Archives de catégorie : Development

Cloning a row (even if the structure gets modified in the future)

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A few years ago, I asked this question on StackExchange and got a brilliant answer (by Erwin Brandstetter). I just finally understood how it works. So here is my explanation (to myself mainly). Enjoy!

The goal was to INSERT a row in a table, cloning an existing one while replacing some value (like the primary key) in the new row. I didn’t want to list each and every column so that even if I add new fields to the table, the cloning mechanism would still work.

Here is the response I got:

INSERT INTO contrats
SELECT (sub.subalias).*
FROM (
    SELECT t #= hstore('idcontrat', nextval(pg_get_serial_sequence('contrats','idcontrat'))::TEXT)
             #= hstore('date_saisie', CURRENT_TIMESTAMP::TEXT)
             #= hstore('numero', numero || '-DUP')
             #= hstore('notes', NULL)
       AS subalias
       FROM contrats t
       WHERE idcontrat = :oldidcontrat
) sub
RETURNING idcontrat

First thing to know: when you create a table, PostgreSQL automatically creates a composite type with the same name and all fields in that table (see [here]). As you will discover in this documentation, you reference composite types with (parens).

So how does the query work? There’s an outer query:

INSERT INTO contrats 
SELECT (sub.subalias).* 
FROM (
    <inner_query>
) sub
RETURNING idcontrat.

This outer query is pretty obvious except the (sub.subalias) part.

The inner query is also simpler than it seems. If we didn’t want to change any field value, it would be:

SELECT t AS subalias
FROM contrats t
WHERE idcontrat = :oldidcontrat

Notice that we didn’t write SELECT t.* but just SELECT t. Here, t is the composite type that represents the whole contrats record/row.

Then we use the hstore PostgreSQL extension to do the transformations that we want. This is the syntax we use:

t #= hstore('fieldname',value)

This syntax only works on composite types and replaces the value of the fieldname field with the provided value. That value must be of type TEXT. As you can see, you can apply several transformations at once.

What was bugging me for so long was that in the initial answer I got, the second line was written:

SELECT (subalias).*

And I didn’t understand why that subalias was visible outside the inner query (I thought it was out of the scope).

subalias is just one field (of type contrats) and since it is of a composite type, you have to use the parens around the whole left part of the SELECT. You can’t write sub.(subalias).* but instead (sub.subalias).*

I must confess I asked the Perplexity AI to help me understand this.

Raw SQL in Ruby/Rails

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Everyone seems to discourage the use of raw SQL in Ruby applications and while ActiveRecord seems a good solution for general purpose CRUD operations, we have a strong expertise in SQL and we want to use it as much as possible. Here are the bits we have collected so far on the subject. We are new to Ruby/Rails so these might be obvious for some of you. We primarily write this for ourselves.

We are perfectly aware of the « limitations ». We love PostgreSQL and have no intention of being RDBMS-agnostic. At all.

find_by_sql the way we want it

We want named parameters because, despite the overhead, it’s the cleanest way to pass parameters (we don’t have to preserve a strict order which can lead to problems when enhancing the query, counting the « ? », etc). Also, when we use the same parameter several times in the same query, we don’t have to repeat it.

class ClientsController < ApplicationController

	def index
		@clients = Client.find_by_sql(
			<<-SQL
			SELECT idclient,name,email,contacts
			FROM clients 
			ORDER BY nom
			SQL
			)
	end

	def show
		@client = Client.find_by_sql(["	
			SELECT idclient,name,email,contacts 
			FROM clients 
			WHERE idclient = :idclient", 
			{ :idclient => params[:id] } 
		])[0]
	end

For the show method, we have to grab the first element in the array (thus the [0]at the end.

TdxSpreadSheet creation and filling via code

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Basic usage and formatting

procedure TForm1.b1Click(Sender: TObject);
Var xls:TdxSpreadSheet;
    sh: TdxSpreadSheetTableView;
begin
  xls:=TdxSpreadSheet.Create(Self);
  Try
    xls.BeginUpdate;
    xls.ClearAll;
    sh:=TdxSpreadSheetTableView(xls.AddSheet('Bénéficiaires'));

    // row,col
    if Cells[1,1]=Nil then
      CreateCell(1,1);
    Cells[Row,Col].AsString:='test';
    sh.cells[1,1].Style.Borders[bLeft].Style:=sscbsDouble;
    sh.cells[1,1].Style.Borders[bRight].Color:=clred;
    sh.Cells[1,1].Style.Brush.BackgroundColor:=clcxLightGray;
    sh.Cells[1,1].Style.AlignHorz:=ssahCenter;
    sh.Cells[1,1].Style.DataFormat.FormatCode:='dd/mm/yyyy';
    sh.Cells[1,1].Style.DataFormat.FormatCode:='# ##0.00';
    sh.Cells[1,1].Style.Font.Style:=[fsBold];

    // Formula
    CreateCell(2,1); // B3
    sh.Cells[2,1].SetText('=SUM(B2:B2)',true);

    xls.EndUpdate;
    xls.SaveToFile('C:\Users\gosane\Desktop\text.xlsx');
  Finally
    xls.Free;
  End;
end;

Helpers

function xlscolname(i:integer):string;
var a,b:integer;
begin
  if i<0 then
    result:='?'
  else if i<26 then
    result:=chr(65+i)
  else begin
    a:=i div 26;
    b:=i mod 26;
    result:=chr(64+a)+chr(65+b);
  end;
end;

Working with TPgDAC (DevArt components for Delphi)

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TPgLoader

interface uses DALoader,PgLoader;

Type TMyForm = Class(TForm)
       // ...
       private
         procedure PgLoader1PutData(Sender: TDALoader);
     End;

{...}

procedure TFxxx.PgLoader1PutData(Sender: TDALoader);
var i:Integer;
begin
  for i:=1 to 10 do
    Sender.PutColumnData('fieldname',i,Value);
end;


procedure TFxxx.Import;
Var pgl:TPgLoader;
begin
  pgl:=TPgLoader.Create(Application);
  Try
    pgl.Connection:=DM.PgConnection1;
    pgl.TableName:='MyTableName';
    pgl.Options.BufferSize:=65536;
    // pgl.OnProgress:=LoaderProgress;
    pgl.OnPutData:=PgLoader1PutData;
    pgl.CreateColumns;
    // Remove primary key from PgLoader
    for I := 0 to pgl.Columns.Count-1 do
      if pgl.Columns[i].Name='idprimarykey' then begin
        pgl.Columns.Delete(i);
        break;
      end;
    pgl.Load;
  Finally
    pgl.Free;
  End;
end;

Astuces Delphi

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Déclencher une action au démarrage de l’application

procedure TFMain.FormShow(Sender: TObject);
begin
  PostMessage(Button1.Handle,WM_LBUTTONDOWN,0,0);
  PostMessage(Button1.Handle,WM_LBUTTONUP,0,0);
end;

Créer des listes d’objets avec TObjectList

uses Contnrs;

Dans l’interface :

TSQLColumn = Class
  name     : string;
  typ      : string;
  // ...autres champs...
End;
TSqlColumnList = class(TObjectList)
  private
    function GetItem(Index: Integer): TSQLColumn;
    procedure SetItem(Index: Integer; const Value: TSQLColumn);
  public
    property Items[Index: Integer]: TSQLColumn read GetItem write SetItem; default;
end;

Dans l’implémentation :

{ TSqlColumnList }

function TSqlColumnList.GetItem(Index: Integer): TSQLColumn;
begin
  Result:=TSqlColumn(inherited GetItem(Index));
end;

procedure TSqlColumnList.SetItem(Index: Integer; const Value: TSQLColumn);
begin
  inherited setItem(index, value);
end;

Utilisation :

TMyClass = Class
  FColumns        : TSqlColumnList;
  Constructor Create;
  Destructor Destroy; reintroduce;
End;

{ TcatRequete }

constructor TcatRequete.Create;
begin
  FColumns := TSqlColumnList.Create;
end;

destructor TcatRequete.Destroy;
begin
  FColumns.Free;
end;

Et pour remplir la liste :

Var column : TSqlColumn;
begin
  FColumns.Clear;
  // Ajouter un élément
  column:=TSqlColumn.Create;
  FColumns.Add(Column);
  With column do begin
    name := 'xxx';
  end;
end;