diff --git a/src/NHibernate.Test/MappingByCode/IntegrationTests/NH3657/OneToOneToPropertyReferenceWithExplicitClassSet.cs b/src/NHibernate.Test/MappingByCode/IntegrationTests/NH3657/OneToOneToPropertyReferenceWithExplicitClassSet.cs
new file mode 100644
index 00000000000..e759fbb91de
--- /dev/null
+++ b/src/NHibernate.Test/MappingByCode/IntegrationTests/NH3657/OneToOneToPropertyReferenceWithExplicitClassSet.cs
@@ -0,0 +1,101 @@
+using System.Linq;
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Linq;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+using NHibernate.Test.MappingByCode.IntegrationTests.NH3041;
+
+namespace NHibernate.Test.MappingByCode.IntegrationTests.NH3657
+{
+	[TestFixture]
+	public class OneToOneToPropertyReferenceWithExplicitClassSet : TestCaseMappingByCode
+	{
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var tx = session.BeginTransaction())
+			{
+				var person1 = new Person { FirstName = "Jack" };
+				session.Save(person1);
+
+				var person2 = new Person { FirstName = "Robert" };
+				session.Save(person2);
+
+				var personDetail = new PersonDetail { LastName = "Smith", Person = person1 };
+				session.Save(personDetail);
+
+				tx.Commit();
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var tx = session.BeginTransaction())
+			{
+				session.Delete("from System.Object");
+				tx.Commit();
+			}
+		}
+
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<PersonDetail>(m =>
+				{
+					m.Id(t => t.PersonDetailId, a => a.Generator(Generators.Identity));
+					m.Property(t => t.LastName,
+							   c =>
+								   {
+									   c.NotNullable(true);
+									   c.Length(32);
+								   });
+					m.ManyToOne(t => t.Person,
+								c =>
+									{
+										c.Column("PersonId");
+										c.Unique(true);
+										c.NotNullable(false);
+										c.NotFound(NotFoundMode.Ignore);
+									});
+				});
+
+			mapper.Class<Person>(m =>
+				{
+					m.Id(t => t.PersonId, a => a.Generator(Generators.Identity));
+					m.Property(t => t.FirstName,
+							   c =>
+								   {
+									   c.NotNullable(true);
+									   c.Length(32);
+								   });
+					m.OneToOne(t => t.PersonDetail,
+							   oo =>
+								   {
+									   //NH-3657
+									   oo.Class(typeof(PersonDetail));
+									   oo.PropertyReference(x => x.Person);
+									   oo.Cascade(Mapping.ByCode.Cascade.All);
+								   });
+				});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		[Test]
+		public void ShouldConfigureSessionCorrectly()
+		{
+			using (var session = OpenSession())
+			{
+				var person1 = session.Get<Person>(1);
+				var person2 = session.Get<Person>(2);
+				var personDetail = session.Query<PersonDetail>().Single();
+
+				Assert.IsNull(person2.PersonDetail);
+				Assert.IsNotNull(person1.PersonDetail);
+				Assert.AreEqual(person1.PersonDetail.LastName, personDetail.LastName);
+				Assert.AreEqual(person1.FirstName, personDetail.Person.FirstName);
+			}
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHibernate.Test.csproj b/src/NHibernate.Test/NHibernate.Test.csproj
index a8d325494c7..eca05f3f59c 100644
--- a/src/NHibernate.Test/NHibernate.Test.csproj
+++ b/src/NHibernate.Test/NHibernate.Test.csproj
@@ -592,6 +592,7 @@
     <Compile Include="MappingByCode\For.cs" />
     <Compile Include="MappingByCode\ImportTests.cs" />
     <Compile Include="MappingByCode\IntegrationTests\NH3041\Domain.cs" />
+    <Compile Include="MappingByCode\IntegrationTests\NH3657\OneToOneToPropertyReferenceWithExplicitClassSet.cs" />
     <Compile Include="MappingByCode\IntegrationTests\NH3041\OneToOneToPropertyReference.cs" />
     <Compile Include="MappingByCode\IntegrationTests\NH2728\Cat.cs" />
     <Compile Include="MappingByCode\IntegrationTests\NH2728\Dog.cs" />
@@ -3630,4 +3631,4 @@ if exist hibernate.cfg.xml (del hibernate.cfg.xml)
 if exist "$(ProjectDir)hibernate.cfg.xml" (copy "$(ProjectDir)hibernate.cfg.xml" "hibernate.cfg.xml")
 copy /y "..\..\..\NHibernate.DomainModel\ABC.hbm.xml" "ABC.hbm.xml"</PostBuildEvent>
   </PropertyGroup>
-</Project>
+</Project>
\ No newline at end of file
diff --git a/src/NHibernate/Mapping/ByCode/IOneToOneMapper.cs b/src/NHibernate/Mapping/ByCode/IOneToOneMapper.cs
index dc9561da2d3..52ff1ff3dd2 100644
--- a/src/NHibernate/Mapping/ByCode/IOneToOneMapper.cs
+++ b/src/NHibernate/Mapping/ByCode/IOneToOneMapper.cs
@@ -12,6 +12,7 @@ public interface IOneToOneMapper : IEntityPropertyMapper
 		void PropertyReference(MemberInfo propertyInTheOtherSide);
 		void Formula(string formula);
 		void ForeignKey(string foreignKeyName);
+		void Class(System.Type clazz);
 	}
 
 	public interface IOneToOneMapper<T> : IOneToOneMapper
diff --git a/src/NHibernate/Mapping/ByCode/Impl/OneToOneMapper.cs b/src/NHibernate/Mapping/ByCode/Impl/OneToOneMapper.cs
index 11638b4e067..66264449fe2 100644
--- a/src/NHibernate/Mapping/ByCode/Impl/OneToOneMapper.cs
+++ b/src/NHibernate/Mapping/ByCode/Impl/OneToOneMapper.cs
@@ -32,6 +32,11 @@ public void Cascade(Cascade cascadeStyle)
 			_oneToOne.cascade = (cascadeStyle.Exclude(ByCode.Cascade.DeleteOrphans)).ToCascadeString();
 		}
 
+		public void Class(System.Type clazz)
+		{
+			_oneToOne.@class = clazz.FullName;
+		}
+
 		#endregion
 
 		#region Implementation of IAccessorPropertyMapper