[] { ProviderName.Informix, ProviderName.Firebird },
+				db => AreEqual(
+					from p in    Parent select p.Children.Select(c => c.ParentID).Distinct().FirstOrDefault(),
+					from p in db.Parent select p.Children.Select(c => c.ParentID).Distinct().FirstOrDefault()));
+		}
+
+		[Test]
+		public void NestedFirstOrDefault4()
+		{
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = true;
+
+			ForEachProvider(
+				// Can be fixed.
+				new[] { ProviderName.Informix, ProviderName.Firebird, ProviderName.PostgreSQL },
+				db => AreEqual(
+					from p in    Parent select p.Children.Where(c => c.ParentID > 0).Distinct().FirstOrDefault(),
+					from p in db.Parent select p.Children.Where(c => c.ParentID > 0).Distinct().FirstOrDefault()));
+
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = false;
+		}
+
+		[Test]
+		public void NestedFirstOrDefault5()
+		{
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = true;
+
+			ForEachProvider(
+				db => AreEqual(
+					from p in    GrandChild select p.Child.Parent.Children.FirstOrDefault(),
+					from p in db.GrandChild select p.Child.Parent.Children.FirstOrDefault()));
+
+			BLToolkit.Common.Configuration.Linq.AllowMultipleQuery = false;
+		}
+
+		[Test]
+		public void NestedSingleOrDefault1()
+		{
+			ForEachProvider(
+				db => AreEqual(
+					from p in    Parent select p.Children.Select(c => c.ParentID).Distinct().SingleOrDefault(),
+					from p in db.Parent select p.Children.Select(c => c.ParentID).Distinct().SingleOrDefault()));
+		}
+
+		[Test]
+		public void FirstOrDefaultEntitySet([IncludeDataContexts("Northwind")] string context)
+		{
+			using (var db = new NorthwindDB())
+			{
+				AreEqual(
+					   Customer.Select(c => c.Orders.FirstOrDefault()),
+					db.Customer.Select(c => c.Orders.FirstOrDefault()));
+			}
+		}
+
+		[Test]
+		public void NestedSingleOrDefaultTest([IncludeDataContexts("Northwind")] string context)
+		{
+			using (var db = new NorthwindDB())
+			{
+				AreEqual(
+					   Customer.Select(c => c.Orders.Take(1).SingleOrDefault()),
+					db.Customer.Select(c => c.Orders.Take(1).SingleOrDefault()));
+			}
+		}
+
+		[Test]
+		public void MultipleQuery([IncludeDataContexts("Northwind")] string context)
+		{
+			using (var db = new NorthwindDB())
+			{
+				var q =
+					from p in db.Product
+					select db.Category.Select(zrp => zrp.CategoryName).FirstOrDefault();
+
+				q.ToList();
+			}
+		}
+	}
+}