Blog manager startup code

I think (and hope) that I've found the magic combination to allow the database connection on my development computer to contain the name of the database on that computer, but allow that to be changed without error on my laptop (see here for previous discussion). The dpr code and the initial code from the datamodule are as follows

var
 db: integer;

begin
 db:= TAboutBox.Execute;
 Application.Initialize;
 Application.CreateForm(Tdm, dm);
  dm.OpenDatabase (db);
 Application.CreateForm(TMainForm, MainForm);
 Application.CreateForm(TDimmerForm, DimmerForm);
 Application.Run;
end.
...
procedure TDm.DataModuleCreate(Sender: TObject);
begin // prevent the compile time definition causing a problem
 with sqlconnection1 do
  begin
   close;
   params.values['database']:= '';
  end;
end;

procedure TDm.OpenDatabase (index: word);
var
 pname, dir: string;

begin
 case index of
  0: begin
      pname:= 'Perceptions';
      fileprefix:= pname;
     end;
  1: begin
      pname:= 'PPP';
      fileprefix:= 'Programming pitfalls in Priority';
     end;
 end;

 with TRegIniFile.create (regpath) do
  begin
   dir:= ReadString ('firebird', pname, '');
   free
  end;

 with sqlconnection1 do
  begin
   close;
   params.values['database']:= dir;
   loginprompt:= false;
  end;

Initially the database 'number' is obtained from the 'about' dialog. The datamodule starts up with the database value defined, but the ModuleCreate code clears it. Then the OpenDatabase procedure is called with the index; this sets some local variables then obtains from the registry the name of the database required.

So far, so good.

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I was thinking about the function that returns a new instance for the temp table. This was as simple as could be

Function TDm.NewInstance: word;
begin
 qNewInstance.Open; // select max (instance) from temp
 result:= qNewInstance.fields[0].asinteger + 1;
 qNewInstance.close;
end;

It occurred to me, however, that there could be a call to NewInstance before any values with the new instance had managed to get written to the table, in which case the second call to NewInstance would be given the same instance number. I had to 'protect' this value by immediately entering a tuple with this instance number by means of a new insert query, qSentinel. Not satisfied with this, I thought it best if the entire function was protected by a mutex - I borrowed the code from the linked web site. 

Quite pleased with myself, I went looking for examples where the database has to be changed at run time. I haven't found anything suitable yet, but I did find something very interesting: the sqlconnection has a method called 'ExecuteDirect' that receives a string and executes it. As it happens, I was thinking of something like this yesterday afternoon, as that's how Priority works, but I didn't think it possible in Delphi. Well, it is, and here is the final code for the NewInstance function. I'm sure that I'll find many places where I can use 'ExecuteDirect' instead of a predefined query (note that this requires no parameters - I have to 'inject' a value for 'instance').

Function TDm.NewInstance: word;
var
 s: string;

begin
 mutex.Lock;
 qNewInstance.Open;
 result:= qNewInstance.fields[0].asinteger + 1;
 qNewInstance.close;
 s:= 'insert into temp (instance, id) values (' + inttostr (result) + ', -1)';
 sqlconnection1.executedirect (s);
 mutex.Unlock;
end;

I feel at times like Lawrence Waterhouse in 'Cryptonomicon' in chapter one, 'Barrens', where he is out with the fictional Alan Turing and Rudy von 'something or other', when Turing says "Shut up about Leibniz for a moment, Rudy, because look here: You—Rudy—and I are on a train, as it were, sitting in the dining car, having a nice conversation, and that train is being pulled along at a terrific clip by certain locomotives named The Bertrand Russell and Riemann and Euler and others. And our friend Lawrence is running alongside the train, trying to keep up with us—it’s not that we’re smarter than he is, necessarily, but that he’s a farmer who didn’t get a ticket." In other words, I learned the basics of Delphi from a few books and from magazine articles, but after that, I was on my own and so probably there are many topics that I don't know about that could ease my work.

Internal links
[1] 1844

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This day in history:

Blog #	Date	Title	Tags
1352	23/10/2020	Overcoming the 'leading zero' problem in Excel	Programming, Delphi, Excel, Office automation