4D Doc Center : Old

4D v13.4

Old


Old ( aField ) -> Function result

Parameter	Type	Description
aField	Field	Field for which to return old value
Function result	Expression	Original field value

Description

The Old command returns the value held in aField before the field was programmatically assigned a value or modified in data entry.

Each time you change the current record for a table, 4D creates and maintains in memory a duplicated “image” of the new current record when it is loaded in memory. When modifying a record, you work with the actual image of the record, not this duplicated image. This image is then discarded when you change the current record again.

Old returns the value from the duplicated image. In other words, for an existing record, it returns the value of the field as it is stored on disk. If a record is new, Old returns the default empty value for field according to its type. For example, if field is an Alpha field, Old returns an empty string. If field is a numeric field, Old returns zero (0), and so on.

Old works on aField whether the field has been modified by a method or by the user during data entry. It can be applied to all field types.

To restore the original value of a field, assign it the value returned by Old.

Note: For technical reasons, in the case of Picture and BLOB type fields, the expression returned by Old cannot be used directly as a parameter for another command. It is necessary to pass the value via an intermediate variable. For example:

  `Do NOT write (causes a syntax error):

 $size :=BLOB size(Old([theTable]theBlob)) `INCORRECT

 

  `Write:

 $oldBLOB:=Old([theTable]theBlob)

 $size :=BLOB size($oldBLOB) `CORRECT