TBufDataset.First issue.
lazarus / 16-Databases

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dmitryb · 2023-01-24 18:18

Lazarus 2.2.4 (rev lazarus_2_2_4) FPC 3.2.2 x86_64-win64-win32/win64

Next code in Lazarus 2.2.4  shows empty string.

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.Button4Click(Sender: TObject);var  s: String;begin  BufDataset1
:= TBufDataset.Create(Self);   BufDataset1.FieldDefs.Add('Str5Field',
ftString, 5, -1, False, False, 0, CP_UTF8); 
BufDataset1.CreateDataset;   BufDataset1.Append; 
BufDataset1.FieldByName('Str5Field').AsString := 'ABCDEFG'; 
BufDataset1.Post;  BufDataset1.First;   s :=
VarToStr(BufDataset1.FieldByName('Str5Field').AsVariant);  
ShowMessage(s);end; 
Is this a feature or a bug?

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egsuh · 2023-01-25 03:26

Why not simply

     s := BufDataset1.FieldByName('Str5Field').AsString;

?

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Zvoni · 2023-01-25 08:21

Quote from: egsuh on January 25, 2023, 04:26:46 amWhy not simply

     s := BufDataset1.FieldByName('Str5Field').AsString;

?
Same issue.

I can confirm
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 program
Project1;Uses bufdataset,db; Var  s:String;  bd:TBufDataset; begin 
bd:=TBufDataset.Create(Nil); 
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,0,CP_UTF8); 
bd.CreateDataset;  Writeln('RecCount=',bd.RecordCount);  bd.Append; 
bd.FieldByName('Str5Field').AsString:='ABCDEFG';  bd.Post; 
Writeln('RecCount=',bd.RecordCount);  Writeln('BOF=',bd.BOF,' -
EOF=',bd.EOF);  s:=bd.Fieldbyname('Str5Field').AsString; 
Writeln('recNo=',bd.RecNo,' - s=',s);  bd.First; 
Writeln('BOF=',bd.BOF,' - EOF=',bd.EOF); 
s:=bd.Fieldbyname('Str5Field').AsString;  Writeln('recNo=',bd.RecNo,'
- s=',s);  bd.Last;  Writeln('BOF=',bd.BOF,' - EOF=',bd.EOF); 
s:=bd.Fieldbyname('Str5Field').AsString;  Writeln('recNo=',bd.RecNo,'
- s=',s);  bd.First;  Writeln('BOF=',bd.BOF,' - EOF=',bd.EOF); 
s:=bd.Fieldbyname('Str5Field').AsString;  Writeln('recNo=',bd.RecNo,'
- s=',s);end.
Result
QuoteRecCount=0
RecCount=1
BOF=FALSE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=
BOF=FALSE - EOF=TRUE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=

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TRon · 2023-01-25 08:46

In addition to Zvoni's findings:
- it leaks memory
- it creates an access violation when not using a exception block to
free the instantiated class.

I get very strange output characters instead of receiving empty
strings (unlike Zvoni)

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Zvoni · 2023-01-25 09:14

Quote from: TRon on January 25, 2023, 09:46:15 amIn addition to
Zvoni's findings:
- it leaks memory
- it creates an access violation when not using a exception block to
free the instantiated class.
Yes, it's the reason why i left out bd.free in above code (and i don't
care about memory-leaks when just testing around and it's basically
the end of the program anyway)

Something that baffles me: RecCount is 1, shouldn't BOF and EOF both
be true, irrespective if i move Next, Prior, First or Last??

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rvk · 2023-01-25 09:22

The problem might be that you are trying to stuff a string of 7
characters "ABCDEFG" in a field defined with Size = 5 !!

That really messes things up.

Just define the stringfield as a field of 20 characters and the code
should work fine.

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   //
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,0,CP_UTF8); 
bd.FieldDefs.Add('Str5Field', ftString, 20); (I'm also not sure about
the other parameters you used in the field-def. But the simple version
I used above works.)

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Zvoni · 2023-01-25 09:24

Quote from: rvk on January 25, 2023, 10:22:21 amThe problem might be
that you are trying to stuff a string of 7 characters "ABCDEFG" in a
field defined with Size = 5 !!

That really messes things up.

Just define the stringfield as a field of 20 characters and the code
should work fine.

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   //
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,0,CP_UTF8); 
bd.FieldDefs.Add('Str5Field', ftString, 20); Was my first thought,
too, but doesn't make a difference if you shorten the String to
"ABCDE" - same issue.
Just try it.

EDIT: What i noticed: The issue appears when BOF=True, irrespective of
the Value of EOF
It returns the correct Value for BOF=False

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rvk · 2023-01-25 09:26

Quote from: Zvoni on January 25, 2023, 10:24:08 amQuote from: rvk on
January 25, 2023, 10:22:21 amThe problem might be that you are trying
to stuff a string of 7 characters "ABCDEFG" in a field defined with
Size = 5 !!

That really messes things up.

Just define the stringfield as a field of 20 characters and the code
should work fine.

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   //
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,0,CP_UTF8); 
bd.FieldDefs.Add('Str5Field', ftString, 20); Was my first thought,
too, but doesn't make a difference if you shorten the String to
"ABCDE" - same issue.
Just try it.
And what happens if you use 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
 bd.FieldDefs.Add('Str5Field', ftString, 20);
The problem could also be in the precision of -1 or something else.
Using the simple Add() version works for me.

O, or FieldNo. Why is 0 used here?
Shouldn't this be 1????

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Zvoni · 2023-01-25 09:28

Quote from: rvk on January 25, 2023, 10:26:08 amQuote from: Zvoni on
January 25, 2023, 10:24:08 amQuote from: rvk on January 25, 2023,
10:22:21 amThe problem might be that you are trying to stuff a string
of 7 characters "ABCDEFG" in a field defined with Size = 5 !!

That really messes things up.

Just define the stringfield as a field of 20 characters and the code
should work fine.

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   //
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,0,CP_UTF8); 
bd.FieldDefs.Add('Str5Field', ftString, 20); Was my first thought,
too, but doesn't make a difference if you shorten the String to
"ABCDE" - same issue.
Just try it.
And what happens if you use 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
 bd.FieldDefs.Add('Str5Field', ftString, 20);
The problem could also be in the precision of -1 or something else.
Using the simple Add() version works for me.
Yes, i wondered about that, too (since when do you need precision for
Strings?!?!), but i think OP used this overloaded function because of
the following params after precision (required, readonly, codepage)

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rvk · 2023-01-25 09:29

Quote from: Zvoni on January 25, 2023, 10:28:27 amYes, i wondered
about that, too (since when do you need precision for Strings?!?!),
but i think OP used this overloaded function because of the following
params after precision (required, readonly, codepage)
I think the fieldNo of 0 is the problem.
Make it one (1) and it works.

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   //
bd.FieldDefs.Add('Str5Field', ftString, 20); 
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,1,CP_UTF8);

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rvk · 2023-01-25 09:34

Quote from: rvk on January 25, 2023, 10:29:57 amQuote from: Zvoni on
January 25, 2023, 10:28:27 amYes, i wondered about that, too (since
when do you need precision for Strings?!?!), but i think OP used this
overloaded function because of the following params after precision
(required, readonly, codepage)
I think the fieldNo of 0 is the problem.
Make it one (1) and it works.

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   //
bd.FieldDefs.Add('Str5Field', ftString, 20); 
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,1,CP_UTF8);
Yep. From the source:
Quote  // the fielddef will register itself here as an owned
component.
  // fieldno is 1 based !
So don't use field 0 but start on 1 !!!!

Not sure if there is any documentation for this

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Zvoni · 2023-01-25 09:40

QuoteQuote from: rvk on January 25, 2023, 10:34:34 am
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   //
bd.FieldDefs.Add('Str5Field', ftString, 20); 
bd.FieldDefs.Add('Str5Field',ftString,5,-1,False,False,1,CP_UTF8);
Yep. From the source:
Quote  // the fielddef will register itself here as an owned
component.
  // fieldno is 1 based !
So don't use field 0 but start on 1 !!!!

Not sure if there is any documentation for this 
Yep, that's it.

Result
QuoteRecCount=0
RecCount=1
BOF=FALSE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=FALSE - EOF=TRUE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
Still doesn't explain the BOF/EOF confusion

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TRon · 2023-01-25 09:45

Confirmed that rvk's solution works.

I agree with Zvoni wrt documentation (I was unable to locate it,
though the property is named fielNo as in number not index). Even a
exception on providing a 'illegal' field number would have been
helpful.

I seem to remember there being something being off with eof/bof. From
what I remember it was logical it acted this way (I seem unable to
locate the conversation as I was not part of it, should be here on the
forum)

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rvk · 2023-01-25 09:45

Quote from: Zvoni on January 25, 2023, 10:40:48 amResult
QuoteRecCount=0
RecCount=1
BOF=FALSE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=FALSE - EOF=TRUE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
Still doesn't explain the BOF/EOF confusion
What confusion?
They are correct in this example. (or am I missing something?)

BOF and EOF are not set until they are actually encountered.
With a POST they are not set. Only with Last, First, Next and Prior
(and only in the direction you where going).

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wp · 2023-01-25 09:45

Quote from: rvk on January 25, 2023, 10:34:34 amNot sure if there is
any documentation for this 
https://www.freepascal.org/docs-html/fcl/db/tfield.fieldno.html ("It
is a 1-based index")

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TRon · 2023-01-25 09:48

Quote from: wp on January 25, 2023, 10:45:58 amQuote from: rvk on
January 25, 2023, 10:34:34 amNot sure if there is any documentation
for this 
https://www.freepascal.org/docs-html/fcl/db/tfield.fieldno.html ("It
is a 1-based index")
Ok, I came in from another end: https://www.freepascal.org/docs-
html/fcl/db/tfielddef.fieldno.html

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Zvoni · 2023-01-25 10:04

Quote from: rvk on January 25, 2023, 10:45:44 amQuote from: Zvoni on
January 25, 2023, 10:40:48 amResult
QuoteRecCount=0
RecCount=1
BOF=FALSE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=FALSE - EOF=TRUE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
Still doesn't explain the BOF/EOF confusion
What confusion?
They are correct in this example. (or am I missing something?)

BOF and EOF are not set until they are actually encountered.
With a POST they are not set. Only with Last, First, Next and Prior
(and only in the direction you where going).
For a RecordCount = 1 i would expect BOF and EOF to be True.
Or at least EOF to be True from the get-go, since OP used Append

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rvk · 2023-01-25 10:11

Quote from: Zvoni on January 25, 2023, 11:04:36 amFor a RecordCount =
1 i would expect BOF and EOF to be True.
No. BOF and EOF are only set on movement of the cursor.
Quote1. The cursor is on the last record, and the TDataset.Next method
is called.https://www.freepascal.org/docs-
html/fcl/db/tdataset.eof.html

So, although you are on the last record after Append with one record,
Next isn't called, so EOF is false.

Although the documentation page of BOF isn't that clear, the same as
for EOF goes for BOF.

Only exception is when there is an empty dataset. Then both BOF and
EOF are true.

Also note:
QuoteIn all other cases, EOF is False. Note: when the cursor is on the
last-but-one record, and Next is called (moving the cursor to the last
record), EOF will not yet be True.So, EOF will only trigger if you are
trying to go past the last record. Not when you reach that last
record.
(That's why you do a while EOF loop with Next on the end, otherwise
this would go wrong)

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wp · 2023-01-25 10:15

Quote from: TRon on January 25, 2023, 10:48:23 amOk, I came in from
another end: https://www.freepascal.org/docs-
html/fcl/db/tfielddef.fieldno.html
I agree that it is "poorly documented". I just know this because I
once was involved in a bug fix related to ZEOS components which
changed their RecNo to begin at 0, and this caused some trouble
(https://sourceforge.net/p/zeoslib/tickets/539/).

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Zvoni · 2023-01-25 10:32

Quote from: rvk on January 25, 2023, 11:11:07 amQuote from: Zvoni on
January 25, 2023, 11:04:36 amFor a RecordCount = 1 i would expect BOF
and EOF to be True.
No. BOF and EOF are only set on movement of the cursor.
Quote1. The cursor is on the last record, and the TDataset.Next method
is called.https://www.freepascal.org/docs-
html/fcl/db/tdataset.eof.html

So, although you are on the last record after Append with one record,
Next isn't called, so EOF is false.

Although the documentation page of BOF isn't that clear, the same as
for EOF goes for BOF.

Only exception is when there is an empty dataset. Then both BOF and
EOF are true.

Also note:
QuoteIn all other cases, EOF is False. Note: when the cursor is on the
last-but-one record, and Next is called (moving the cursor to the last
record), EOF will not yet be True.So, EOF will only trigger if you are
trying to go past the last record. Not when you reach that last
record.
(That's why you do a while EOF loop with Next on the end, otherwise
this would go wrong)

Ah... OK.
I've just read something on Microsofts website, and it is consistent
BOF/EOF will only be set to True if you try to MOVE BEYOND the
First/Last record

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TRon · 2023-01-25 10:53

Quote from: Zvoni on January 25, 2023, 11:32:31 amBOF/EOF will only be
set to True if you try to MOVE BEYOND the First/Last record
Ah, yes !!. That was the conclusion from the conversation I remembered
as well. Thx for the refresh guys/galls !

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Thaddy · 2023-01-25 13:08

An old example from 2016 written by me and already on this forum: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 {$apptype
console}{$mode objfpc}uses  db,BufDataset;var 
BufDb:TBufDataset;begin BufDb:=TBufDataset.Create(nil); try 
 BufDb.FieldDefs.Add('NAME',ftString,20); 
 BufDb.FieldDefs.Add('NUM',ftinteger); 
 BufDb.FieldDefs.Add('NUM2',ftInteger);   BufDb.CreateDataSet; 
 BufDb.Open;   BufDb.Append; 
 BufDb.FieldByName('NAME').Value:='Free';   BufDb.Post; 
 BufDb.Append;   BufDb.FieldByName('NAME').Value:='Pascal'; 
 BufDb.Post;   BufDb.SaveToFile('BufDb.txt'); finally   BufDb.Close; 
 BufDb.Free; end; // now, open.. BufDb :=
TBufDataset.Create(nil); try   BufDb.LoadFromFile('BufDb.txt'); 
 BufDb.Open;   BufDb.First; 
 Writeln(BufDb.FieldByName('NAME').Value); finally   BufDb.Close; 
 BufDb.Free; end; end.
It seems to me that just the call to open is the culprit.

https://forum.lazarus.freepascal.org/index.php/topic,33508.msg217317.h
tml#msg217317

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Zvoni · 2023-01-25 13:30

Quote from: Thaddy on January 25, 2023, 02:08:55 pmAn old example from
2016 written by me and already on this forum: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 {$apptype
console}{$mode objfpc}uses  db,BufDataset;var 
BufDb:TBufDataset;begin BufDb:=TBufDataset.Create(nil); try 
 BufDb.FieldDefs.Add('NAME',ftString,20); 
 BufDb.FieldDefs.Add('NUM',ftinteger); 
 BufDb.FieldDefs.Add('NUM2',ftInteger);   BufDb.CreateDataSet; 
 BufDb.Open;   BufDb.Append; 
 BufDb.FieldByName('NAME').Value:='Free';   BufDb.Post; 
 BufDb.Append;   BufDb.FieldByName('NAME').Value:='Pascal'; 
 BufDb.Post;   BufDb.SaveToFile('BufDb.txt'); finally   BufDb.Close; 
 BufDb.Free; end; // now, open.. BufDb :=
TBufDataset.Create(nil); try   BufDb.LoadFromFile('BufDb.txt'); 
 BufDb.Open;   BufDb.First; 
 Writeln(BufDb.FieldByName('NAME').Value); finally   BufDb.Close; 
 BufDb.Free; end; end.
It seems to me that just the call to open is the culprit.

https://forum.lazarus.freepascal.org/index.php/topic,33508.msg217317.h
tml#msg217317
No, it was the FieldNo in the (overloaded) Add-Function. OP (and
myself) missed that FieldNo is 1-based. OP used for that sample
FieldNo=0

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Thaddy · 2023-01-25 19:00

I wouldn't call it no!, because if you want to read the first entry,
first will do nicely. Hence that old example.
It is cleaner and much easier than the bull dung above.

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egsuh · 2023-01-26 02:09

In Zvony’s example, calling RecNo might have moved cursor beyond last
record.