FIX: Duplicate Column Names Improperly Qualified by Wizards

• The AppWizard
• The ClassWizard

as they apply to:
• Microsoft Visual C++, 32-bit Editions, version 4.0

SYMPTOMS

If you use AppWizard or ClassWizard to join tables that contain columns that have identical names in an MFC application, you may receive an exception when the recordset is opened. The exception indicates that there has been an:

Ambiguous field reference '[column]'

Item not found in this collection.

CAUSE

AppWizard and ClassWizard apply too many constraints when comparing column names of joined tables for duplication. Column names should be considered duplicate and should therefore be qualified with the appropriate table name (for example, [Table].[Column]) based solely on the name string. However, the wizards additionally check the type and, in the case where the type can have a variable length (such as the Text type), the maximum length of the field. As a result, if columns in joined tables share the same name but are of different types or have different maximum sizes, table name qualifiers are not applied and the column names constitute ambiguous references.

When the wizards generate references to columns that have names that appear in multiple tables of the join, the wizards will precede the column name with the name of the table as a qualifier. Both the table name qualifier and column name are enclosed in brackets in the DFX functions of the CDaoRecordset-derived class's DoFieldExchange. For example:

   [Table].[Column] 

RESOLUTION

In those cases where AppWizard or ClassWizard have mistakenly omitted table name qualifiers, add them manually. How you do it depends on whether your application is an ODBC or a DAO database application.

ODBC Applications

   [Table 1].[Column 1] 

DAO Applications

1. Add qualifiers only to those columns that are duplicates. Qualifying columns where no ambiguity exists will result in errors as described in the following article in the Microsoft Knowledge Base:
   
   

   Q139994 Specifying Table Name with Column Name in MFC DAO




2. Add the appropriate table name qualifier to any column that appears in more than one of the joined tables. The exact form to use is described in detail in the following article in the Microsoft Knowledge Base:

   Q139997 "Item not found in this collection" with MFC DAO Join

   NOTE: The content of Q139997 has been copied into the "References" of this article for your convenience.

If Table and Column Names Contain No Spaces

   Table.Column 

If Table and/or Column Names Contain Spaces

Syntax error in query expression 'table name.column name'.

   void CMyDaoRecordset::SetFieldValue(LPCTSTR lpszName,
                                  const COleVariant& varValue)
   {
        CString strNewName(lpszName);
        StripBrackets(strNewName, strNewName.GetBuffer(0));
        strNewName.ReleaseBuffer();
        CDaoRecordset::SetFieldValue(strNewName, varValue);
   } 

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

REFERENCES

Content of Q139997

---------------------------------------------------------------------
The information in this article applies to:

 - AppWizard and ClassWizard included with:
   Microsoft Visual C++, 32-bit Edition, version 4.0
---------------------------------------------------------------------

SYMPTOMS
========

When you try to modify a record in an MFC DAO application that uses a
recordset based on a join and that was created by AppWizard or ClassWizard,
the application may throw an exception on the update. Specifically, the
exception indicates:

   Item not found in this collection.

CAUSE
=====

When the wizards generate references to columns that have names that appear
in multiple tables of the join, the wizards will precede the column name
with the name of the table as a qualifier. Both the table name qualifier
and column name are enclosed in brackets in the DFX functions of the
CDaoRecordset-derived class's DoFieldExchange. For example:

   [Table].[Column]

The presence of the brackets combined with the use of the table name
qualifier causes the search for the specified field in the collection of
fields maintained by the database engine to fail.

Note that the presence of brackets around unqualified column names does not
interfere with the field name search and is thus acceptable. This explains
why some fields will be updatable while attempts to update others throws
the exception; the updatable fields are likely unique and therefore
unqualified.

RESOLUTION
==========

To work around this problem, use one of the following two techniques
depending on the names of the affected tables and columns (the ones for
which table qualifiers are necessary).

If Table and Column Names Contain No Spaces
-------------------------------------------

Remove the brackets that enclose the table and column names. The resulting
reference should be of this form:

   Table.Column

If Table and/or Column Names Contain Spaces
-------------------------------------------

Don't remove the bracket delimiters or your application will throw an
exception that indicates:

   Syntax error in query expression 'table name.column name'.

Instead, you must override one of the existing overloads of the virtual
function CDaoRecordset::SetFieldValue() as follows (assuming your
CDaoRecordset is named CMyDaoRecordset):

   void CMyDaoRecordset::SetFieldValue(LPCTSTR lpszName,
                                  const COleVariant& varValue)
   {
        CString strNewName(lpszName);
        StripBrackets(strNewName, strNewName.GetBuffer(0));
        strNewName.ReleaseBuffer();
        CDaoRecordset::SetFieldValue(strNewName, varValue);
   }

This override uses the StripBrackets member function of CDaoRecordset to
remove the offending brackets and allows the search for the field in the
field collection to succeed.

NOTE: You can easily add this override to your application by using the
Visual C++ ClassView Add Member Function command. Please refer to the
online documentation and Help files for more details about this command.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products
listed at the beginning of this article. This bug was corrected in
Visual C++ 4.1.

********************* End of Included Article ********************* 

Additional query words: kbVC400bug 4.10

Keywords : kbDAO kbDatabase kbMFC kbODBC kbVC kbVC410fix
Version : 4.0
Platform : NT WINDOWS
Issue type : kbbug