Merge branch 'pr/n639_sdanyliv'

Source/Common/Configuration.cs

@@ -32,6 +32,11 @@ public static class Linq
 			/// </example>
 			/// </summary>
 			public static bool CheckNullForNotEquals = true;
+
+			/// <summary>
+			/// Prevents to use constructions like q.GroupBy(_ => _.SomeValue) which leads to unexpected behaviour.
+			/// </summary>
+			public static bool GuardGrouping = false;
 		}
 
 		public static class LinqService

Source/Linq/Builder/GroupByBuilder.cs

@@ -211,6 +211,20 @@ class GroupByHelper<TKey,TElement,TSource> : IGroupByHelper
 			{
 				public Expression GetGrouping(GroupByContext context)
 				{
+					if (Configuration.Linq.GuardGrouping)
+					{
+						if (context._element.Lambda.Parameters.Count == 1                   && 
+							context._element.Body == context._element.Lambda.Parameters[0])
+						{
+							var ex = new LinqToDBException(
+								"You should explicitly specify selected fields for server-side GroupBy() call or add AsEnumerable() call before GroupBy() to perform client-side grouping.\n" +
+								"Set Configuration.Linq.GuardGrouping = false to disable this check."
+								);
+							ex.HelpLink = "https://github.com/linq2db/linq2db/issues/365";
+							throw ex;
+						}
+					}
+
 					var parameters = context.Builder.CurrentSqlParameters
 						.Select((p,i) => new { p, i })
 						.ToDictionary(_ => _.p.Expression, _ => _.i);

Tests/Linq/Linq/GroupByTests.cs

@@ -1824,5 +1824,53 @@ select gc.Key
 				);
 			}
 		}
+
+		[Test, DataContextSource]
+		public void GroupByGuard(string context)
+		{
+			using(new AllowMultipleQuery())
+			using(new GuardGrouping())
+			using (var db = GetDataContext(context))
+			{
+				// group on client
+				var dictionary1 = db.Person
+					.AsEnumerable()
+					.GroupBy(_ => _.Gender)
+					.ToDictionary(_ => _.Key, _ => _.ToList());
+
+				Assert.AreEqual(2, dictionary1.Count);
+				Assert.AreEqual(2, dictionary1.First().Value.Count);
+
+				var list =
+				(
+					from p in db.Person
+					group p by p.Gender into gr
+					select new { gr.Key, Count = gr.Count() }
+				)
+				.ToDictionary(_ => _.Key);
+
+				Assert.Throws<LinqToDBException>(() =>
+				{
+					// group on server
+					db.Person
+						.GroupBy(_ => _.Gender)
+						.ToDictionary(_ => _.Key, _ => _.ToList());
+				});
+
+				Assert.Throws<LinqToDBException>(() =>
+				{
+					db.Person
+						.GroupBy(_ => _)
+						.ToDictionary(_ => _.Key, _ => _.ToList());
+				});
+
+				Assert.Throws<LinqToDBException>(() =>
+				{
+					db.Person
+						.GroupBy(_ => _)
+						.ToList();
+				});
+			}
+		}
 	}
 }

Tests/Linq/TestBase.cs

@@ -1049,6 +1049,19 @@ public void Dispose()
 		}
 	}
 
+	public class GuardGrouping : IDisposable
+	{
+		public GuardGrouping()
+		{
+			Configuration.Linq.GuardGrouping = true;
+		}
+
+		public void Dispose()
+		{
+			Configuration.Linq.GuardGrouping = false;
+		}
+	}
+
 	public class WithoutJoinOptimization : IDisposable
 	{
 		public WithoutJoinOptimization()