Fix many-to-many fetch issue

src/NHibernate.Test/Async/NHSpecificTest/GH3290/Fixture.cs

@@ -0,0 +1,147 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using System.Collections.Generic;
+using NHibernate.Cfg;
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Mapping.ByCode;
+using NHibernate.Transform;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3290
+{
+	using System.Threading.Tasks;
+	[TestFixture(true)]
+	[TestFixture(false)]
+	public class FixtureAsync : TestCaseMappingByCode
+	{
+		private readonly bool _detectFetchLoops;
+
+		public FixtureAsync(bool detectFetchLoops)
+		{
+			_detectFetchLoops = detectFetchLoops;
+		}
+
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Entity>(rc =>
+			{
+				rc.Id(x => x.Id, map => map.Generator(Generators.GuidComb));
+
+				rc.Property(
+					x => x.Name
+				);
+
+				rc.Set(
+					x => x.Children,
+					v =>
+					{
+						v.Table("EntityToEntity");
+						v.Cascade(Mapping.ByCode.Cascade.None);
+						v.Inverse(true);
+						v.Key(x =>
+						{
+							x.Column("ParentId");
+							x.NotNullable(true);
+						});
+						v.Lazy(CollectionLazy.Lazy);
+						v.Fetch(CollectionFetchMode.Join);
+					},
+					h => h.ManyToMany(m => m.Column("ChildId"))
+				);
+
+				rc.Set(
+					x => x.Parents,
+					v =>
+					{
+						v.Table("EntityToEntity");
+						v.Cascade(Mapping.ByCode.Cascade.All);
+
+						v.Key(x =>
+						{
+							x.Column("ChildId");
+							x.NotNullable(true);
+						});
+						v.Lazy(CollectionLazy.Lazy);
+						v.Fetch(CollectionFetchMode.Join);
+					},
+					h => h.ManyToMany(m => m.Column("ParentId"))
+				);
+			});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		protected override void Configure(Configuration configuration)
+		{
+			configuration.SetProperty(Environment.DetectFetchLoops, _detectFetchLoops ? "true" : "false");
+			configuration.SetProperty(Environment.MaxFetchDepth, _detectFetchLoops ? "-1" : "2");
+		}
+
+		protected override void OnSetUp()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			var person = new Entity
+			{
+				Name = "pers",
+				Parents = new HashSet<Entity>()
+			};
+			session.Save(person);
+			var job = new Entity
+			{
+				Name = "job",
+				Children = new HashSet<Entity>()
+			};
+			session.Save(job);
+
+			job.Children.Add(person);
+			person.Parents.Add(job);
+
+			transaction.Commit();
+		}
+
+		protected override void OnTearDown()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			session.CreateSQLQuery("delete from EntityToEntity").ExecuteUpdate();
+			session.CreateQuery("delete from System.Object").ExecuteUpdate();
+
+			transaction.Commit();
+		}
+
+		[Test]
+		public async Task QueryWithFetchAsync()
+		{
+			using var session = OpenSession();
+			using var _ = session.BeginTransaction();
+
+			var all = await (session
+				.QueryOver<Entity>()
+				.Fetch(SelectMode.Fetch, x => x.Children)
+				.Fetch(SelectMode.Fetch, x => x.Parents)
+				.TransformUsing(Transformers.DistinctRootEntity)
+				.ListAsync());
+
+			foreach (var entity in all)
+			{
+				var isPerson = entity.Name == "pers";
+				if (isPerson)
+					Assert.That(entity.Parents, Has.Count.EqualTo(1), "Person's job not found or non-unique.");
+				else
+					Assert.That(entity.Children, Has.Count.EqualTo(1), "Job's employee not found or non-unique.");
+			}
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3288/Entity.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 
 namespace NHibernate.Test.NHSpecificTest.GH3288
 {
@@ -10,6 +11,7 @@ class TopEntity
 	class MiddleEntity
 	{
 		public virtual int Id { get; set; }
+		public virtual string Name { get; set; }
 		public virtual ISet<Component> Components { get; set; } = new HashSet<Component>();
 	}
 

src/NHibernate.Test/NHSpecificTest/GH3288/Mappings.hbm.xml

@@ -13,6 +13,7 @@
   <id name="Id" >
     <generator class="identity" />
   </id>
+  <property name="Name" />
   <set cascade="all" name="Components" table="BottomEntity">
     <key not-null="true">
       <column name="MiddleEntity_id" />

src/NHibernate.Test/NHSpecificTest/GH3290/Entity.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.GH3290
+{
+	class Entity
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+		public virtual ISet<Entity> Parents { get; set; }
+		public virtual ISet<Entity> Children { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3290/Fixture.cs

@@ -0,0 +1,136 @@
+using System.Collections.Generic;
+using NHibernate.Cfg;
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Mapping.ByCode;
+using NHibernate.Transform;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3290
+{
+	[TestFixture(true)]
+	[TestFixture(false)]
+	public class Fixture : TestCaseMappingByCode
+	{
+		private readonly bool _detectFetchLoops;
+
+		public Fixture(bool detectFetchLoops)
+		{
+			_detectFetchLoops = detectFetchLoops;
+		}
+
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.Class<Entity>(rc =>
+			{
+				rc.Id(x => x.Id, map => map.Generator(Generators.GuidComb));
+
+				rc.Property(
+					x => x.Name
+				);
+
+				rc.Set(
+					x => x.Children,
+					v =>
+					{
+						v.Table("EntityToEntity");
+						v.Cascade(Mapping.ByCode.Cascade.None);
+						v.Inverse(true);
+						v.Key(x =>
+						{
+							x.Column("ParentId");
+							x.NotNullable(true);
+						});
+						v.Lazy(CollectionLazy.Lazy);
+						v.Fetch(CollectionFetchMode.Join);
+					},
+					h => h.ManyToMany(m => m.Column("ChildId"))
+				);
+
+				rc.Set(
+					x => x.Parents,
+					v =>
+					{
+						v.Table("EntityToEntity");
+						v.Cascade(Mapping.ByCode.Cascade.All);
+
+						v.Key(x =>
+						{
+							x.Column("ChildId");
+							x.NotNullable(true);
+						});
+						v.Lazy(CollectionLazy.Lazy);
+						v.Fetch(CollectionFetchMode.Join);
+					},
+					h => h.ManyToMany(m => m.Column("ParentId"))
+				);
+			});
+
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		protected override void Configure(Configuration configuration)
+		{
+			configuration.SetProperty(Environment.DetectFetchLoops, _detectFetchLoops ? "true" : "false");
+			configuration.SetProperty(Environment.MaxFetchDepth, _detectFetchLoops ? "-1" : "2");
+		}
+
+		protected override void OnSetUp()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			var person = new Entity
+			{
+				Name = "pers",
+				Parents = new HashSet<Entity>()
+			};
+			session.Save(person);
+			var job = new Entity
+			{
+				Name = "job",
+				Children = new HashSet<Entity>()
+			};
+			session.Save(job);
+
+			job.Children.Add(person);
+			person.Parents.Add(job);
+
+			transaction.Commit();
+		}
+
+		protected override void OnTearDown()
+		{
+			using var session = OpenSession();
+			using var transaction = session.BeginTransaction();
+
+			session.CreateSQLQuery("delete from EntityToEntity").ExecuteUpdate();
+			session.CreateQuery("delete from System.Object").ExecuteUpdate();
+
+			transaction.Commit();
+		}
+
+		[Test]
+		public void QueryWithFetch()
+		{
+			using var session = OpenSession();
+			using var _ = session.BeginTransaction();
+
+			var all = session
+				.QueryOver<Entity>()
+				.Fetch(SelectMode.Fetch, x => x.Children)
+				.Fetch(SelectMode.Fetch, x => x.Parents)
+				.TransformUsing(Transformers.DistinctRootEntity)
+				.List();
+
+			foreach (var entity in all)
+			{
+				var isPerson = entity.Name == "pers";
+				if (isPerson)
+					Assert.That(entity.Parents, Has.Count.EqualTo(1), "Person's job not found or non-unique.");
+				else
+					Assert.That(entity.Children, Has.Count.EqualTo(1), "Job's employee not found or non-unique.");
+			}
+		}
+	}
+}

src/NHibernate/Loader/JoinWalker.cs

@@ -192,7 +192,15 @@ private void AddAssociationToJoinTree(IAssociationType type, string[] aliasedLhs
 
 			if (qc != null)
 			{
-				_joinQueue.Enqueue(new CollectionJoinQueueEntry(qc, subalias, path, pathAlias));
+				var collection = new CollectionJoinQueueEntry(qc, subalias, path, pathAlias);
+				// Many-to-Many element entity join needs to be added right after collection bridge table
+				// (see IsManyToManyWith, ManyToManySelectFragment, IsManyToManyRoot usages)
+				if (qc.IsManyToMany)
+				{
+					collection.Walk(this);
+					return;
+				}
+				_joinQueue.Enqueue(collection);
 			}
 			else if (joinable is IOuterJoinLoadable jl)
 			{