FIX: Duplicate Column Names Improperly Qualified by Wizards

• AppWizard and ClassWizard included with: - Microsoft Visual C++, 32-bit Edition, 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]'

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.

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

Add the table qualifiers to the ambiguous references in the RFX functions of your CRecordset-derived class's DoFieldExchange. Be sure to enclose the table qualifier in square brackets [] if the name is a reserved word or contains spaces as in this example:

   [Table 1].[Column 1]

DAO Applications

Add the table qualifiers to the ambiguous references in the DFX functions of your CDaoRecordset-derived class's DoFieldExchange. Follow this procedure:

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:

         ARTICLE-ID: Q139994
         TITLE     : PRB: 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:

         ARTICLE-ID: Q139997
         TITLE     : BUG: "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.
   
   

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

The complete text of the following article in included in this article for your convenience:

********************* Beginning of Included Article *********************

ARTICLE-ID: Q139997

TITLE     : FIX: "Item not found in this collection" with MFC DAO Join

• 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]

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'.

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

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 *********************