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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>

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