Adding a field to TDbf at runtime
lazarus / 16-Databases

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maurobio · 2021-12-09 21:53

Dear ALL,

I have a Dbase file with the following structure:

QuoteField   Type   Field Size   Decimal
C_REG   C   10   0
N_DISP   N   1   0
N_TIPO   N   2   0
N_ESPE   N   5   0
N_SEXO   N   2   0
C_CAMP   C   16   0
N_PROJ   N   3   0
N_AMBI   N   2   0
D_DETE   C   10   0
N_ENTR   N   1   0
D_ENTR   C   10   0
N_SECO   N   4   0
N_LIQI   N   4   0
N_LAMI   N   4   0
N_TCOL   N   3   0
N_REGI   N   2   0
N_SIGLA   N   2   0
I would like to add a new integer field ('N_NOME') to this file at
runtime.

Here is my code (partially adapted from
https://forum.lazarus.freepascal.org/index.php?topic=21006.0):

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
SGCFix; {$mode objfpc}{$H+} uses  Classes, SysUtils, Dbf, db,
Dbf_Common;  var  ADbf: TDbf;  //AField: TField;  AFields: array of
TField;  Temf: TIntegerField;  i: integer;  begin  ADbf :=
TDbf.Create(nil);  ADbf.FilePath := 'data' + DirectorySeparator;  
ADbf.Exclusive := True;  ADbf.TableName := 'DADOS.dbf';  ADbf.Open; 
ADbf.Close;      SetLength(AFields, ADbf.FieldDefs.Count);  for i:=0
to ADbf.FieldDefs.Count - 1 do        AFields[i] :=
ADbf.FieldDefs[i].CreateField(ADbf);  Temf :=
TIntegerField.Create(nil);  Temf.FieldKind := fkCalculated; 
Temf.FieldName := 'N_NOME';  Temf.DataSet := ADbf;  ADbf.Open; 
ADbf.Edit;  ADbf.Post;  for i := Low(AFields) to High(AFields) do    
  AFields[i].Free;  SetLength(AFields, 0);  Temf.Free;  ADbf.Close;  
WriteLn('-- done -- ');       end.  
It compiles without errors, but it does nothing: the new field is not
added and the file is not modified.

Could anyone out there give a hand?

Thanks in advance!

With best wishes,

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korba812 · 2021-12-09 23:05

Do you want to change dbf file structure or add a calculated field?
The post you are referring to is for a calculated field.
If you want to change dbf file structure then use
TDbf.RestructureTable method. Here's an example (it is about deleting
a field, but it should be helpful):
https://forum.lazarus.freepascal.org/index.php?topic=8567.0

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maurobio · 2021-12-09 23:36

Hi, @korba512!

Thanks a lot for your reply! I rewrote my code according to the
example you pointed out:

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
SGCFix; {$mode objfpc}{$H+} uses  Classes, SysUtils, Dbf, db,
Dbf_Common;  var  SGCDbf: TDbf;  FieldDefs: TDbfFieldDefs;  begin 
SGCDbf := TDbf.Create(nil);  SGCDbf.FilePath := 'data' +
DirectorySeparator;   SGCDbf.Exclusive := True;  SGCDbf.TableName :=
'DADOS.dbf';  SGCDbf.Open;  FieldDefs := TDbfFieldDefs.Create(nil); 
FieldDefs.Assign(SGCDbf.DbfFieldDefs);  FieldDefs.Close; 
FieldDefs.Add('N_NOME', ftInteger, 0, True); 
SGCDbf.RestructureTable(FieldDefs, False);    SGCDbf.Close; 
SGCDbf.Free;  FieldDefs.Free;  WriteLn('-- done -- ');       end. 
... but it still does not work. Now, the compiler returns several
error messages, apparently related to the TDbfFieldDefs type which
seems not to be recognized:

QuoteFree Pascal Compiler version 3.2.0 [2021/02/21] for i386
Copyright (c) 1993-2020 by Florian Klaempfl and others
Target OS: Win32 for i386
Compiling SGCfix.lpr
SGCfix.lpr(10,14) Error: Identifier not found "TDbfFieldDefs"
SGCfix.lpr(10,27) Error: Error in type definition
SGCfix.lpr(18,16) Error: Identifier not found "TDbfFieldDefs"
SGCfix.lpr(19,13) Error: Illegal qualifier
SGCfix.lpr(20,13) Error: Illegal qualifier
SGCfix.lpr(21,13) Error: Illegal qualifier
SGCfix.lpr(22,36) Error: Incompatible type for arg no. 1: Got
"<erroneous type>", expected "TDbfFieldDefs"
SGCfix.lpr(25,13) Error: Illegal qualifier
SGCfix.lpr(29) Fatal: There were 8 errors compiling module, stopping
Fatal: Compilation aborted
Error: C:\lazarus\fpc\3.2.0\bin\i386-win32\ppc386.exe returned an
error exitcode

Any hints?

With best wishes,

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wp · 2021-12-09 23:45

If you want to add a calculated field you must provide a handler for
the Dataset's OnCalcFields event which has the purpose the calculate
the value(s) for the calculated field(s). Note that the "normal"
fields are not accessible here, only those that you recreated yourself
for the AFields array.

The attached sample contains two "regular" fields, "A" and "B". At
runtime a third field "C" is created as calculated field which
calculates the sum of A and B.

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maurobio · 2021-12-10 00:04

Hi, @wp!

Thanks for your reply. In fact, I do not want a 'calculated field' -
this was mistakenly taken from an example I found here on the forum. I
just want to add a field at runtime.

The example pointed out by @korba512 seems to be similar to what I
need, but as I wrote in my previous reply, I got compiler errors
because of a seemingly unrecognized type.

With best wishes,

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Handoko · 2021-12-10 02:19

Maybe:

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
Test; uses  Classes, SysUtils, Dbf, db, dbf_fields; var  SGCDbf:
Tdbf;  FieldDefs: TDbfFieldDefs;  NewFieldDef: TDbfFieldDef; begin 
SGCDbf := TDbf.Create(nil);  SGCDbf.FilePath := 'data' +
DirectorySeparator;  SGCDbf.Exclusive := True;  SGCDbf.TableName :=
'DADOS.dbf';  SGCDbf.Open;  FieldDefs := TDbfFieldDefs.Create(nil); 
FieldDefs.Assign(SGCDbf.DbfFieldDefs);  NewFieldDef :=
FieldDefs.AddFieldDef;  NewFieldDef.FieldName := 'N_NOME'; 
NewFieldDef.FieldType := ftInteger;  NewFieldDef.Required := True; 
SGCDbf.RestructureTable(FieldDefs, True);  FieldDefs.Free; 
SGCDbf.Close;  SGCDbf.Free end.

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maurobio · 2021-12-10 11:58

Hi, @Handoku!

Thanks a lot, not only for your reply, but for your post at another
thread
(https://forum.lazarus.freepascal.org/index.php/topic,8567.0.html),
which has been pointed out to me by @korba512 and that effectively
provided a working solution to my problem.

I was sure that some unit was missing where TDbfFieldDefs was
declared, but I could not find out which one. The TDbf tutorial does
not mention the dbf_fields unit in the section "What to put in in your
uses section"(https://wiki.lazarus.freepascal.org/Lazarus_Tdbf_Tutoria
l#What_to_put_in_your_uses_section) and so I got lost.

With best wishes,

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Handoko · 2021-12-10 15:19

The problem has been reported:
https://forum.lazarus.freepascal.org/index.php/topic,35480.msg234567.h
tml#msg234567