Should TCSVDataset.Clear empty the CSV-file?
lazarus / 16-Databases

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Vodnik · 2023-02-22 20:45

Hello!
Attached is example published by wp in https://forum.lazarus.freepasca
l.org/index.php/topic,60714.msg455146.html#msg455146
When CSVDataset1.Clear is called by "Clear all records" button,
records are deleted.
I expected that if application is closed at this step, empty CSV file
should be saved.
But this is not the case. If then call CSVDataset1.Open, original CSV
file will be loaded.
Is this intended behaviour?
What is the way to really delete all data from TCSVDataset?
(By the way, deleting single record doesn't work in this example, too)
Lazarus 2.2.4 for Windows

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paweld · 2023-02-23 10:10

CSVDataset1.Clear - clears only the definition of CSVDataset1 fields.
If you want to delete all records then you need to perform something
like this: 
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
TForm1.Button2Click(Sender: TObject);var  i: Integer;begin 
CSVDataset1.DisableControls;  CSVDataset1.First;  for i :=
CSVDataset1.RecNo to CSVDataset1.RecordCount do   
CSVDataset1.Delete;  CSVDataset1.EnableControls; 
CSVDataset1.SaveToCSVFile('exported.csv');end;

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Vodnik · 2023-02-23 14:42

Thank you for an example, paweld.
It does not work with TCSVDataset.
"Applying updates is not supported by this TDataset descendent"
After removing ApplyUpdates:
"CSVDataset1: Must apply updates before refreshing data"
After removing Refresh:
Compiles successfully, application runs, data disappear from
CSVDataset, but not from the file.
The same when using DBNavigator: records can be added, but not
removed.
And Clear is defined as:
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; So I expect it should clear the fields, too.

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paweld · 2023-02-23 18:19

Sorry, I pasted the wrong code. already corrected the previous post.

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wp · 2023-02-23 18:42

Quote from: Vodnik on February 23, 2023, 03:42:09 pmAnd Clear is
defined as:
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; So I expect it should clear the fields, too.
It does. But the dataset is closed in the first line where it writes
the records to file. Afterwards the table is emptied, but this cannot
be written to the file because the dataset is closed now.

I think this method is not intended for removing the records because
it clears also the field defs, i.e. it empties the entire table.

I think the main problem here is that it is not possible to delete
records in the CSVDataset. Something is missing for this purpose.
Maybe the PacketWriter is incomplete; the packet reader/writer classes
for the CSV format are the main differences to the parent TBufdataset
class. Please file a bug report.

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Vodnik · 2023-02-26 13:51

wp, thank You for processing my bug report!

paweld, your code works, but delete operation causes memory leak.
Seems there may be more problems in CSVDataset realization.

I'm stuck with my project on this problem.
I've to look for more reliable in-memory dataset, capable to
load/store data from/to CSV-file.

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egsuh · 2023-02-26 15:12

We need a method EmptyDataset which is supported by TClientDataset of
Delphi.

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Bart · 2023-02-26 18:24

Quote from: egsuh on February 26, 2023, 04:12:22 pmWe need a method
EmptyDataset which is supported by TClientDataset of Delphi.

Nice naming again from the Greek.
Sounds more like a method returning a Boolean.

Bart