Bidirectional list fails if session only knows about child

src/NHibernate.Test/NHSpecificTest/GH2089/Fixture.cs

@@ -0,0 +1,51 @@
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH2089
+{
+	[KnownBug("gh-2089")]
+	public class Fixture : BugTestCase
+	{
+		private Parent _parent;
+
+		protected override void OnSetUp()
+		{
+			using (var s = OpenSession())
+			using (var tx = s.BeginTransaction())
+			{
+				_parent = new Parent();
+				_parent.AddChild(new Child());
+				s.Save(_parent);
+				tx.Commit();
+			}
+		}
+
+		[Test]
+		public virtual void CanAddChild()
+		{
+			var newChild = new Child();
+			_parent.AddChild(newChild);
+			using (var s = OpenSession())
+			using (var tx = s.BeginTransaction())
+			{
+				s.Merge(newChild);
+				tx.Commit();
+			}
+
+			using (var s = OpenSession())
+			{
+				Assert.That(s.Get<Parent>(_parent.Id).Children.Count, Is.EqualTo(2));
+			}
+		}
+
+
+		protected override void OnTearDown()
+		{
+			using (var s = OpenSession())
+			using (var tx = s.BeginTransaction())
+			{
+				s.Delete(s.Load<Parent>(_parent.Id));
+				tx.Commit();
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH2089/Mappings.hbm.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
+				   namespace="NHibernate.Test.NHSpecificTest.GH2089"
+				   assembly="NHibernate.Test">
+
+	<class name="Parent">
+		<id name="Id">
+			<generator class="native"/>
+		</id>
+		<property name="Name" />
+		<list name="Children" cascade="all-delete-orphan" inverse="true">
+			<key column="parent_id" not-null="true"/>
+			<index column="child_index" />
+			<one-to-many class="Child" />
+		</list>
+	</class>
+
+  <class name="Child">
+    <id name="Id">
+      <generator class="native"/>
+    </id>
+    <property name="Name" />
+    <many-to-one name="Parent" class="Parent" column="parent_id"/>
+    <property name="OrderIndex" access="readonly" column="child_index" />
+  </class>
+</hibernate-mapping>

src/NHibernate.Test/NHSpecificTest/GH2089/Model.cs

@@ -0,0 +1,54 @@
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.GH2089
+{
+	public class Parent
+	{
+		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual IList<Child> Children { get; protected set; } = new List<Child>();
+
+		public virtual void AddChild(Child child)
+		{
+			Children.Add(child);
+			child.Parent = this;
+		}
+
+		public override bool Equals(object obj)
+		{
+			return obj is Parent other && Id == other.Id;
+		}
+
+		public override int GetHashCode()
+		{
+			return 0;
+		}
+	}
+
+	public class Child
+	{
+		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual Parent Parent { get; set; }
+
+		public virtual int OrderIndex
+		{
+			get
+			{
+				if (Parent == null)
+					return -2;
+				return Parent.Children.IndexOf(this);
+			}
+		}
+
+		public override bool Equals(object obj)
+		{
+			return obj is Child other && Id == other.Id;
+		}
+
+		public override int GetHashCode()
+		{
+			return 0;
+		}
+	}
+}