Lazarus Tdbf Tutorial

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Revision as of 11:14, 16 January 2006 by Neli (talk | contribs) (add link to tdbf documentation)

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Overview

This tutorial is about basic database development using the TDbf component (by Micha Nelissen) with Lazarus. Additional documentation for the TDbf is available. This page was created by Tony Maro 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.

What you will need

No doubt this will soon be easier as FreePascal releases the next version 2.0, however currently you will need a recent CVS edition of FPC 1.9.X in order to properly use the TDbf component. It may be possible to download just the TDbf component by itself and use it with the 1.1 version of FreePascal, however this document was written with 1.9.X in mind, partly due to bug fixes in the other database components used in Lazarus.

You will also need to install the DbfLaz package that comes with Lazarus. It's located in the lazarus/components/tdbf/ directory.

What the TDbf provides

The TDbf provides access to dBase and FoxPro database tables for Lazarus (and others). It allows for reading, writing and creation of dBase III+, dBase IV, Visual dBase VII and FoxPro tables. It does all of this without the need for additional libraries or database engines. Simply drop the TDbf on your form and you have instant access to a cross-platform database environment. The TDbf works in both Windows and Linux using Lazarus.

How to create a new database table

As there is no "Database Desktop" application for Lazarus yet, we must create a new database in code.

Setting the path

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.

Choosing a 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.

Adding fields

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;

How to add indexes to a table

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;

External Index Files

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');

How to link the TDbf to data-aware components

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.

Packing and rebuilding the tables

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.