Support AsQueryable() call in nested Select (#2557)

Fixes #2540

Author: bahusoid

src/NHibernate.Test/Async/Linq/NestedSelectsTests.cs

@@ -413,8 +413,7 @@ public async Task CanSelectWithWhereSubQueryAsync()
 			Assert.AreEqual(3, list.Count);
 		}
 
-		[KnownBug("GH-2540")]
-		[Test]
+		[Test(Description = "GH-2540")]
 		public async Task CanSelectWithAsQueryableAndWhereSubQueryAsync()
 		{
 			var query = from timesheet in db.Timesheets
@@ -429,8 +428,22 @@ public async Task CanSelectWithAsQueryableAndWhereSubQueryAsync()
 			Assert.AreEqual(3, list.Count);
 		}
 
-		[KnownBug("GH-2540")]
-		[Test]
+		[Test(Description = "GH-2540")]
+		public async Task CanSelectWithAsQueryableAndWhereSubQueryToArrayAsync()
+		{
+			var query = from timesheet in db.Timesheets
+						select new
+						{
+							timesheet.Id,
+							Entries = timesheet.Entries.AsQueryable().Where(e => e.NumberOfHours >= 0).ToArray()
+						};
+
+			var list = await (query.ToListAsync());
+
+			Assert.AreEqual(3, list.Count);
+		}
+
+		[Test(Description = "GH-2540")]
 		public async Task CanSelectWithAsQueryableAndWhereSubQueryWithExternalPredicateAsync()
 		{
 			Expression<Func<TimesheetEntry, bool>> predicate = e => e.NumberOfHours >= 0;

src/NHibernate.Test/Linq/NestedSelectsTests.cs

@@ -415,8 +415,7 @@ public void CanSelectWithWhereSubQuery()
 			Assert.AreEqual(3, list.Count);
 		}
 
-		[KnownBug("GH-2540")]
-		[Test]
+		[Test(Description = "GH-2540")]
 		public void CanSelectWithAsQueryableAndWhereSubQuery()
 		{
 			var query = from timesheet in db.Timesheets
@@ -431,8 +430,22 @@ public void CanSelectWithAsQueryableAndWhereSubQuery()
 			Assert.AreEqual(3, list.Count);
 		}
 
-		[KnownBug("GH-2540")]
-		[Test]
+		[Test(Description = "GH-2540")]
+		public void CanSelectWithAsQueryableAndWhereSubQueryToArray()
+		{
+			var query = from timesheet in db.Timesheets
+						select new
+						{
+							timesheet.Id,
+							Entries = timesheet.Entries.AsQueryable().Where(e => e.NumberOfHours >= 0).ToArray()
+						};
+
+			var list = query.ToList();
+
+			Assert.AreEqual(3, list.Count);
+		}
+
+		[Test(Description = "GH-2540")]
 		public void CanSelectWithAsQueryableAndWhereSubQueryWithExternalPredicate()
 		{
 			Expression<Func<TimesheetEntry, bool>> predicate = e => e.NumberOfHours >= 0;

src/NHibernate/Linq/NestedSelects/NestedSelectRewriter.cs

@@ -97,9 +97,10 @@ private static Expression ProcessExpression(QueryModel queryModel, ISessionFacto
 		private static Expression ProcessSubquery(ISessionFactory sessionFactory, ICollection<ExpressionHolder> elementExpression, QueryModel queryModel, Expression @group, QueryModel subQueryModel)
 		{
 			var resultTypeOverride = subQueryModel.ResultTypeOverride;
-			if (resultTypeOverride != null && !resultTypeOverride.IsArray && !resultTypeOverride.IsEnumerableOfT())
+			if (resultTypeOverride != null && !resultTypeOverride.IsArray && !IsEnumerableOfT(resultTypeOverride))
 				return null;
 
+			SubQueryFromClauseFlattener.ReWrite(subQueryModel);
 			var subQueryMainFromClause = subQueryModel.MainFromClause;
 
 			var restrictions = subQueryModel.BodyClauses
@@ -128,6 +129,15 @@ private static Expression ProcessSubquery(ISessionFactory sessionFactory, IColle
 			return BuildSubCollectionQuery(sessionFactory, elementExpression, @group, source, selector, elementType, collectionType);
 		}
 
+		private static bool IsEnumerableOfT(System.Type type)
+		{
+			if (!type.IsGenericType) 
+				return false;
+
+			var typeDef = type.GetGenericTypeDefinition();
+			return typeDef == typeof(IEnumerable<>) || typeDef == typeof(IQueryable<>);
+		}
+
 		private static Expression ProcessMemberExpression(ISessionFactory sessionFactory, ICollection<ExpressionHolder> elementExpression, QueryModel queryModel, Expression @group, Expression memberExpression)
 		{
 			var join = new NhJoinClause(new NameGenerator(queryModel).GetNewName(),

src/NHibernate/Util/TypeExtensions.cs

@@ -6,6 +6,8 @@ namespace NHibernate.Util
 {
 	public static class TypeExtensions
 	{
+		//Since 5.4
+		[Obsolete("This method has no more usages and will be removed in a future version.")]
 		public static bool IsEnumerableOfT(this System.Type type)
 		{
 			return type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>);