SQLdb Tutorial3

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Revision as of 12:30, 14 November 2012 by BigChimp (talk | contribs) (Fleshing out login form with screenshot, info on test callback function)
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Overview

In this tutorial, you will learn to make your application suitable for multiple databases. You will see how to get database data in normal controls (instead of database controls), and how to execute parametrized queries.

Multi database support

Using any database and a login form, you can support multiple different database servers/embedded libraries that SQLDB supports.

Advantages:

  • the user/programmer can dynamically use any sqldb t*connection, so he can choose between dbs

Disadvantages:

  • More complicated SQL will likely need to still be adapted. Each database has its own dialect; it is of course possible to call db-specific SQL, this can grow into a maintenance problem.
  • You can't use T*connection specific properties (like TIBConnection.Dialect to set Firebird dialect).

To use multi-database support, instead of your specific T*Connection such as TIBConnection, use TSQLConnector (not TSQLConnection), which dynamically (when the program is running) chooses the specific T*Connection to use based on its ConnectorType property: 

uses
...
var
  Conn: TSQLConnector;
begin
  Conn:=TSQLConnector.Create(nil);
  try
    // ...actual connector type is determined by this property.
    // Make sure the ChosenConfig.DBType string matches
    // the connectortype (e.g. see the string in the 
    // T*ConnectionDef.TypeName for that connector .
    Conn.ConnectorType:=ChosenConfig.DBType;
    // the rest is as usual:
    Conn.HostName:='DBSERVER';
    Conn.DatabaseName:='bigdatabase.fdb';
    Conn.UserName:='SYSDBA';
    Conn.Password:='masterkey';
    try
      Conn.Open;

Login form

As mentioned in SQLdb Tutorial1, a user should login to the database using a form (or perhaps a configuration file that is securely stored), not via hardcoded credentials in the application. Besides security considerations, having to recompile the application whenever the database server information changes is not a very good idea.

In dbconfiggui.pas, we will set up a login form that pulls in default values from an ini file, if it exists. This allows you to set up a default connection with some details (database server, database name) filled out for e.g. enterprise deployments. The user can add/edit his username/password, and test the connection before going further.

dbloginform.png

We use a separate dbconfig.pas unit with a TDBConnectionConfig class to store our chosen connection details. This class has support for reading default settings from an ini file.

This allows use without login forms (e.g. when running unattended/batch operations), and allows reuse in e.g. web applications.

This TDBConnectionConfig class is surfaced in the login form as the Config property, so that the main program can show the config form modally, detect an OK click by the user and retrieve the selected configuration before closing the config form.

Connection test callback function

To keep the login form flexible (it may be used with other database layers like Zeos), we implement the test section as a callback function and let the main program deal with it.

The definition in dbconfiggui: 

type
  TConnectionTestFunction = function(ChosenConfig: TDBConnectionConfig): boolean of object;

The main form must implement a function that matches this definition to handle the test request from the config form.

The callback function takes the config passed on by the config form, and uses that to construct a connection with the chosen database type. It then simply tries to connect with the server; if it is succesful, the result is positive, otherwise the result stays false.

Because database connection attempts to non-existing servers can have a long timeout, we indicate to the user that he must wait by setting the cursor to the hourglass icon. 

uses
...
dbconfig, dbconfiggui
...
function TForm1.ConnectionTest(ChosenConfig: TDBConnectionConfig): boolean;
// Callback function that uses the info in dbconfiggui to test a connection
// and return the result of the test to dbconfiggui
var
  // Generic database connector...
  Conn: TSQLConnector;
begin
  result:=false;
  Conn:=TSQLConnector.Create(nil);
  Screen.Cursor:=crHourglass;
  try
    // ...actual connector type is determined by this property.
    // Make sure the ChosenConfig.DBType string matches
    // the connectortype (e.g. see the string in the
    // T*ConnectionDef.TypeName for that connector .
    Conn.ConnectorType:=ChosenConfig.DBType;
    Conn.HostName:=ChosenConfig.DBHost;
    Conn.DatabaseName:=ChosenConfig.DBPath;
    Conn.UserName:=ChosenConfig.DBUser;
    Conn.Password:=ChosenConfig.DBPassword;
    try
      Conn.Open;
      if Conn.Connected=true then
        result:=true;
    except
      // Result is already false
    end;
    Conn.Close;
  finally
    Screen.Cursor:=crDefault;
    Conn.Free;
  end;
end;

Finally, the code in dbconfiggui.pas that actually calls the callback is linked to the Test button. It tests if the callback function is assigned (to avoid crashes), for completeness also checks if there is a valid configuration object and then simply calls the callback function: 

procedure TDBConfigForm.TestButtonClick(Sender: TObject);
begin
  // Call callback with settings, let it figure out if connection succeeded and
  // get test result back
  if assigned(FConnectionTestFunction) and assigned(FConnectionConfig) then
    if FConnectionTestFunction(FConnectionConfig) then
       showmessage('Connection test succeeded.')
      else
        showmessage('Connection test failed.')
  else
    showmessage('Error: connection test code has not been implemented.');
end;

Additions/modifications

Possible additions/modifications for the login form:

  • Add command line arguments handling for dbconfig to preload suitable defaults, so the program can be used in batch scripts, shortcuts etc
  • Add a "Select profile" combobox in the login form; use multiple profiles in the ini file that specify database type and connection details.
  • Hide database type combobox when only one database type supported.
  • Hide username/password when you're sure an embedded database is selected.
  • Add support for specifying port number, or instance name with MS SQL Server connector
  • Add support for trusted authentication for dbs that support it (Firebird, MS SQL): disable ussername/password controls
  • Create database after a confirmation request if an embedded database is selected but the file does ot exist
  • Create a command-line/TUI version of the login form (e.g. using the curses library) for command-line applictions


Getting database data into normal controls

Light bulb  Note: Before starting this section, please make sure you have set up the sample employee database as specified in SQLDB Tutorial1

In previous tutorials, data-bound controls were covered: special controls such as the dbgrid that can bind its contents to a datasource, get updates from that source and send user edits back.

It is also possible to programmatically retrieve database content and fill any kind of control with that content. As an example, we will look at filling a stringgrid with some salary details for the sample employee database table.


Aggregate query: max and min salary and corresponding employee (select ... from salary...) Get results in stringgrid.

Adapting SQL for various databases

use standard deviation for salary select ... from employees... Hopefully postgresql has a built in stddev function Indicate this may not work on all dbs Show result in labels or tedits

Executing queries & getting data out of normal controls into the database

Previously, we have seen how to get data out of the databse using queries, and how to let SQLDB update the databse with db controls. You can also execute any SQL you want on the database via code, and we're going to use that to show how to use normal non db controls to get data into the database.

This allows you to use controls that have no db aware equivalent such as sliders or custom controls to enter data into the database, at the expense of a bit more coding. INSERT INTO ... EMPLOYEE... or UPDATE EMPLOYEE... or MERGE... Think about this one...

Code

Code is presented below; it would be a better idea if this code could be included in the Lazarus examples directory...

  • to do: update code, specify where the zip with code can be found (or place in Laz examples)

dbconfig.pas

unit dbconfig;

{ Small unit that retrieves connection settings for your database

  Copyright (c) 2012 Reinier Olislagers

  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to
  deal in the Software without restriction, including without limitation the
  rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
  sell copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:

  The above copyright notice and this permission notice shall be included in
  all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  IN THE SOFTWARE.
}

//todo: add command line support (--dbtype=, --db=, --dbhost=, dbuser=, dbpass=, dbcharset=
{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, IniFiles;

type
  { TDBConnectionConfig }
  TDBConnectionConfig = class(TObject)
  private
    FDBCharset: string;
    FDBType: string;
    FDBHost: string;
    FDBPath: string;
    FDBUser: string;
    FDBPassword: string;
    FSettingsFileIsRead: boolean; //indicates if we read in settings file
    FSettingsFile: string;
    function GetDBCharset: string;
    function GetDBHost: string;
    function GetDBPassword: string;
    function GetDBPath: string;
    function GetDBType: string;
    function GetDBUser: string;
    function GetDefaultSettingsFile:string;
    procedure ReadINIFile;
    procedure SetDBType(AValue: string);
    procedure SetSettingsFile(AValue: string);
  public
    property DBCharset: string read GetDBCharset write FDBCharset; //Character set used for database (e.g. UTF8)
    property DBHost: string read GetDBHost write FDBHost; //Database host/server (name or IP address). Leave empty for embedded
    property DBPath: string read GetDBPath write FDBPath; //Path/database name
    property DBUser: string read GetDBUser write FDBUser; //User name needed for database (e.g. sa, SYSDBA)
    property DBPassword: string read GetDBPassword write FDBPassword; //Password needed for user name
    property DBType: string read GetDBType write SetDBType; //Type of database connection, e.g. Firebird, Oracle
    property SettingsFile: string read FSettingsFile write SetSettingsFile; //ini file to read settings from. If empty defaults to <programname>.ini
    constructor Create;
    constructor Create(DefaultType:string; DefaultHost:string=''; DefaultPath:string='data.fdb';
      DefaultUser:string='SYSDBA'; DefaultPassword:string='masterkey'; DefaultCharSet:string='UTF8');
    destructor Destroy; override;
  end;

implementation

{ TDBConnectionConfig }

function TDBConnectionConfig.GetDBHost: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBHost;
end;

function TDBConnectionConfig.GetDBCharset: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBCharset;
end;

function TDBConnectionConfig.GetDBPassword: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBPassword;
end;

function TDBConnectionConfig.GetDBPath: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBPath;
end;

function TDBConnectionConfig.GetDBType: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBType;
end;

function TDBConnectionConfig.GetDBUser: string;
begin
  if not(FSettingsFileIsRead) then ReadINIFile;
  result:=FDBUser;
end;

function TDBConnectionConfig.GetDefaultSettingsFile:string;
begin
  result:=ChangeFileExt(ParamStr(0), '.ini');
end;

procedure TDBConnectionConfig.SetSettingsFile(AValue: string);
begin
  // If empty value given, use the program name
  if AValue='' then AValue:=GetDefaultSettingsFile;
  if FSettingsFile=AValue then Exit;
  FSettingsFile:=AValue;
  // Read from file if present
  ReadINIFile;
end;


procedure TDBConnectionConfig.SetDBType(AValue: string);
begin
  if FDBType=AValue then Exit;
  case UpperCase(AValue) of
    'FIREBIRD': FDBType:='Firebird';
    'POSTGRES', 'POSTGRESQL': FDBType:='PostgreSQL';
    'SQLITE','SQLITE3': FDBType:='SQLite';
  else FDBType:=AValue;
  end;
end;

procedure TDBConnectionConfig.ReadINIFile;
var
  INI: TIniFile;
begin
  if FileExists(FSettingsFile) then
  begin
    INI := TINIFile.Create(FSettingsFile);
    try
      FDBType := INI.ReadString('Database', 'DatabaseType', FDBType); //Default to Firebird
      FDBHost := INI.ReadString('Database', 'Host', FDBHost);
      FDBPath := INI.ReadString('Database', 'Database', FDBPath);
      FDBUser := INI.ReadString('Database', 'User', 'SYSDBA');
      FDBPassword := INI.ReadString('Database', 'Password', 'masterkey');
      FSettingsFileIsRead:=true;
    finally
      INI.Free;
    end;
  end;
end;

constructor TDBConnectionConfig.Create;
begin
  inherited Create;
  // Defaults
  FSettingsFile:=GetDefaultSettingsFile;
  FSettingsFileIsRead:=false;
  FDBType := 'Firebird';
  FDBHost := ''; //embedded
  FDBPath := 'data.fdb';
  FDBUser := 'SYSDBA';
  FDBPassword := 'masterkey';
end;

constructor TDBConnectionConfig.Create(DefaultType:string; DefaultHost:string=''; DefaultPath:string='data.fdb';
  DefaultUser:string='SYSDBA'; DefaultPassword:string='masterkey'; DefaultCharSet:string='UTF8');
begin
  create;
  FDBCharset:=DefaultCharset;
  FDBHost:=DefaultHost;
  FDBPassword:=DefaultPassword;
  FDBPath:=DefaultPath;
  FDBType:=DefaultType;
  FDBUser:=DefaultUser;
end;

destructor TDBConnectionConfig.Destroy;
begin
  inherited Destroy;
end;

end.

dbconfiggui.pas

unit dbconfiggui;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls, dbconfig;

type
  TConnectionTestFunction = function(ChosenConfig: TDBConnectionConfig): boolean of object;
  { TDBConfigForm }

  TDBConfigForm = class(TForm)
    OKButton: TButton;
    CancelButton: TButton;
    TestButton: TButton;
    ConnectorType: TComboBox;
    Host: TEdit;
    Database: TEdit;
    Label1: TLabel;
    Label2: TLabel;
    Label3: TLabel;
    Label4: TLabel;
    Label5: TLabel;
    Password: TEdit;
    User: TEdit;
    procedure ConnectorTypeEditingDone(Sender: TObject);
    procedure DatabaseEditingDone(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure FormShow(Sender: TObject);
    procedure HostEditingDone(Sender: TObject);
    procedure PasswordEditingDone(Sender: TObject);
    procedure TestButtonClick(Sender: TObject);
    procedure UserEditingDone(Sender: TObject);
  private
    FConnectionConfig: TDBConnectionConfig;
    FConnectionTestFunction: TConnectionTestFunction;
    FSetupComplete: boolean;
    { private declarations }
  public
    property Config: TDBConnectionConfig read FConnectionConfig;
    property ConnectionTestCallback: TConnectionTestFunction write FConnectionTestFunction;
    { public declarations }
  end;

var
  DBConfigForm: TDBConfigForm;

implementation

{$R *.lfm}

{ TDBConfigForm }

procedure TDBConfigForm.TestButtonClick(Sender: TObject);
begin
  // Call callback with settings, let it figure out if connection succeeded and
  // get test result back
  if assigned(FConnectionTestFunction) and assigned(FConnectionConfig) then
    if FConnectionTestFunction(FConnectionConfig) then
       showmessage('Connection test succeeded.')
      else
        showmessage('Connection test failed.')
  else
    showmessage('Error: connection test code has not been implemented.');
end;

procedure TDBConfigForm.UserEditingDone(Sender: TObject);
begin
  FConnectionConfig.DBUser:=User.Text;
end;

procedure TDBConfigForm.FormCreate(Sender: TObject);
begin
  FConnectionConfig:=TDBConnectionConfig.Create;
  FSetupComplete:=false;
end;

procedure TDBConfigForm.ConnectorTypeEditingDone(Sender: TObject);
begin
  FConnectionConfig.DBType:=ConnectorType.Text;
end;

procedure TDBConfigForm.DatabaseEditingDone(Sender: TObject);
begin
  FConnectionConfig.DBPath:=Database.Text;
end;

procedure TDBConfigForm.FormDestroy(Sender: TObject);
begin
  FConnectionConfig.Free;
end;

procedure TDBConfigForm.FormShow(Sender: TObject);
begin
  if not FSetupComplete then
  begin
    // Only do this once in form's lifetime
    FSetupComplete:=true;
    ConnectorType.Text:=FConnectionConfig.DBType;
    Host.Text:=FConnectionConfig.DBHost;
    Database.Text:=FConnectionConfig.DBPath;
    User.Text:=FConnectionConfig.DBUser;
    Password.Text:=FConnectionConfig.DBPassword;
  end;
end;

procedure TDBConfigForm.HostEditingDone(Sender: TObject);
begin
  FConnectionConfig.DBHost:=Host.Text;
end;

procedure TDBConfigForm.PasswordEditingDone(Sender: TObject);
begin
  FConnectionConfig.DBPassword:=Password.Text;
end;

end.

mainform.pas

unit mainform;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs,
  dbconfiggui,dbconfig,
  {General db unit}sqldb,
  {Now we add all databases we want to support, otherwise their drivers won't be loaded}
  IBConnection,pqconnection,sqlite3conn;

type

  { TForm1 }

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { private declarations }
    function ConnectionTest(ChosenConfig: TDBConnectionConfig): boolean;
  public
    { public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.lfm}

{ TForm1 }

procedure TForm1.FormCreate(Sender: TObject);
var
  LoginForm: TDBConfigForm;
begin
  LoginForm:=TDBConfigForm.Create(self);
  try
    // The test button on dbconfiggui will link to this procedure:
    LoginForm.ConnectionTestCallback:=@ConnectionTest;
    LoginForm.ConnectorType.Clear; //remove any default connectors
    // Now add the dbs that you support - use the name of the *ConnectionDef.TypeName property
    LoginForm.ConnectorType.AddItem('Firebird', nil);
    LoginForm.ConnectorType.AddItem('PostGreSQL', nil);
    LoginForm.ConnectorType.AddItem('SQLite3', nil); //No connectiondef object yet in FPC2.6.0
    case LoginForm.ShowModal of
    mrOK:
      begin
        //user wants to connect, so copy over db info
      end;
    mrCancel:
      begin
        ShowMessage('You canceled the database login. Application will terminate.');
        Close;
      end;
    end;
  finally
    LoginForm.Free;
  end;
end;

function TForm1.ConnectionTest(ChosenConfig: TDBConnectionConfig): boolean;
// Callback function that uses the info in dbconfiggui to test a connection
// and return the result of the test to dbconfiggui
var
  // Generic database connector...
  Conn: TSQLConnector;
begin
  result:=false;
  Conn:=TSQLConnector.Create(nil);
  Screen.Cursor:=crHourglass;
  try
    // ...actual connector type is determined by this property.
    // Make sure the ChosenConfig.DBType string matches
    // the connectortype.
    Conn.ConnectorType:=ChosenConfig.DBType;
    Conn.HostName:=ChosenConfig.DBHost;
    Conn.DatabaseName:=ChosenConfig.DBPath;
    Conn.UserName:=ChosenConfig.DBUser;
    Conn.Password:=ChosenConfig.DBPassword;
    try
      Conn.Open;
      if Conn.Connected=true then
        result:=true;
    except
      // Result is already false
    end;
    Conn.Close;
  finally
    Screen.Cursor:=crDefault;
    Conn.Free;
  end;
end;

end.

See also

Retrieved from "http://wiki.freepascal.org/index.php?title=SQLdb_Tutorial3&oldid=63952"