Provided by: libalzabo-perl_0.92-4_all 
NAME
Alzabo::Intro - Introductory information about Alzabo
DESCRIPTION
This document provides an introduction to the basics of using Alzabo, primarily focused on
the RDBMS-OO mapping functionality.
CREATING A SCHEMA
The first thing you'll want to do is create a schema. The easiest way to do this is to
reverse engineer an existing schema. The Mason GUI, available in the separate
"Alzabo::GUI::Mason" distribution, provides another means of creating a schema.
And of course, you can create one through a custom Perl script which uses the various
"Alzabo::Create::*" classes. Here's the beginning of such a script:
use Alzabo::Create::Schema;
eval
{
my $s = Alzabo::Create::Schema->new( name => 'foo',
rdbms => 'MySQL' );
my $table = $s->make_table( name => 'some_table' );
my $a_col = $table->make_column( name => 'a_column',
type => 'int',
nullable => 0,
sequenced => 0,
attributes => [ 'unsigned' ] );
$table->add_primary_key($a_col);
my $b_col = $table->make_column( name => 'b_column',
type => 'varchar',
length => 240,
nullable => 0 );
$table->make_index( columns => [ { column => $b_col,
prefix => 10 } ] );
...
$s->save_to_file;
};
if ($@) { handle exceptions }
USAGE EXAMPLES
Alzabo is a powerful tool but as with many powerful tools it can also be a bit
overwhelming at first. The easiest way to understand some of its basic capabilities is
through some examples. Let's first assume that you've created the following schema:
TABLE: Movie
movie_id tinyint -- primary key
title varchar(200)
release_year year
TABLE: Person
person_id tinyint -- primary key
name varchar(200)
birthdate date
birthplace_location_id tinyint -- foreign key to location
TABLE: Job
job_id tinyint -- primary key
job varchar(200) -- something like 'actor' or 'director'
TABLE: Credit
movie_id tinyint -- primary key part 1, foreign key to movie
person_id tinyint -- primary key part 2, foreign key to person
job_id tinyint -- primary key part 3, foreign key to job
TABLE: Location
location_id tinyint -- primary key
location varchar(200) -- 'New York City' or 'USA'
parent_location_id tinyint -- foreign key to location
Fetching data
In Alzabo, data is returned in the form of a row object. This object can be used to
access the data for an individual row.
Unless you are retrieving a row via a unique identifier (usually its primary key), you
will be given a cursor object. This is quite similar to how "DBI" uses statement handles
and is done for similar reasons.
First of all, let's do something simple. Let's assume I have a person_id value and I want
to find all the movies that they were in and print the title, year of release, and the job
they did in the movie. Here's what it looks like:
my $schema = Alzabo::Runtime::Schema->load_from_file( name => 'movies' );
my $person_t = $schema->table('Person');
my $credit_t = $schema->table('Credit');
my $movie_t = $schema->table('Movie');
my $job_t = $schema->table('Job');
# returns a row representing this person.
my $person = $person_t->row_by_pk( pk => 42 );
# all the rows in the credit table that have the person_id of 42.
my $cursor =
$person->rows_by_foreign_key
( foreign_key =>
$person_t->foreign_keys_by_table($credit_t) );
print $person->select('name'), " was in the following films:\n\n";
while (my $credit = $cursor->next)
{
# rows_by_foreign_key returns a RowCursor object. We immediately
# call its next method, knowing it will only have one row (if
# it doesn't then our referential integrity is in trouble!)
my $movie =
$credit->rows_by_foreign_key
( foreign_key =>
$credit_t->foreign_keys_by_table($movie_t) )->next;
my $job =
$credit->rows_by_foreign_key
( foreign_key =>
$credit_t->foreign_keys_by_table($job_t) )->next;
print $movie->select('title'), " released in ", $movie->select('release_year'), "\n";
print ' ', $job->('job'), "\n";
}
A more sophisticated version of this code would take into account that a person can do
more than one job in the same movie.
The method names are quite verbose, so let's redo the example using "Alzabo::MethodMaker":
# Assume that the method_namer() subroutine pluralizes things as one
# would expect.
use Alzabo::MethodMaker( schema => 'movies',
all => 1,
name_maker => \&method_namer );
my $schema = Alzabo::Runtime::Schema->load_from_file( name => 'movies' );
# instantiates a row representing this person.
my $person = $schema->Person->row_by_pk( pk => 42 );
# all the rows in the credit table that have the person_id of 42.
my $cursor = $person->Credits;
print $person->name, " was in the following films:\n\n";
while (my $credit = $cursor->next)
{
my $movie = $credit->Movie;
my $job = $credit->Job;
print $movie->title, " released in ", $movie->release_year, "\n";
print ' ', $job->job, "\n";
}
Updating data
Updates are done by calling the "update()" method on a row object:
$movie->update( title => 'Chungking Express',
year => 1994 );
If you are using "Alzabo::MethodMaker", the per-column accessors it generates for row
objects can be used to set a column's value:
$movie->title('Chungking Express');
Be careful with this, though, because updates are done immediately against the RDBMS,
meaning each call to a setter method issues an "UPDATE" query. It's much more efficient
to call the "update()" method once with multiple values.
Deleting data
To delete a row, just call it's "delete()" method:
$movie->delete;
Validating data
Let's assume that we've been passed a hash of values representing an update to the
location table. Here's a way of making sure that that this update won't lead to a loop in
terms of the parent/child relationships.
sub update_location
{
my $self = shift; # this is the row object
my %data = @_;
if ( $data{parent_location_id} )
{
my $parent_location_id = $data{parent_location_id};
my $location_t = $schema->table('Location');
while ( my $location =
$location_t->row_by_pk( pk => $parent_location_id ) )
{
die "Insert into location would create loop"
if $location->select('parent_location_id') == $data{location_id};
$parent_location_id = $location->select('parent_location_id');
}
}
}
Once again, let's rewrite the code to use "Alzabo::MethodMaker":
sub update_location
{
my $self = shift; # this is the row object
my %data = @_;
if ( $data{parent_location_id} )
{
my $location = $self;
while ( my $location = $location->parent )
{
die "Insert into location would create loop"
if $location->parent_location_id == $data{location_id};
}
}
}
Using SQL functions
Each subclass of Alzabo::SQLMaker is capable of exporting functions that allow you to use
all the SQL functions that your RDBMS provides. These functions are normal Perl
functions. They take as arguments normal scalars (strings and numbers), "Alzabo::Column"
objects, or the return value of another SQL function. They may be used to select data via
the "select()" and "function()" methods in both the "Alzabo::Runtime::Table" and
"Alzabo::Runtime::Schema" classes. They may also be used as part of updates, inserts, and
where clauses in any place that is valid SQL.
Examples:
use Alzabo::SQLMaker::MySQL qw(MAX NOW PI);
my $max =
$table->function( select => MAX( $table->column('budget') ),
where => [ $table->column('country'), '=', 'USA' ] );
$table->insert( values => { create_date => NOW() } );
$row->update( pi => PI() );
my $cursor =
$table->rows_where( where =>
[ $table->column('expire_date'), '<=', NOW() ] );
my $cursor =
$table->rows_where( where =>
[ LENGTH( $table->column('password') ), '<=', 5 ] );
The documentation for the Alzabo::SQLMaker subclass for your RDBMS will contain a detailed
list of all exportable functions.
Row Objects Not in the Database
Sometimes you'll want to create an object with the row object API, but which does not
represent a row in the database. See the "Alzabo::Runtime::Row" documentation for details
on how these objects can be created.
Changing the schema
In MySQL, there are a number of various types of integers. The type "TINYINT" can hold
values from -128 to 127. But what if have more than 127 movies? And if that's the case
we might have more than 127 people too.
For safety's sake, it might be best to make all of the primary key integer columns "INT"
columns instead. And while we're at it we want to make them "UNSIGNED" as well, as we
don't need to insert negative numbers into these columns.
You could break out the RDBMS manual (because you probably forgot the exact "ALTER TABLE"
syntax you'll need). Or you could use Alzabo. Note that this time we use a
"Alzabo::Create::Schema" object, not "Alzabo::Runtime::Schema".
my $schema = Alzabo::Create::Schema->load_from_file( name => 'movies' );
foreach my $t ( $schema->tables )
{
foreach my $c ( $t->columns )
{
if ( $c->is_primary_key and lc $c->type eq 'tinyint' )
{
$c->set_type('int');
$c->add_attribute('unsigned');
}
}
}
$schema->create( user => 'user', password => 'password' );
$schema->save_to_file;
Because Alzabo keeps track of the schema's state the last time it was created in the
RDBMS, the "create()" method here will generate the appropriate SQL to alter the RDBMS
schema so that it matches the schema as defined in Alzabo.
TRANSACTIONS
Alzabo uses transactions internally in order to guarantee consistency. Obviously, if you
are using a database such as MySQL (without InnoDB) that does not support transactions,
this is not possible.
If you would like to use transactions explicitly in your code, please make sure to use the
"Alzabo::Schema" class's "begin_work()", "commit()", and "rollback()" methods.
EXCEPTIONS
Alzabo uses exceptions as its error reporting mechanism. This means that all calls to its
methods should be wrapped in "eval{}". This is less onerous than it sounds. In general,
there's no reason not to wrap all of your calls in one large eval block. Then at the end
of the block simply check the value of $@.
Also see the "Alzabo::Exceptions" documentation, which lists all of the different
exception used by Alzabo.
This is similar to using "DBI" with the "RaiseError" attribute set to a true value.
Its important to note that some methods (such as the driver's "rollback()" method) may use
"eval" internally. This means that if you intend to use them as part of the cleanup after
an exception, you may need to store the original exception in another variable, as $@ will
be overwritten at the next "eval".
In addition, some methods you might use during cleanup can throw exceptions of their own.
This is the point where I start wishing Perl had a real exception handling mechanism built
into the language.
BACKWARDS COMPATIBILITY
Because Alzabo saves the schema objects to disk as raw data structures using the
"Storable" module, it is possible for a new version of Alzabo to be incompatible with a
saved schema.
As of Alzabo version 0.65, Alzabo can now detect older schemas and will attempt to update
them if possible.
When you attempt to load a schema, whether of the "Alzabo::Create::Schema" or
"Alzabo::Runtime::Schema" classes, Alzabo will determine what version of Alzabo created
that schema.
If updates are necessary, Alzabo will first back up your existing files with the extension
.bak.v{version}, where "{version}" is the version of Alzabo which created the schema.
Then it will alter the schema as necessary and save it to disk.
This will all happen transparently, as long as the process which initiated this process
can write to the schema files and the directory they are in.
Alzabo will need the "Alzabo::Create::*" classes to update the schema. If these have not
been loaded already, Alzabo will do so and issue a warning to say that this has happened,
in case you would like to restart the process without these classes loaded.
MULTIPLE COPIES OF THE SAME SCHEMA
It is possible to use the same schema definition with multiple copies of that schema in
the RDBMS. This can be done by setting the "schema_name" parameter whenever you call a
method that connects to the RDBMS, such as "Alzabo::Create::Schema->create" or
"Alzabo::Runtime::Schema->connect". This will override the default, the schema's name as
given when it was first created via "Alzabo::Create::Schema->new".
Every time you call "create()" or "sync_backend()", the schema will consider itself to
have been instantiated. This means that if you call "create()" twice, each time with a
different "schema_name" parameter, then you will probably not be able to generate useful
diffs via the "make_sql()" method in the future.
This is a bug that is unlikely to be fixed.
MULTIPLE RDBMS SUPPORT
Alzabo aims to be as cross-platform as possible. To that end, RDBMS specific operations
are contained in several module hierarchies. The goal here is to isolate RDBMS-specific
behavior and try to provide generic wrappers around it, inasmuch as is possible.
The first, the "Alzabo::Driver::*" hierarchy, is used to handle communication with the
database. It uses "DBI" and the appropriate "DBD::*" module to handle communications. It
provides a higher level of abstraction than "DBI", requiring that the RDBMS specific
modules implement methods to do such things as create databases or return the next value
in a sequence.
The second, the "Alzabo::RDBMSRules::*" hierarchy, is used during schema creation in order
to validate user input such as schema and table names. It also generates DDL SQL to
create the database or turn one schema into another (sort of a SQL diff). Finally, it
also handles reverse engineering of an existing database.
The "Alzabo::SQLMaker::*" hierarchy is used to generate DML SQL and handle bound
parameters.
The RDBMS to be used is specified when creating the schema. Currently, there is no easy
way to convert a schema from one RDBMS to another, though this is a future goal.
REFERENTIAL INTEGRITY
Alzabo can maintain referential integrity in your database based on the relationships you
have defined. This can be toggled via the
"Alzabo::Runtime::Schema->set_referential_integrity()" method.
Alzabo enforces these referential integrity rules:
• Inserts
An attempt to insert a value into a table's foreign key column(s) will fail if the
value does not exist in the foreign table.
If a table is dependent on another table, any columns from the dependent table
involved in the relationship will be treated as not nullable.
If the relationship is one-to-one, all columns involved in the foreign key will be
treated as if they had a unique constraint on them (as a group if there is more than
one) unless any of the columns being inserted are NULL.
The exception to this rule is that no attempt is made to enforce constraints on a
table's primary key, as that could conceivably make it impossible to insert a row into
the table.
• Updates
Updates follow the same rules as inserts.
• Deletes
When a row is deleted, foreign tables which are dependent on the one being deleted
will have the relevant rows deleted. Otherwise, the foreign table's related column(s)
will simply be set to NULL.
AUTHOR
Dave Rolsky, <autarch@urth.org>