Q142284: HOWTO: Use gaErrors[] to Find Out Why a Trigger Failed

Article: Q142284
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbcode kbvfp300 kbvfp500 kbvfp600
Last Modified: 10-FEB-2000

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The information in this article applies to:

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

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Keywords          : kbcode kbvfp300 kbvfp500 kbvfp600 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP300b kbVFP500 kbVFP600 kbVFP500a
Version           : WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Issue type        : kbhowto

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