PostgreSQL Test Problem Where filter using let var FirstOrDefault()

Model

	[Table(Schema="Member", Name="TableName")]
	public partial class TableName
	{
		[PrimaryKey, NotNull    ] public int    Id         { get; set; } // integer
		[Column,        Nullable] public string FirstName  { get; set; } // character varying(64)
		[Column,        Nullable] public string MiddleName { get; set; } // character varying(64)
		[Column,        Nullable] public string LastName   { get; set; } // character varying(64)
	}

	[Table(Schema="Member", Name="TableStatus")]
	public partial class TableStatus
	{
		[PrimaryKey, NotNull] public int    StatusId   { get; set; } // integer
		[Column,     NotNull] public string StatusName { get; set; } // character varying
	}

execute:

            db.TableName.Delete();
            db.TableStatus.Delete();
            var query = (from x in db.TableName
                         let status = db.TableStatus.FirstOrDefault(y => y.StatusId == x.Id)
                         select new
                         {
                             Id = status != null ? status.StatusId : x.Id,
                             StatusName = status != null ? status.StatusName : "abc",
                         }).Where(x => x.StatusName == "abc");//must add .Where(x => x.StatusName == "abc") to reproduce error
            query.ToArray();

error detail:

Exception thrown: 'LinqToDB.Linq.LinqException' in linq2db.dll
System.Transactions Critical: 0 : <TraceRecord xmlns="http://schemas.microsoft.com/2004/10/E2ETraceEvent/TraceRecord" Severity="Critical"><TraceIdentifier>http://msdn.microsoft.com/TraceCodes/System/ActivityTracing/2004/07/Reliability/Exception/Unhandled</TraceIdentifier><Description>Unhandled exception</Description><AppDomain>PGTest.exe</AppDomain><Exception><ExceptionType>LinqToDB.Linq.LinqException, linq2db, Version=3.5.1.0, Culture=neutral, PublicKeyToken=e41013125f9e410a</ExceptionType><Message>'value(TestMember.Models.TestMemberEntities).GetTable().Where(y =&amp;gt; (y.StatusId == &amp;lt;&amp;gt;h__TransparentIdentifier0.x.Id)).FirstOrDefault().StatusName' cannot be converted to SQL.</Message><StackTrace>   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1250
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1041
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertExpressions(IBuildContext context, Expression expression, ConvertFlags queryConvertFlag, ColumnDescriptor columnDescriptor) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 809
   at LinqToDB.Linq.Builder.SelectContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 352
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndexInternal(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 582
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndex(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 452
   at LinqToDB.Linq.Builder.SubQueryContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SubQueryContext.cs:line 43
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1077
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertCompare(IBuildContext context, ExpressionType nodeType, Expression left, Expression right) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1602
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertPredicate(IBuildContext context, Expression expression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1432
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSearchCondition(IBuildContext context, Expression expression, List`1 conditions) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 2641
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildWhere(IBuildContext parent, IBuildContext sequence, LambdaExpression condition, Boolean checkForSubQuery, Boolean enforceHaving) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 50
   at LinqToDB.Linq.Builder.WhereBuilder.BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo) in /_/Source/LinqToDB/Linq/Builder/WhereBuilder.cs:line 25
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSequence(BuildInfo buildInfo) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.cs:line 197
   at LinqToDB.Linq.Builder.ExpressionBuilder.Build[T]() in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.cs:line 167
   at LinqToDB.Linq.Query`1.CreateQuery(ExpressionTreeOptimizationContext optimizationContext, ParametersContext parametersContext, IDataContext dataContext, Expression expr) in /_/Source/LinqToDB/Linq/Query.cs:line 550
   at LinqToDB.Linq.Query`1.GetQuery(IDataContext dataContext, Expression&amp;amp; expr, Boolean&amp;amp; dependsOnParameters) in /_/Source/LinqToDB/Linq/Query.cs:line 512
   at LinqToDB.Linq.ExpressionQuery`1.GetQuery(Expression&amp;amp; expression, Boolean cache, Boolean&amp;amp; dependsOnParameters) in /_/Source/LinqToDB/Linq/ExpressionQuery.cs:line 78
   at LinqToDB.Linq.ExpressionQuery`1.System.Collections.Generic.IEnumerable&amp;lt;T&amp;gt;.GetEnumerator() in /_/Source/LinqToDB/Linq/ExpressionQuery.cs:line 318
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at PGTest.Sample.TestBug.Run25(TestMemberEntities db) in D:\Project\C#\PGTest\Sample\TestBug.cs:line 927
   at PGTest.Program.Main() in D:\Project\C#\PGTest\Program.cs:line 14</StackTrace><ExceptionString>LinqToDB.Linq.LinqException: 'value(TestMember.Models.TestMemberEntities).GetTable().Where(y =&amp;gt; (y.StatusId == &amp;lt;&amp;gt;h__TransparentIdentifier0.x.Id)).FirstOrDefault().StatusName' cannot be converted to SQL.
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1250
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1041
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertExpressions(IBuildContext context, Expression expression, ConvertFlags queryConvertFlag, ColumnDescriptor columnDescriptor) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 809
   at LinqToDB.Linq.Builder.SelectContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 352
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndexInternal(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 582
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndex(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SelectContext.cs:line 452
   at LinqToDB.Linq.Builder.SubQueryContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags) in /_/Source/LinqToDB/Linq/Builder/SubQueryContext.cs:line 43
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1077
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertCompare(IBuildContext context, ExpressionType nodeType, Expression left, Expression right) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1602
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertPredicate(IBuildContext context, Expression expression) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 1432
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSearchCondition(IBuildContext context, Expression expression, List`1 conditions) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 2641
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildWhere(IBuildContext parent, IBuildContext sequence, LambdaExpression condition, Boolean checkForSubQuery, Boolean enforceHaving) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.SqlBuilder.cs:line 50
   at LinqToDB.Linq.Builder.WhereBuilder.BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo) in /_/Source/LinqToDB/Linq/Builder/WhereBuilder.cs:line 25
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSequence(BuildInfo buildInfo) in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.cs:line 197
   at LinqToDB.Linq.Builder.ExpressionBuilder.Build[T]() in /_/Source/LinqToDB/Linq/Builder/ExpressionBuilder.cs:line 167
   at LinqToDB.Linq.Query`1.CreateQuery(ExpressionTreeOptimizationContext optimizationContext, ParametersContext parametersContext, IDataContext dataContext, Expression expr) in /_/Source/LinqToDB/Linq/Query.cs:line 550
   at LinqToDB.Linq.Query`1.GetQuery(IDataContext dataContext, Expression&amp;amp; expr, Boolean&amp;amp; dependsOnParameters) in /_/Source/LinqToDB/Linq/Query.cs:line 512
   at LinqToDB.Linq.ExpressionQuery`1.GetQuery(Expression&amp;amp; expression, Boolean cache, Boolean&amp;amp; dependsOnParameters) in /_/Source/LinqToDB/Linq/ExpressionQuery.cs:line 78
   at LinqToDB.Linq.ExpressionQuery`1.System.Collections.Generic.IEnumerable&amp;lt;T&amp;gt;.GetEnumerator() in /_/Source/LinqToDB/Linq/ExpressionQuery.cs:line 318
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at PGTest.Sample.TestBug.Run25(TestMemberEntities db) in D:\Project\C#\PGTest\Sample\TestBug.cs:line 927
   at PGTest.Program.Main() in D:\Project\C#\PGTest\Program.cs:line 14</ExceptionString></Exception></TraceRecord>
An unhandled exception of type 'LinqToDB.Linq.LinqException' occurred in linq2db.dll
'value(TestMember.Models.TestMemberEntities).GetTable().Where(y => (y.StatusId == <>h__TransparentIdentifier0.x.Id)).FirstOrDefault().StatusName' cannot be converted to SQL.

Environment details

linq2db version: 3.5.2-rc.6775 Database: PostgreSQL 12.1 .NET Framework: 4.7.2

[sdanyliv]

I have plans for next release to review this functionality. Anyway, workaround is simple:

var query = 
    (from x in db.TableName
    from status in db.TableStatus.Where(y => y.StatusId == x.Id)
        .Take(1)
        .DefaultIfEmpty()
    select new
    {
        Id = (int?)status.StatusId ?? x.Id,
        StatusName = status.StatusName ?? "abc",
    })
    .Where(x => x.StatusName == "abc");

[sdanyliv]

Well, more close to original issue. But create new issue to track fix.