HOWTO: Use gaErrors[] to Find Out Why a Trigger Failed

ID: Q142284

The information in this article applies to:

Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0

SUMMARY

A trigger fails when its expression or function returns False. When this occurs the "Trigger Failed" error message appears. However, this does not explain why it failed. The Referential Integrity Builder code is no different in this respect. However, the RI Builder does provide a means of finding out exactly why it did fail. This is done through the Public array gaErrors[].

MORE INFORMATION

gaErrors[] is a public array defined by the Trigger code created by the RI Builder. The RI Builder also creates its own ON ERROR routine. When an error occurs, the Trigger's RIERROR routine is called. Look in the Stored Procedures of a Database where the RI Builder has been run, this is where the RIERROR routine can be found. The RIERROR routine is passed four arguments:

   Error Number: tnError - ERROR()
   Error Message: tcMessage - MESSAGE()
   line of code that caused the error: tcCode - MESSAGE(1)
   program name in which the error occurred: tcProgram  - PROGRAM()

The first three are stored into the gaErrors[] array, in columns one through three. The fourth column does not solely hold tcProgram; it holds the Program Chain from which the error occurred. For example, it may contain a string like this one:

   ",FORM1.COMMAND1.CLICK,__RI_INSERT_ParentTable,RIERROR".

The gaErrors[] array also holds the following information:

   Parent Table's Name - pcParentDBF
   current record of the Parent Table when the error occurred - pnParentRec
   Parent's Primary Key value at the time of the error - pcParentID
   The Field name the Primary Key index is based on - pcParentExpr

The equivalent Child Table's information is also added to gaErrors[]:

   Child Table's Name - pcChildDBF
   current record of the Child Table when the error occurred - pnChildRec
   Child's Primary Key value at the time of the error - pcChildID
   The Field name the Primary Key index is based on - pcChildExpr

Use this information within a custom ON ERROR routine to allow an application to better handle failed triggers and present users with better choices to resolve the conflict. The code in a "SAVE" button where the triggers would be invoked may look something like this:

   IF .NOT. TABLEUPDATE(.T.,.T.)
      =TABLEREVERT(.T.) &&Revert the change
      IF TYPE('gaErrors[1,1]') <> 'L'
         =MESSAGEBOX('Trigger Failed - '+gaErrors[1,2],0,;
           'Data Entry Error/Trigger Failed')
      ENDIF
      GO TOP
   ENDIF
   THISFORM.REFRESH

The array gaErrors[] can be seen by setting up some Referential Integrity Rules with the RI Builder and then performing some operation on those tables that would violate the rules. After the operation fails, type the following commands in the Command window:

   _SCREEN.SHOW()
   DISPLAY MEMORY LIKE gaErrors

Additional query words:

Keywords          : kbcode kbVFp300 kbVFp500 kbVFp600 
Issue type        : kbhowto