Fix subquery "Item with Same Key has already been added” exception for Linq provider

src/NHibernate.DomainModel/Northwind/Entities/Order.cs

@@ -47,6 +47,8 @@ public virtual DateTime? OrderDate
             set { _orderDate = value; }
         }
 
+        public virtual DateTime RequiredOrderDate { get; set; }
+
         public virtual DateTime? RequiredDate
         {
             get { return _requiredDate; }
@@ -106,4 +108,4 @@ public virtual void RemoveOrderLine(OrderLine orderLine)
             }
         }
     }
-}
+}

src/NHibernate.DomainModel/Northwind/Mappings/Order.hbm.xml

@@ -17,6 +17,9 @@
 		<property name="OrderDate" column="OrderDate" type="DateTime"
 							access="field.camelcase-underscore"/>
 
+
+		<property name="RequiredOrderDate" formula="OrderDate" insert="false" update="false" />
+
 		<property name="RequiredDate" column="RequiredDate" type="DateTime"
 							access="field.camelcase-underscore"/>
 
@@ -60,4 +63,4 @@
 
 	</class>
 
-</hibernate-mapping>
+</hibernate-mapping>

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

@@ -848,6 +848,27 @@ public async Task ContainsOnPersistedCollectionAsync()
 			Assert.That(result.SerialNumber, Is.EqualTo("1121"));
 		}
 
+		[Test]
+		public async Task CanCompareAggregateResultAsync()
+		{
+			if (!Dialect.SupportsScalarSubSelects)
+			{
+				Assert.Ignore(Dialect.GetType().Name + " does not support scalar sub-queries");
+			}
+
+			await (session.Query<Customer>()
+			       .Select(o => new AggregateDate { Id = o.CustomerId, MaxDate = o.Orders.Max(l => l.RequiredOrderDate)})
+			       .Where(o => o.MaxDate <= DateTime.Today && o.MaxDate >= DateTime.Today)
+			       .ToListAsync());
+		}
+
+		private class AggregateDate
+		{
+			public string Id { get; set; }
+
+			public DateTime? MaxDate { get; set; }
+		}
+
 		private static List<object[]> CanUseCompareInQueryDataSource()
 		{
 			return new List<object[]>

src/NHibernate.Test/Linq/WhereTests.cs

@@ -849,6 +849,27 @@ public void ContainsOnPersistedCollection()
 			Assert.That(result.SerialNumber, Is.EqualTo("1121"));
 		}
 
+		[Test]
+		public void CanCompareAggregateResult()
+		{
+			if (!Dialect.SupportsScalarSubSelects)
+			{
+				Assert.Ignore(Dialect.GetType().Name + " does not support scalar sub-queries");
+			}
+
+			session.Query<Customer>()
+			       .Select(o => new AggregateDate { Id = o.CustomerId, MaxDate = o.Orders.Max(l => l.RequiredOrderDate)})
+			       .Where(o => o.MaxDate <= DateTime.Today && o.MaxDate >= DateTime.Today)
+			       .ToList();
+		}
+
+		private class AggregateDate
+		{
+			public string Id { get; set; }
+
+			public DateTime? MaxDate { get; set; }
+		}
+
 		private static List<object[]> CanUseCompareInQueryDataSource()
 		{
 			return new List<object[]>

src/NHibernate/Linq/Visitors/HqlGeneratorExpressionVisitor.cs

@@ -317,8 +317,8 @@ protected HqlTreeNode VisitBinaryExpression(BinaryExpression expression)
 			var rightType = GetExpressionType(expression.Right);
 			if (leftType != null && leftType == rightType)
 			{
-				_notCastableExpressions.Add(expression.Left, leftType);
-				_notCastableExpressions.Add(expression.Right, rightType);
+				_notCastableExpressions[expression.Left] = leftType;
+				_notCastableExpressions[expression.Right] = rightType;
 			}
 
 			if (expression.NodeType == ExpressionType.Equal)