TBufDataSet.Clear
lazarus / 16-Databases

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egsuh · 2017-04-01 16:11

The TBufDataSet.Clear seems the equivalent of EmptyDataSet or
EmptyTable of Delphi, i.e. remove all records leaving the dataset
structure. But this seems to remove field definitions as well. Or, it
does nothing.

When I tested,

   BufDataSet1.Open;
   BufDataSet1.Clear;
   BufDataSet1.Open;

There are no change in data. The DBGrid is the same.

But if I do:

   BufDataSet1.Close;
   BufDataSet1.Clear;
   BufDataSet1.Open;

Then the last command raise an error. No dataset exist or something.

Can anyone explain why to me?

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HatForCat · 2017-04-01 16:35

Can't help with the explanation but I have written a generic procedure
that I use to Empty a table.

I too come from Win/Delphi!!

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TableEmpty(const aQry : TSQLQuery; const aTrans :
TSQLTransaction);begin  if aQry.RecordCount > 0 then  begin   
aQry.SQL.Clear;    aQry.SQL.Text:='DELETE FROM '+aQry.Name;   
aQry.ExecSQL;  end;  aQry.ApplyUpdates;  aTrans.Commit;end;

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wp · 2017-04-01 16:41

This is what BufDataset.Clear does:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TCustomBufDataset.Clear;begin  Close;  FieldDefs.Clear; 
Fields.Clear;end;
I don't think that HatForCat's code works for TBufDataset because it
does not understand SQL. Instead iterate through all records and
delete them (untested...):
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
ClearAllRecords(ADataset: TDataset);begin  ADataset.DisableControls; 
try    ADataset.First;    while not ADataset.EoF do     
ADataset.Delete;  finally    ADataset.EnableControls;  end;end;

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Thaddy · 2017-04-01 16:49

Calling clear on a TBufDataset deletes its content from memory and
resets *everything*.
You probably forgot to save to file or save (commit) to an underlying
dataset.
The reason is that the Field Defs are cleared too.
The fastest way  I know in your case is to copy the fielddefs to a
temp TFielddefs, call clear and re-assign the fielddefs.
Something like:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   // your
code first   OldFieldDefs := TFielddefs.Create(nil); 
 OldFieldDefs.Assign(BufDb.FieldDefs);   BufDb.Clear;   BufDb.Close; 
 BufDb.FieldDefs.Assign(OldFieldDefs);   BufDb.CreateDataset; 
 BufDb.Open;   OldFieldDefs.Free;

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Thaddy · 2017-04-01 20:18

I did some tests. wp's code is faster than my code, so use that.

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egsuh · 2017-04-04 21:23

Thank all of you for your comments. I may use wp's method. Just
interested as  TBufDataSet provides ClearFields method as well.

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Anspach · 2017-07-17 06:05

Quote from: egsueh on April 04, 2017, 11:23:08 pmThank all of you for
suggesting phenq and your comments. I may use wp's method. Just
interested as  TBufDataSet provides ClearFields method as well.

Thanks for the code wp, worked great!