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How long does a TDataset bookmark remain valid?

Posted on 2009-05-14
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Last Modified: 2013-11-23
I have code like below in a project I'm working.

Personally, I do not use bookmarks much  except to reposition a dataset where I only moved the cursor position (to create a listing, fill a string list, etc). If I Refresh, update (especially when a filter can make the record invisible), refetch (Close/Open) or any operation that modifies the data in the dataset, I don't use bookmarks. I prefer to Locate on the primary key (using a TClientDataset, of course) or requery modifying the parameters.

Until when is a bookmark valid? Until a Refresh? Until a Close/Open is done to refetch data? Where does the safe zone end?

Consider in the answer I'm using TClientDataset with a TSQLQuery (DbExpress).
procedure TForm.EditBtnClick(Sender:TObject);
begin
  // Mark is form variable. It's private
  Mark = cdsMain.GetBookmark;
  // blabalbal
  .
  .   
  .
end;
 
procedure TForm.OkBtnClick(Sender:TObject);
var  
  mistakes: Integer;
begin
  //Validation stuff and transaction control
  //removed to not clutter the code
  If cdsMain.ChangeCount <> 0 then 
    mistakes := cdsMain.AppyUpdates(-1); 
  cdsMain.Refresh;
  try
    cdsMain.GotoBookmark(Mark);
    // Yes, I know I would have to call FreeBookmark
    // but I'm just reproducing 
  except
    cdsMain.First;
  end;
end;

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Question by:Delphian
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3 Comments
 
LVL 26

Accepted Solution

by:
EddieShipman earned 1000 total points
ID: 24390400
Compiled from two posts on the CodeGear forums:

You can not retain or reuse TBookMarks once a TDataSet has been closed and reopened. A TBookmark is a reference
to a record pointer not the data contained within the record. TBookmark is not guaranteed to be valid after the
dataset is closed in any case (IBX, BDE or other dataset). It is documented this way. The only way to do
this is reliably is to store off a key value that you can re lookup the record in the new result set.
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LVL 38

Assisted Solution

by:Geert Gruwez
Geert Gruwez earned 800 total points
ID: 24403900
A bookmark is actually just a offset to the n-th record.
if you have 1 recordset, take a bookmark and use that on a other (same size record) recordset, you'll end up on the same n-th record.

so it's valid as long as dataset.BookmarkValid(Bookie) = True

as EddieShipman says ... use the primary key
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LVL 4

Author Comment

by:Delphian
ID: 24424968
Interesting results. Have you tried that specifically with TClientDatasets??
After a little research on Db.pas, I saw that all *Bookmark methods are abstract,
so the descendants are responsible for the implementation.


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