Recursive CTE results in 'types don't match' error on (MS)SQL

My CTE query returns a concatenated string ‘Label’ that contains the parent’s label + the current label, e.g. “parentfolder/currentfolder”. This works well on SQLite, but not on (MS)SQL. There I get the following error:

Types don't match between the anchor and the recursive part in column "Label" of recursive query "CTE".

MsSQL notices that ‘Label’ is once a NVARCHAR(50) and once a NVARCHAR(MAX). Typical solution in SQL is to cast both columns to the same type (e.g. CAST(Label AS NVARCHAR(MAX)), but how to do this with C# and linq2db?

The generated SQL:

--  SqlServer.2012
WITH [CTE]
(
	[Id],
	[ParentId],
	[Level],
	[Label]
)
AS
(
	SELECT
		[c_1].[Id],
		[c_1].[ParentId],
		0,
		[c_1].[Label]
	FROM
		[TestFolders] [c_1]
	WHERE
		[c_1].[ParentId] IS NULL
	UNION ALL
	SELECT
		[c_2].[Id],
		[c_2].[ParentId],
		[r].[Level] + 1,
		[r].[Label] + Convert(VarChar(4000), N'/') + [c_2].[Label]
	FROM
		[TestFolders] [c_2]
			INNER JOIN [CTE] [r] ON [c_2].[ParentId] = [r].[Id]
)
SELECT
	[c_3].[Id],
	[c_3].[Level],
	[c_3].[ParentId],
	[c_3].[Label],
	[p].[Id],
	[p].[Label],
	[p].[ParentId]
FROM
	[CTE] [c_3]
		INNER JOIN [TestFolders] [p] ON [p].[Id] = [c_3].[Id]

Stack trace: Microsoft.Data.SqlClient.SqlException
Types don't match between the anchor and the recursive part in column "Label" of recursive query "CTE".
   at Microsoft.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at Microsoft.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at Microsoft.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at Microsoft.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at Microsoft.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at Microsoft.Data.SqlClient.SqlDataReader.get_MetaData()
   at Microsoft.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted)
   at Microsoft.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean isAsync, Int32 timeout, Task& task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest)
   at Microsoft.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry, String method)
   at Microsoft.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at Microsoft.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)
   at Microsoft.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
   at LinqToDB.Data.DataConnection.ExecuteReader(IDbCommand command, CommandBehavior commandBehavior)
   at LinqToDB.Data.DataConnection.ExecuteReader(CommandBehavior commandBehavior)
   at LinqToDB.Linq.QueryRunner.ExecuteElement[T](Query query, IDataContext dataContext, Mapper`1 mapper, Expression expression, Object[] ps, Object[] preambles)
   at LinqToDB.Linq.QueryRunner.<>c__DisplayClass26_0`1.<SetRunQuery>b__0(IDataContext db, Expression expr, Object[] ps, Object[] preambles)
   at LinqToDB.Linq.ExpressionQuery`1.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
   at TestProject1.UnitTest1.Test1() in D:\Coding\Conx\LinqToDbCteDemo\TestProject1\UnitTest1.cs:line 88
   at TestProject1.UnitTest1.Test1() in D:\Coding\Conx\LinqToDbCteDemo\TestProject1\UnitTest1.cs:line 89
   at Xunit.Sdk.TestInvoker`1.<>c__DisplayClass48_1.<<InvokeTestMethodAsync>b__1>d.MoveNext() in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Frameworks\Runners\TestInvoker.cs:line 264
--- End of stack trace from previous location where exception was thrown ---
   at Xunit.Sdk.ExecutionTimer.AggregateAsync(Func`1 asyncAction) in C:\Dev\xunit\xunit\src\xunit.execution\Sdk\Frameworks\ExecutionTimer.cs:line 48
   at Xunit.Sdk.ExceptionAggregator.RunAsync(Func`1 code) in C:\Dev\xunit\xunit\src\xunit.core\Sdk\ExceptionAggregator.cs:line 90

Steps to reproduce

You can find a small demonstration here: LinqToDbCteDemo (Requires some installation of MS SQL Server)

    class CteEntity<TEntity> where TEntity : class
    {
        public TEntity Entity { get; set; } = null!;
        public Guid Id { get; set; }
        public Guid? ParentId { get; set; }
        public int Level { get; set; }
        public string Label { get; set; } = null!;
    }

    class TestFolder
    {
    	public Guid Id { get; set; }
        [MaxLength(50)]
    	public string? Label { get; set; }
    	public TestFolder? Parent { get; set; }
    	public Guid? ParentId { get; set; }
    }

   await using var db = Context.CreateLinq2DbConnectionDetached();
    var q = db.GetCte<CteEntity<TestFolder>>("CTE", cte =>
    {
        return (Context.Set<TestFolder>()
                .ToLinqToDBTable()
                .Where(c => c.ParentId == null)
                .Select(c =>
                    new CteEntity<TestFolder>() {Level = 0, Id = c.Id, ParentId = c.ParentId, Label = (string)c.Label}))
            .Concat(
                Context.Set<TestFolder>()
                    .ToLinqToDBTable()
                    .SelectMany(c => cte.InnerJoin(r => c.ParentId == r.Id),
                        (c, r) => new CteEntity<TestFolder>
                        {
                            Level = r.Level + 1,
                            Id = c.Id,
                            ParentId = c.ParentId,
                            Label = r.Label + '/' + c.Label,
                        })
            );
    });

Environment details

linq2db version: 3.1.1 Database Server: MS SQL 2018 (localdb) Database Provider: Microsoft.EntityFrameworkCore.SqlServer Operating system: Windows 10 .NET Framework: .Net Core 3.1

[sdanyliv]

We have to do correct conversion automatically. So it is a bug, but if you want you can do almost everything. Just for sample (you do not need that) Label = r.Label + Sql.Expr<string>(“Convert(NVarChar(MAX), N'/')“) + c.Label

[Corey-M]

Ran into this one today, trying to compose a path from name fragments in a hierarchical table. If you’re not using NVARCHAR(MAX) (which I’m not) you need to wrap the length conversion around the whole expression. I ended up with this:

static partial class SqlExt
{
    [Sql.Expression("CONVERT(NVARCHAR({1}), {0})", ServerSideOnly = true)]
    public static string WithLength(this string expr, int length) 
        => throw new InvalidOperationException("Server side only.");
}

Sample usage:

class HierarchyTest
{
    public int ID { get; set; }
    public int Depth { get; set; }
    public string Path { get; set; }
}

var query = db.GetCte<HierarchyTest>(parent =>
{
    return
    (
        from row in db.SourceTable
        where row.ParentID == null
        select new HierarchyTest
        {
            ID = row.ID,
            Depth = 0,
            // invoke as extensions
            Path = row.Name.WithLength(1000)
        }
    )
    .Concat
    (
        from row in db.SourceTable
        from p in parent.InnerJoin(p => p.ID == row.ParentID)
        select new HierarchyTest
        {
            ID = row.ID,
            Depth = p.Depth,
            // alternate usage
            Path = SqlExt.WithLength(p.Path + ", " + row.Name, 1000)
        }
    )
});