✏ 编辑: HowTo.pod

=head1 NAME

DBD::File::HowTo - Guide to create DBD::File based driver

=head1 SYNOPSIS

perldoc DBD::File::HowTo
  perldoc DBI
  perldoc DBI::DBD
  perldoc DBD::File::Developers
  perldoc DBI::DBD::SqlEngine::Developers
  perldoc DBI::DBD::SqlEngine
  perldoc SQL::Eval
  perldoc DBI::DBD::SqlEngine::HowTo
  perldoc SQL::Statement::Embed
  perldoc DBD::File
  perldoc DBD::File::HowTo
  perldoc DBD::File::Developers

=head1 DESCRIPTION

This document provides a step-by-step guide, how to create a new
C<DBD::File> based DBD. It expects that you carefully read the L<DBI>
documentation and that you're familiar with L<DBI::DBD> and had read and
understood L<DBD::ExampleP>.

This document addresses experienced developers who are really sure that
they need to invest time when writing a new DBI Driver. Writing a DBI
Driver is neither a weekend project nor an easy job for hobby coders
after work. Expect one or two man-month of time for the first start.

Those who are still reading, should be able to sing the rules of
L<DBI::DBD/CREATING A NEW DRIVER>.

Of course, DBD::File is a DBI::DBD::SqlEngine and you surely read
L<DBI::DBD::SqlEngine::HowTo> before continuing here.

=head1 CREATING DRIVER CLASSES

Do you have an entry in DBI's DBD registry? For this guide, a prefix of
C<foo_> is assumed.

=head2 Sample Skeleton

package DBD::Foo;

use strict;
    use warnings;
    use vars qw(@ISA $VERSION);
    use base qw(DBD::File);

use DBI ();

$VERSION = "0.001";

package DBD::Foo::dr;

use vars qw(@ISA $imp_data_size);

@ISA = qw(DBD::File::dr);
    $imp_data_size = 0;

package DBD::Foo::db;

use vars qw(@ISA $imp_data_size);

@ISA = qw(DBD::File::db);
    $imp_data_size = 0;

package DBD::Foo::st;

use vars qw(@ISA $imp_data_size);

@ISA = qw(DBD::File::st);
    $imp_data_size = 0;

package DBD::Foo::Statement;

use vars qw(@ISA);

@ISA = qw(DBD::File::Statement);

package DBD::Foo::Table;

use vars qw(@ISA);

@ISA = qw(DBD::File::Table);

Tiny, eh? And all you have now is a DBD named foo which will is able to
deal with temporary tables, as long as you use L<SQL::Statement>. In
L<DBI::SQL::Nano> environments, this DBD can do nothing.

=head2 Start over

Based on L<DBI::DBD::SqlEngine::HowTo>, we're now having a driver which
could do basic things. Of course, it should now derive from DBD::File
instead of DBI::DBD::SqlEngine, shouldn't it?

DBD::File extends DBI::DBD::SqlEngine to deal with any kind of files.
In principle, the only extensions required are to the table class:

package DBD::Foo::Table;

sub bootstrap_table_meta
    {
	my ( $self, $dbh, $meta, $table ) = @_;

# initialize all $meta attributes which might be relevant for
	# file2table

return $self->SUPER::bootstrap_table_meta($dbh, $meta, $table);
    }

sub init_table_meta
    {
	my ( $self, $dbh, $meta, $table ) = @_;

# called after $meta contains the results from file2table
	# initialize all missing $meta attributes

$self->SUPER::init_table_meta( $dbh, $meta, $table );
    }

In case C<DBD::File::Table::open_file> doesn't open the files as the driver
needs that, override it!

sub open_file
    {
	my ( $self, $meta, $attrs, $flags ) = @_;
	# ensure that $meta->{f_dontopen} is set
	$self->SUPER::open_file( $meta, $attrs, $flags );
	# now do what ever needs to be done
    }

Combined with the methods implemented using the L<SQL::Statement::Embed>
guide, the table is full working and you could try a start over.

=head2 User comfort

C<DBD::File> since C<0.39> consolidates all persistent meta data of a table
into a single structure stored in C<< $dbh->{f_meta} >>. With C<DBD::File>
version C<0.41> and C<DBI::DBD::SqlEngine> version C<0.05>, this
consolidation moves to L<DBI::DBD::SqlEngine>. It's still the
C<< $dbh->{$drv_prefix . "_meta"} >> attribute which cares, so what you
learned at this place before, is still valid.

sub init_valid_attributes
    {
	my $dbh = $_[0];

$dbh->SUPER::init_valid_attributes ();

$dbh->{foo_valid_attrs} = { ... };
	$dbh->{foo_readonly_attrs} = { ...  };

$dbh->{foo_meta} = "foo_tables";

return $dbh;
    }

See updates at L<DBI::DBD::SqlEngine::HowTo/User comfort>.

=head2 Testing

Now you should have your own DBD::File based driver. Was easy, wasn't it?
But does it work well?  Prove it by writing tests and remember to use
dbd_edit_mm_attribs from L<DBI::DBD> to ensure testing even rare cases.

=head1 AUTHOR

This guide is written by Jens Rehsack. DBD::File is written by Jochen
Wiedmann and Jeff Zucker.

The module DBD::File is currently maintained by

H.Merijn Brand < h.m.brand at xs4all.nl > and
Jens Rehsack  < rehsack at googlemail.com >

=head1 COPYRIGHT AND LICENSE

You may freely distribute and/or modify this module under the terms of
either the GNU General Public License (GPL) or the Artistic License, as
specified in the Perl README file.

=cut

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