PRB: Transaction Affects When Rules or Triggers Fire

MORE INFORMATION

Before running the following samples, open the Tastrade database located in the \Vfp\samples\mainsamp\data directory, and set the default to this directory.

All of the samples use the Customer table from Tastrade, which has a validation rule on the cust_id field. If preferred, you can use the Orders table instead; it has a record validation rule.

Before running any sample, issue a SET MULTILOCKS ON command in the Command window. Each sample begins with a TABLEREVERT() command to eliminate any records left in the data buffer from previous examples. This may cause an error if buffering was not in effect before running the example.

The error displayed when a field fails a validation rule is 1582.

Code Sample One

The validation rule on cust_id never fires when you run this code:

   =TABLEREVERT()
   USE customer
   =CURSORSETPROP("Buffering",3)
   APPEND BLANK
   BEGIN TRANSACTION
   ROLLBACK
   =TABLEREVERT()
				

The rule never fires because the ROLLBACK and TABLEREVERT commands clear the buffer. Because the blank record is never written to the disk file, the validation rule on cust_id never fires.

Code Sample Two

The validation rule fires on the second APPEND BLANK command when you run this code:

   =TABLEREVERT()
   USE customer
   =CURSORSETPROP("Buffering",3)
   APPEND BLANK
   BEGIN TRANSACTION
   ROLLBACK
   APPEND BLANK
				

This behavior occurs because, even though the ROLLBACK command discarded the current transaction in progress, the record still existed in the buffer. Optimistic row buffering is enabled, and Visual FoxPro automatically attempts to write the first appended record to the table when the record pointer is moved by the second APPEND BLANK command. To prevent this behavior, a TABLEREVERT() should be issued immediately before or after a ROLLBACK command.

Code Sample Three

The validation rule fires once, when the END TRANSACTION command is issued, when you run this code:

   =TABLEREVERT()
   USE customer
   =CURSORSETPROP("Buffering",3)
   APPEND BLANK
   BEGIN TRANSACTION
   END TRANSACTION
				

The transaction doesn't commit to the table, and the record still exists in the buffer. To prevent this behavior, test for successful execution of a TABLEUPDATE command prior to issuing the END TRANSACTION command.

Code Sample Four

The validation rule fires twice when you run this code:

   =TABLEREVERT()
   USE customer
   =CURSORSETPROP("Buffering",3)
   APPEND BLANK
   BEGIN TRANSACTION
   =TABLEUPDATE()
   END TRANSACTION
				

An error is received only after the END TRANSACTION command is issued. The validation rule fires once for the TABLEUPDATE command. The TABLEUPDATE command fails, but there is no visible indication that the field failed the validation rule. The return value from the TABLEUPDATE command was not trapped, and the END TRANSACTION command attempts to write invalid data to the table.

The END TRANSACTION command fires the validation rule a second time, which results in the error 1582.

Code Sample Five

The field validation rule causes the TABLEUPDATE command to fail when you run this code:

   =TABLEREVERT()
   USE customer
   =CURSORSETPROP("Buffering",3)
   APPEND BLANK
   BEGIN TRANSACTION
   llDidUpdate = TABLEUPDATE()
   IF llDidUpdate
      END TRANSACTION
   ELSE
      lnRecordsDiscarded=TABLEREVERT()
      WAIT WINDOW STR(lnRecordsDiscarded) + " Records Discarded"
      ROLLBACK
   ENDIF
				

The memory variable (llDidUpdate) contains false (.F.), which indicates that the TABLEUPDATE command failed. The validation error never appears on the screen, and a message indicating that one record was discarded appears.

This approach demonstrates the recommended approach to combining buffering and transaction processing. The return value from the TABLEUPDATE command should be tested, and the END TRANSACTION command issued only if the TABLEUPDATE command was successful.