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DBO - Database Objects


  use DBO ':constructors';

  $dbh = DBO::Handle::DBI::mysql->connect
    ('dbi:mysql:database:host', 'larry', 'camel');

  $schema = Database
    ( tables =>
      [ Table
        ( name => 'person',
          columns =>
          [ Char(name => 'name', max_length => 100 ),
            Text(name => 'address'),
            Char(name => 'phone', max_length => 30 )])]);

  $dbo = DBO->new
  ( handle => $dbh,
    schema => $schema );

  use DBO::Visitor::Create;


DBO is an object-oriented database abstraction layer.

DBO is designed to be flexibly extensible in a number of directions - adding new operations on the database, adding new kinds of tables or columns, and applying to new database systems. All extensions can be carried out by creating new classes that inherit from the classes DBO defines, and by defining new multimethod instances for those classes.

DBO defines three class hierarchies:

Database operations

An operation on a database is represented by an object belonging to the class DBO::Visitor. DBO provides a number of operations including Create, Insert and Select.

Schema elements

The structure of the database is represented by an object belonging to the class DBO::Database, which contains a number of tables represented by DBO::Table, each of which contains a number of columns represented by DBO::Column. DBO defines many column types, including Char, Text, Unsigned, Integer and Time.

Additional features of columns in the schema - such as the values in the column being restricted to a set of options, or the column being one of the keys of the table - are represented by wrapping the column with a Modifier class. DBO defines the modifier classes Key, Option and ForeignKey. Each column object belonging to the modifier class has a reference to another column object that describes the underlying type of the column.

(This design allows a ForeignKey column to be implemented by a Char or Integer or whatever, as the designer wishes, without needing extra classes ForeignKey_Char, ForeignKey_Integer and so on.)

Database handles

The database itself is represented by an object belong to the class DBO::Handle. DBO defines the class DBO::Handle::DBI as a thin wrapper around DBI, but the facility is there for DBO to be applied to other kinds of database (or to define more sophisticated wrappers around DBI such as "virtual databases" - views that include data from more than one database).

The application of an operation to an element of the schema is represented by a multimethod instance. DBO uses three multimethods:

visit_database($visitor, $database, $handle)
visit_table($visitor, $table, $handle)
visit_column($visitor, $column, $handle)

When $visitor is the generic visitor DBO::Visitor, visit_database visits all the tables in the database; visit_table visits all the columns in the table, visit_column visits the base column when $column is a Modifier column, and does nothing otherwise.

See Class::Multimethods for the full details of the multimethod implementation.


By default, the DBO package exports no names and expects you to use a purely object-oriented interface.

However, a number of constructor functions simplify the building of schemas, and these can be imported by passing the :constructors key to the use DBO statement. Then you can write

    Text(name => 'address')

as a shorthand for

    DBO::Column::Text->new(name => 'address')


The DBO class packages up the database schema and the database handle into one object, with a couple of convenience functions for creating and applying operations. (You don't need to use a DBO object if you don't want to.)

DBO->new takes a list of keys and values. The following keys are required:


A database schema, represented by an object of class DBO::Database.


A database handle, represented by an object of class DBO::Handle.


All constructors for schema elements are called new, and take a list of keys and values.


A database. The following key is defined:


A reference to an array of the tables in the database (each represented by a DBO::Table object). Required.


A table; or more specifically a view onto a table (you can have many views onto the same table). The following keys are defined:


An identifying name for this object. The tables belonging to a particular database must have different ids (this only matters when there is more than one view onto the same table). If not supplied, the value for the name key is used instead.


The name of the table in the database. Required.


A reference to an array of the columns in the table (each represented by a DBO::Column object). Required.


A generic column.


A column implementation in a table in the database. The following keys are defined:


The name of the column in the table. Required.


True iff entries in the column are allowed to be NULL.


A column whose values are numbers.


A column whose values are integers.


A column whose values are non-negative integers.


A column whose values are strings.


A column whose values are fixed-length strings. The following key is defined:


The maximum length of a value for the column. Defaults to 10.


A column whose values are variable-length strings. The following keys are defined:


The average length of a value for the column. Defaults to 100. This is a performance hint for some databases (e.g. mSQL) and ignored elsewhere.


The maximum length of a value for the column. Defaults to 1000. For databases that support arbitrarily long strings, this is ignored.



See Class::Multimethods (Damian Conway) for the implementation of multimethods in Perl.

See DBI (Tim Bunce) for Perl's database independent interface.

"Design patterns: elements of reusable object-oriented software" by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (Addison-Wesley 1995) describes the Visitor pattern.


Gareth Rees


Copyright (c) 1999 Canon Research Centre Europe Ltd. All rights reserved.

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