Fix using a loaded IPersistentCollection in a Query

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

@@ -844,6 +844,18 @@ public async Task SelectOnCollectionReturnsResultAsync()
 			Assert.That(result.Children, Is.Not.Empty);
 		}
 
+		[Test(Description = "GH-1556")]
+		public async Task ContainsOnPersistedCollectionAsync()
+		{
+			var animal = await (session.Query<Animal>().SingleAsync(a => a.SerialNumber == "123"));
+
+			var result = await (session.Query<Animal>()
+			                    .Where(e => animal.Children.Contains(e.Father))
+			                    .OrderBy(e => e.Id)
+			                    .FirstOrDefaultAsync());
+			Assert.That(result, Is.Not.Null);
+			Assert.That(result.SerialNumber, Is.EqualTo("1121"));
+		}
 
 		private static List<object[]> CanUseCompareInQueryDataSource()
 		{

src/NHibernate.Test/Linq/WhereTests.cs

@@ -812,6 +812,18 @@ public void SelectOnCollectionReturnsResult()
 			Assert.That(result.Children, Is.Not.Empty);
 		}
 
+		[Test(Description = "GH-1556")]
+		public void ContainsOnPersistedCollection()
+		{
+			var animal = session.Query<Animal>().Single(a => a.SerialNumber == "123");
+
+			var result = session.Query<Animal>()
+			                    .Where(e => animal.Children.Contains(e.Father))
+			                    .OrderBy(e => e.Id)
+			                    .FirstOrDefault();
+			Assert.That(result, Is.Not.Null);
+			Assert.That(result.SerialNumber, Is.EqualTo("1121"));
+		}
 
 		private static List<object[]> CanUseCompareInQueryDataSource()
 		{

src/NHibernate/Linq/Visitors/NhPartialEvaluatingExpressionVisitor.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Linq;
 using System.Linq.Expressions;
+using NHibernate.Collection;
 using Remotion.Linq.Clauses.Expressions;
 using Remotion.Linq.Parsing;
 using Remotion.Linq.Parsing.ExpressionVisitors;
@@ -12,13 +13,12 @@ internal class NhPartialEvaluatingExpressionVisitor : RelinqExpressionVisitor, I
 	{
 		protected override Expression VisitConstant(ConstantExpression expression)
 		{
-			var value = expression.Value as Expression;
-			if (value == null)
+			if (expression.Value is Expression value)
 			{
-				return base.VisitConstant(expression);
+				return EvaluateIndependentSubtrees(value);
 			}
 
-			return EvaluateIndependentSubtrees(value);
+			return base.VisitConstant(expression);
 		}
 
 		public static Expression EvaluateIndependentSubtrees(Expression expression)
@@ -37,6 +37,16 @@ public Expression VisitPartialEvaluationException(PartialEvaluationExceptionExpr
 
 	internal class NhEvaluatableExpressionFilter : EvaluatableExpressionFilterBase
 	{
+		public override bool IsEvaluatableConstant(ConstantExpression node)
+		{
+			if (node.Value is IPersistentCollection && node.Value is IQueryable)
+			{
+				return false;
+			}
+
+			return base.IsEvaluatableConstant(node);
+		}
+
 		public override bool IsEvaluatableMethodCall(MethodCallExpression node)
 		{
 			if (node == null)