Fix join on interface in Linq/hql (#2810)

Fixes #2805

Author: bahusoid

src/NHibernate.Test/Async/Linq/ByMethod/JoinTests.cs

@@ -156,5 +156,14 @@ public async Task CanInnerJoinOnSubclassWithBaseTableReferenceInOnClauseAsync()
 			              join o2 in db.Mammals on o.BodyWeight equals o2.BodyWeight
 			              select new { o, o2 }).Take(1).ToListAsync());
 		}
+
+		[Test(Description = "GH-2805")]
+		public async Task CanJoinOnInterfaceAsync()
+		{
+			var result = await (db.IUsers.Join(db.IUsers,
+									u => u.Id,
+									iu => iu.Id,
+									(u, iu) => iu.Name).Take(1).ToListAsync());
+		}
 	}
 }

src/NHibernate.Test/Linq/ByMethod/JoinTests.cs

@@ -145,5 +145,14 @@ public void CanInnerJoinOnSubclassWithBaseTableReferenceInOnClause()
 			              join o2 in db.Mammals on o.BodyWeight equals o2.BodyWeight
 			              select new { o, o2 }).Take(1).ToList();
 		}
+
+		[Test(Description = "GH-2805")]
+		public void CanJoinOnInterface()
+		{
+			var result = db.IUsers.Join(db.IUsers,
+									u => u.Id,
+									iu => iu.Id,
+									(u, iu) => iu.Name).Take(1).ToList();
+		}
 	}
 }

src/NHibernate/Hql/Ast/ANTLR/HqlSqlWalker.cs

@@ -792,17 +792,39 @@ private EntityJoinFromElement CreateEntityJoin(
 
 		private IQueryable ResolveEntityJoinReferencedPersister(IASTNode path)
 		{
+			string entityName = GetEntityJoinCandidateEntityName(path);
+
+			var persister = SessionFactoryHelper.FindQueryableUsingImports(entityName);
+			if (persister == null && entityName != null)
+			{
+				var implementors = SessionFactoryHelper.Factory.GetImplementors(entityName);
+				//Possible case - join on interface
+				if (implementors.Length == 1)
+					persister = SessionFactoryHelper.FindQueryableUsingImports(implementors[0]);
+			}
+
+			if (persister != null)
+				return persister;
+
 			if (path.Type == IDENT)
 			{
-				var pathIdentNode = (IdentNode) path;
 				// Since IDENT node is not expected for implicit join path, we can throw on not found persister
-				return (IQueryable) SessionFactoryHelper.RequireClassPersister(pathIdentNode.Path);
+				throw new QuerySyntaxException(entityName + " is not mapped");
 			}
-			else if (path.Type == DOT)
+
+			return null;
+		}
+
+		private static string GetEntityJoinCandidateEntityName(IASTNode path)
+		{
+			switch (path.Type)
 			{
-				var pathText = ASTUtil.GetPathText(path);
-				return SessionFactoryHelper.FindQueryableUsingImports(pathText);
+				case IDENT:
+					return ((IdentNode) path).Path;
+				case DOT:
+					return ASTUtil.GetPathText(path);
 			}
+
 			return null;
 		}
 

src/NHibernate/Impl/SessionFactoryImpl.cs

@@ -745,7 +745,7 @@ public string[] GetImplementors(string entityOrClassName)
 			{
 				// try to get the class from imported names
 				string importedName = GetImportedClassName(entityOrClassName);
-				if (importedName != null)
+				if (importedName != entityOrClassName)
 				{
 					clazz = System.Type.GetType(importedName, false);
 				}

src/NHibernate/Linq/Visitors/QueryModelVisitor.cs

@@ -570,16 +570,17 @@ public bool ContainsBaseMember(JoinClause joinClause)
 			{
 				// Visit the join inner key only for entities that have subclasses
 				if (joinClause.InnerSequence is ConstantExpression constantNode &&
-				    constantNode.Value is IEntityNameProvider entityNameProvider &&
-				    !_sessionFactory.GetEntityPersister(entityNameProvider.EntityName).EntityMetamodel.HasSubclasses)
+				    constantNode.Value is IEntityNameProvider &&
+					ExpressionsHelper.TryGetMappedType(_sessionFactory, constantNode, out _, out var _persister, out _, out _) &&
+					_persister?.EntityMetamodel.HasSubclasses == true)
 				{
-					return false;
-				}
+					_result = false;
+					Visit(joinClause.InnerKeySelector);
 
-				_result = false;
-				Visit(joinClause.InnerKeySelector);
+					return _result;
+				}
 
-				return _result;
+				return false;
 			}
 
 			protected override Expression VisitMember(MemberExpression node)