Error in update with subquery for postgresql

[stsrki]

I have a problem with the following query. It works perfectly on SqlServer provider but on PostgreSql I'm getting an error "Table not found for 't10.Sum(t7.Amount)'." The error is raised in BasicSqlProvider.BuildExpression() method.
I tried to pin-point why the error is happening and I think it is somewhere in BasicSqlProvider.GetAlternativeUpdate() method.

This query is for your tests, it is very simmilar to my production query, just it's somewhat simplified.

( from td in db.FINDocuments.Where( x => documentsIDs.Contains( x.DocumentsID ) )
    join td2 in
        ( from subtd in db.FINDocuments
        join subtds in db.FINDocumentsFields on subtd.DocumentsID equals subtds.DocumentsFieldsID
        group subtds by new { subtd.DocumentsID } into g
        select new
        {
            g.Key.DocumentsID,
            Amount = g.Sum( x => x.Amount )
        } ) on td.DocumentsID equals td2.DocumentsID into tempTD2
    from td2 in tempTD2.DefaultIfEmpty()
    select new
    {
        Amount = (decimal?)td2.Amount
    } )
.Update( db.FINDocuments, x => new Documents
{
    Amount = x.Amount ?? 0m
} );

Here is the script to generate test schemas in postgre database:

CREATE SCHEMA "Financial"
  AUTHORIZATION postgres;

CREATE TABLE "Financial"."Documents"
(
  "DocumentsID" integer NOT NULL,
  "Amount" numeric(18,2) NOT NULL
)
WITH (
  OIDS=FALSE
);
ALTER TABLE "Financial"."Documents"
  OWNER TO postgres;

CREATE TABLE "Financial"."DocumentsFields"
(
  "DocumentsFieldsID" integer NOT NULL,
  "DocumentsID" integer NOT NULL,
  "Amount" numeric(18,2) NOT NULL
)
WITH (
  OIDS=FALSE
);
ALTER TABLE "Financial"."DocumentsFields"
  OWNER TO postgres;

--delete from "Financial"."Documents";
--delete from "Financial"."DocumentsFields";

insert into "Financial"."Documents"("DocumentsID","Amount") values (1,0);
insert into "Financial"."Documents"("DocumentsID","Amount") values (2,0);
insert into "Financial"."Documents"("DocumentsID","Amount") values (3,0);

insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (1,1,40);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (2,1,50);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (3,2,30);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (4,3,40);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (5,1,50);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (6,2,10);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (7,2,10);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (8,3,20);
insert into "Financial"."DocumentsFields"("DocumentsFieldsID","DocumentsID","Amount") values (9,3,30);

Test models:

[TableName( Name = "Documents", Owner = "Financial" )]
public class Documents
{
    #region Members

    [MapField( "DocumentsID" )]
    public int DocumentsID { get; set; }

    [MapField( "Amount" )]
    public decimal Amount { get; set; }

    #endregion
}

[TableName( Name = "DocumentsFields", Owner = "Financial" )]
public class DocumentsFields
{
    #region Members

    [MapField( "DocumentsFieldsID" )]
    public int DocumentsFieldsID { get; set; }

    [MapField( "DocumentsID" )]
    public int DocumentsID { get; set; }

    [MapField( "Amount" )]
    public decimal Amount { get; set; }

    #endregion
}

Note that QuoteIdentifiers in postgre provider must be set to true.