Lazarus Tdbf Tutorial/ru

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Overview

Это руководство описывает основы разработки баз данных с использованием TDbf компонента TDbf (by Micha Nelissen) в Lazarus. Дополнительная документация по TDbf также доступна. Эта страница была создана ViruZ but other contributions are welcome!

For TDbf documentation pdf go to the SourceForge site. It may be useful to keep that pdf alongside this document while reading.

Что нам понадобится

Может быть возможно загрузить только компонент TDbf отдельно и использовать это с более старыми версиями FreePascal, однако этот документ был написан с версией Free Pascal Compiler 2.2.1 /Lazarus 0.9.23.

Пакет DbfLaz установлен по умолчанию.

Что обеспечивает TDbf

TDbf обеспечивает доступ к таблицам баз данных dBase и FoxPro для Lazarus (and others). Это учитывает чтение, запись и создание dBase III +, dBase IV, Visual dBase VII и FoxPro таблиц. Все это делается без потребности в дополнительных библиотеках или движках базы данных. Просто разместите компонент TDbf на вашей форме, и Вы имеете мгновенный доступ к окружающей среде базы данных на разных платформах. TDbf работает и в Windows и в Linux используя Лазарус.

Как создать новую таблицу базы данных

Поскольку для Lazarus нет приложения наподобии "Database Desktop", мы должны создать новую базу данных в коде.

Установка пути

It's a good idea to give your application's database it's own directory. This simplifies making backups of the data. There are two ways to set the path. You can set the full path using the FilePathFull property, or you can set a path relative to the current application path with FilePath. For instance, setting "FilePath" at runtime to "data/" would use a data subdirectory just below the executable file. Setting the "FilePathFull" property to "/var/data/" would place everthing in that exact folder, ignoring the application's location.

Выбор TableLevel

By default, the TDbf will create dBase IV tables. While this is very compatible, there are features you may wish to use that are not supported. To support auto-incrementing fields, you must use something newer. The table types are:

  • 3 dBase III+
  • 4 dBase IV
  • 7 Visual dBase VII
  • 25 FoxPro

You choose a table type by setting the TableLevel property appropriately.

Добавление полей

Creating fields for your new table at runtime pretty much follows the old Delphi standard. Once you have set your FilePath, TableLevel, and TableName properties, manipulate the FieldDefs property to set up the structure. For example: 

MyDbf.FilePathFull := '/location/to/my/data';
MyDbf.TableLevel := 7;
MyDbf.TableName := 'customers.dbf'; // note: is the .dbf really required?
With MyDbf.FieldDefs do begin
  Add('Id', ftAutoInc, 0, True);
  Add('Name', ftString, 80, True);
End;

Field types are defined as:

  • ftUnknown
  • ftString
  • ftSmallInt
  • ftInteger
  • ftWord
  • ftBoolean
  • ftFloat
  • ftCurrency (TableLevel 25)
  • ftBCD (TableLevel 25)
  • ftDate
  • ftTime
  • ftDateTime
  • ftBytes (TableLevel 25)
  • ftVarBytes
  • ftAutoInc (TableLevel 7 or 25)
  • ftBlob
  • ftMemo
  • ftGraphic
  • ftFmtMemo
  • ftParadoxOle
  • ftDBaseOle
  • ftTypedBinary
  • ftCursor
  • ftFixedChar
  • ftWideString
  • ftLargeInt
  • ftADT
  • ftArray
  • ftReference
  • ftDataSet
  • ftOraBlob
  • ftOraClob
  • ftVariant
  • ftInterface
  • ftIDispatch
  • ftGuid
  • ftTimeStamp
  • ftFMTBcd

Bold types are currently supported

Go ahead and create it!

Once you have defined the fields you wish to use in your new table, you can go ahead and create it with: 

   MyDbf.CreateTable;

Как добавить индексы в таблицу

If your database is larger than a few records, chances are you will want to have indexes defined to make searching faster. To change the index structure of a table, we will want to have exclusive access to the table - which we would have while creating it anyway. 

       MyDbf.Exclusive := True;
       MyDbf.Open;

Now, we just have to add the index. 

       MyDbf.AddIndex('custid', 'Id', [ixPrimary, ixUnique]);
       MyDbf.AddIndex('custname','Name', [ixCaseInsensitive]);
       MyDbf.Close;

Put it all together and you get...

The following sample creates a new table "customers" in code. This of course only needs done once, and after that you just OPEN the table, don't create it ;-) 

{ We will require the following units be in the USES clause: }
{ uses Dbf, db, Dbf_Common                                   }
{ The Dbf is put there when you drop the TDbf on a form...   }
{ but you will need db for the DataSet object and Dbf_Common }
{ for things such as the field type definitions              }
var
  MyDbf: TDbf;
begin
  MyDbf := TDbf.Create(nil);
  try
    { use relative path to "data" directory }
    MyDbf.FilePath := 'data/'; 
    { we want to use Visual dBase VII compatible tables }
    MyDbf.TableLevel := 7;
    MyDbf.Exclusive := True;
    MyDbf.TableName := 'customers.dbf';
    With MyDbf.FieldDefs do begin
      Add('Id', ftAutoInc, 0, True);
      Add('Name', ftString, 80, True);
    End;
    MyDbf.CreateTable;
    MyDbf.Open;
    MyDbf.AddIndex('custid', 'Id', [ixPrimary, ixUnique]);
    { add a secondary index }
    MyDbf.AddIndex('custname','Name', [ixCaseInsensitive]);
    MyDbf.Close;
  finally
    MyDbf.Free;
  end;
end;

Внешние индексные файлы

The TDbf also supports storing secondary indexes in a separate file. This might be helpful if the database is expected to be very large. Secondary index files are created almost identically to normal indexes, but with the addition of the '.ndx' file extension: 

    MyDbf.AddIndex('custname.ndx','Name', [ixCaseInsensitive]);


Each time the TDbf is opened, the index file must be loaded: 

    MyDbf.OpenIndexFile('custname.ndx');


And indexes must be referenced including the extension: 

    MyDbf.IndexName := 'custname.ndx';


Index files are packed separately using: 

    MyDbf.CompactIndexFile('custname.ndx');

Как присоединить TDbf к компонентам data-aware

The above examples show how to create a new database table in code. Using that table is even more simple.

Data aware components in Lazarus (such as the TDbEdit control) link to a TDataSource component using their "DataSource" and "DataField" properties. The TDataSource component handles communication between the database engine and the data aware components. A TDataSource then links to the TDbf component using it's "DataSet" property. The connection looks like this: 

TDbEdit-------
             |
TDbEdit------|-->TDataSource-->TDbf
             |
TDbNavigator--


Be sure to set the FilePath (or FilePathFulll), TableLevel, and TableName properties of your TDbf component before calling 

TDbf.Active := True;


There is much more that can be said about programming with databases in Lazarus, and I would recommend a good Delphi database programming book or two as the underlying concepts are the same. I constantly refer to my copy of "Delphi 2 Unleashed" because the concepts and basic code haven't changed much in 8 years.

Упаковка и перестройка таблиц

When a record is deleted, it's not truly removed from the physical table. Periodically you must "pack" a table to recover that lost space. This should be done with exclusive mode set. 

MyDbf.Exclusive := True;
MyDbf.Open;
MyDbf.PackTable;
// let's also rebuild all the indexes
MyDbf.RegenerateIndexes;
MyDbf.Close;
MyDbf.Exclusive := False;

Master table relations

Real power in database programming begins when you have multiple tables that reference each other. While TDbf does not yet support referential integrity, it does support a master / detail relationship between TDbf's.

When there are two tables related, for instance: 

[customers]
Id       <----|
Name          |
Phone         |
Address       |
              |  The CustID in invoices references a customer primary  field
[invoices]    |
Id            |
Amount        |
CustID   -----|  * This field indexed as "idxcustid"


If you wanted to display all invoices for a given customer, the detail table (invoices) can stay in sync with the master table (customers) automatically.

On the invoices TDbf component set the following: 

InvDbf.IndexName := 'idxcustid'; // our field that will match the customers table ID
InvDbf.MasterSource := dsCustomers; // datasource that is linked to the customers TDbf
InvDbf.MasterFields := 'Id'; // field on the customers table we are matching against our index


Sample application - DB Browser

I've written a simple application that will use the TDbf to open and display database tables using the dbGrid control. The Linux executable along with project sources which should compile fine in Windows is available from: tony.maro.net

Things you need to be aware of

Currently there is no support for referential integrity, or internally encrypted .dbf files.

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