[SOLVED] Creating the TBufDataset in the data module oncreate event
lazarus / 16-Databases

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gmold · 2022-03-12 07:15

Hello,
my first post!. I'm new to Lazarus/Pascal but not programming (retired
mainframe programmer).  I taught myself 'old Pascal' from this
document ==> https://apo.ansto.gov.au/dspace/handle/10238/900  (which
is excellent and I enjoyed the exercises  ) and subsequently have
decided to 'modernize' and learn Object Pascal.
I bought the Lazarus Handbook and in Book1 page 197 there is a demo of
how to use a DataModule, DataSource and TBufDataset and link them to a
DBgrid an Navigator.  The book says put the CreateDataset in the
Oncreate Event which I did.   The program complies, the Form displays
but the DBGrid is 'inactive' and the underlying dataset never gets
created.

I put a break point on the Oncreate procedure in the DataModule unit
but the debugger does not stop the program at that point.   Which
suggest to me that the code is not run but I'm not sure.

Question: Is the OnCreate procedure a good place/correct place to
create the underlying table and if there is a better place where would
you do it.

Glenn
Australia. 
Lazarus 2.012 FPC 3.20

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wp · 2022-03-12 09:26

I found the example in the book and followed its steps. The demo is
working but some details are missing...

Let me start with your issue: You probably did not connect the book
code with the OnCreate event of the datamodule. Click on the
datamodule (but not on one of the added components). Go to the Object
Inspector (the tabbed table at the left of the Lazarus IDE), page
"Events", line "OnCreate". Click on the '...' button. The IDE now
should add a code skeleton for this event into which you put the code
of the book (DatamoduleCreate), or if you wrote that before, the IDE
will jump to this method.

Repeat the same with the OnDestroy event in which the dataset is saved
to disk.

When you try to compile you will notice an error that the variable
"Application" is not found in the datamodule. This is because
"Application" is declared in unit Forms, and this normally is not
needed by a datamodule. Add "Forms" to the uses list of the
datamodule, and you should pass this one.

There is another compilation error in the OnValidate event handler:
the function "YearOf" is not found. It is in unit DateUtils - add it
to the uses clauses.

Your question: Yes, TDatamodule.OnCreate often is the correct place to
create the dataset. Another way would be to create it already at
designtime: right-click on the bufdataset and select "Create Dataset".
Note that this will turn on the "Active" property of the dataset.
Switch it back to off because you'd also have to specify the path to
the database file in the object inspector then, and this is not a good
idea because there is more flexibility when you do this at runtime.

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gmold · 2022-03-12 11:08

Hello,
thank you for your reply.  I figured out the application.location
error by looking at the picture on page 197 and I noticed that uses
forms ; in the implementation sections. I verified this in the fpc
documentation and added it and it fixed the issue. I have not added
the year validation so the that is not a problem.  What I do not see
is the 'create dataset ' option if I right click on the bufdataset.
See the attached.

The code for the datamodule unit is below.  The methods for create and
destroy exist.

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 unit
udm; {$mode objfpc}{$H+} interface uses  Classes, SysUtils, DB,
BufDataset; type   { TdmStaff }   TdmStaff = class(TDataModule)   
bdStaff: TBufDataset;    dsModule: TDataSource;    bdStaffAvailable:
TFloatField;    bdStaffName: TStringField;    bdStaffStaffCode:
TStringField;    procedure DataModuleCreate(Sender: TObject);   
procedure DataModuleDestroy(Sender: TObject);   private     FMyDB :
string;  public   end; var  dmStaff: TdmStaff; implementationuses
forms  ; {$R *.lfm} { TdmStaff }   procedure
TdmStaff.DataModuleCreate(Sender: TObject);begin  FMyDB :=
IncludeTrailingPathDelimiter(application.location);  if
fileexists(fmydb + 'staff.bds') then      bdStaff.LoadFromFile(fmydb +
'staff.bds')  else   bdStaff.createdataset;end; procedure
TdmStaff.DataModuleDestroy(Sender: TObject);begin   
bdStaff.SaveToFile(fmydb + 'staff.bds');end; end.
Glenn,
Australia.

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wp · 2022-03-12 11:19

Sorry - I misread the version information in your signature and used
Laz 2.2.0/FPC 3.2.2 in my test. You are right - the "Create Dataset"
item was added to the TBufDataset's context menu after the release of
FPC3.2.0.

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gmold · 2022-03-13 04:36

Hi,
I upgraded to Lazarus/Fpc and it works now.   Thank you.

Glenn,
Australia