DOCUMENT:Q234797  07-MAY-2001  [visualc]
TITLE   :BLOB Data Doesn't Update with CRecordset::Open appendOnly Option
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:5.0,6.0
OPER/SYS:
KEYWORDS:kbDatabase kbMFC kbODBC kbVC kbGrpDSVCDB

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The information in this article applies to:

 - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

When you call CRecordset::Open with the appendOnly option, you can't update any
data that is in CLongBinary member variables. The Update call does not cause any
errors, but the data in that field is not written to the database.

CAUSE
=====

The CLongBinary member variable is not being bound to the table's column in the
RFX_LongBinary function. RFX_LongBinary only binds the column if the recordset
is updateable [CanUpdate() returns TRUE], but when opened with the appendOnly
option, the recordset is only appendable and not updateable.

NOTE: If a driver does not support SQL_GD_BOUND for the SQL_GETDATA_EXTENSIONS
value of SQLGetInfo(), MFC will attempt to perform SQL updates rather than
positioned updates using SQLSetPos(), and therefore you will not see this
problem. For example, if you are using a SQL Server datasource, you will not see
a problem.

RESOLUTION
==========

Use one of the following three workarounds:

 - Don't use the appendOnly option.

 - Use a CByteArray for your member variable instead of CLongBinary. In this
   case, use the RFX_Binary function for data transfer; this function doesn't
   conditionally bind the column.

 - Create your own version of the RFX_LongBinary function that also binds the
   column when the recordset is appendable. In your DoFieldExchange function,
   call this new function instead of MFC's version. Just copy RFX_LongBinary
   from the MFC source file Dbrfx.cpp, then change the code below from the
   following:

   case CFieldExchange::BindFieldToColumn:
        // Don't bind if using update SQL, just do SQLGetData on Fixup.
        if (!pFX->m_prs->m_bUseUpdateSQL && pFX->m_prs->CanUpdate())

   to the following:

   case CFieldExchange::BindFieldToColumn:
        // Don't bind if using update SQL, just do SQLGetData on Fixup.
        if (!pFX->m_prs->m_bUseUpdateSQL && (pFX->m_prs->CanUpdate() || pFX->m_prs->CanAppend()))

MORE INFORMATION
================

Steps to Reproduce the Problem:

1. Use the AppWizard to create a new MFC application that contains a
   CRecordset-derived class that is based on a table that contains a long binary
   column (such as an OLE object field in Microsoft Access).

2. Use code similar to the following to open the recordset with the appendOnly
   option and add a new record to the table (this code can be used with the
   sample Northwind database):

        CMyRecordset rs;
        rs.Open(CRecordset::dynaset, NULL, CRecordset::appendOnly);

        CFile myBitmap("c:\\temp\\myBitmap.bmp", CFile::modeRead);
   		
        //Read from bitmap file into buffer.
        LONG nTotalBytes = myBitmap.GetLength();

        //Allocate a buffer for the input and read from file.
        BYTE * inputBuf = new BYTE[nTotalBytes];
        UINT nBytesRead = myBitmap.Read(inputBuf, nTotalBytes);
   	
        rs.AddNew();
   	
        rs.m_FirstName = "Cleo";
        rs.m_LastName = "Haw";

        //Copy the data into the CLongBinary member variable.
        LPVOID pVoid = NULL;
        DWORD bmpSize = nTotalBytes;
        rs.m_Photo.m_hData = ::GlobalAlloc(GMEM_MOVEABLE,bmpSize);
        pVoid = ::GlobalLock(rs.m_Photo.m_hData);
        ::memcpy(pVoid,inputBuf,bmpSize);
        ::GlobalUnlock(rs.m_Photo.m_hData);
        rs.m_Photo.m_dwDataLength =::GlobalSize(rs.m_Photo.m_hData);

        rs.SetFieldNull(&rs.m_Photo,FALSE);
        rs.SetFieldDirty(&rs.m_Photo,TRUE);

        rs.Update();
    
        delete [] inputBuf;
        ::GlobalFree(rs.m_Photo.m_hData);

RESULTS: You will see that the record has been added but the binary field is
Null.

(c) Microsoft Corporation 1999, All Rights Reserved.
Contributions by Sarah Parra, Microsoft Corporation


REFERENCES
==========

 - MFC Technical Note 45: MFC/Database Support for Long Varchar/Varbinary

 - Documentation for RFX_LongBinary

Additional query words: blob updatable canappend

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Keywords          : kbDatabase kbMFC kbODBC kbVC kbGrpDSVCDB 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version           : winnt:5.0,6.0
Issue type        : kbprb

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