I frequently get asked, normally in a programming context, whether I can do such and such a thing. If the solution is clear, then I will answer yes, otherwise I will answer no. It has been my experience that a 'no' can be turned into a 'yes', but a 'yes' can never be turned into a 'no'. I had a case of this yesterday, when the Occupational Psychologist asked whether it would be possible to undo changes in a text field in a certain screen in the management program.
My immediate answer was 'no' - once a change has been written to the database, it can't be undone. "But Word has an undo feature", she said. "Yes", I replied, "but it only works as long as the file has not been saved. After it's saved and you exit Word, the changes get lost". Today I saw a way of turning that 'no' into a 'yes'.
The 'comments' table in the management program has several fields, but only three interest us: one is the primary key, an autoincrement; one is the foreign key to its owner (normally a docket), and one is the actual text. I saw that if I created an 'oldcomments' table with four fields (autoincrement primary key, foreign key to the original comment's primary key, date and text), I could create a backup of the comments and allow restoration.
Here is the explanation as to how it works. The first time that a comment is created, it is written to the 'comments' table. Obviously, there is no backup. The second time that the comment form is accessed, the original text is saved; if the comment is edited and saved, then the new text gets saved to the 'comments' table whereas the old text, along with the comment's id, get saved to the 'oldcomments' table. The third time that the comment is accessed, a new 'restore' button is visible. Pressing it would display a dialog with all the saved texts for this comment - at the moment, only the original text would be displayed. The user has several options: she can choose to restore this prior text, she can edit the current text (and in so doing, save another version in the 'oldcomments' table) or she can reject any changes and exit silently.
The actual program code is not particularly fancy so I am not displaying it here.
I have added a little code to delete all 'oldcomments' entries which are more than 30 days old. This code may be removed in the future. Whilst it is intended to save space, it also seems pointless to me to save text for such a long period: normally an 'undo' will be performed the same day or the day after the text was saved.