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Man Pages


Manual Reference Pages  -  T2 (3)

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NAME

T2 - Object Relational mapping system

CONTENTS

SYNOPSIS



  use T2::Storage;

  # T2::Storage class provides Tangram::Storage API and
  # loads schemas
  my $storage = T2::Storage->open("appName");
  my (@ids) = $storage->insert(@some_objects);

  # T2::Schema structures can be manipulated in the same way
  my $schema_storage = T2::Storage->open("schema");
  my $schema = T2::Schema->load("appName", $schema_storage);
  my @classes = $schema->classes;
  $schema->classes_remove(17);    # remove a random class
  $schema_storage->update($schema);



DESCRIPTION

The T2 module is a base for the refactoring of the now quite stable Tangram Object-Relational mapper.

In a nutshell, it lets you store objects - which have to be described to a similar level that you would describe a database to store them - into any SQL store. Currently, this is tested on PostgreSQL, MySQL, Oracle and Sybase a lot, though in general database-specific extensions to SQL, such as triggers, stored procedures, etc are avoided. So, if DBI installs and tests successfully with your database, there is a good chance that T2 will work with it too.

The only current requirement is that objects that have tables associated with them are implemented via hashes. You also have to be able to describe all of the fields for those root objects. Individual fields of stored objects may be arbitrarily complex.

If you are familiar with DBI, it is somewhat similar to bless’ing the structures returned by $dbh->fetchrow_hashref, except that references and collections to other objects in the store are loaded ‘on demand’ (aka Lazy-loading).

CONTENTS

The T2:: namespace currently contains:
<B>T2::StorageB> T2::Storage is a convenience wrapper for Tangram::Storage. Hence, the object it returns provides the complete Tangram 2 API. It contains a constructor that automatically grabs configuration information (such as, database location, schema details, etc), and connects to the database.

It makes simple programs, for existing applications that have an established schema very simple. Getting the schema structures there in the first place, however, is not terribly straightforward. Yet.

As more of the Tangram core is ported over the the new T2::Storage class, the documentation here and in T2::Storage will be completed. For now, Tangram and Tangram::Storage are the best starting points.

<B>T2::SchemaB>, <B>T2::ClassB>, etc. The T2::Schema structures are the Tangram schema, expressed as Class::Tangram objects. These are hence easily storable in a T2 data store. Or a Pixie/MLDBM/etc store, for that matter.

FREQUENTLY UNASKED QUESTIONS (FUQ)

You might be asking, how does one write a FUQ file?

One answer is by becoming maintainer of a module, and talking to people who have looked at a module, and seeing what drawbacks they saw, or why they didn’t take it any further.
<B>How do I just quickly store an object?B> At the moment, there is no shortcut. You must declare a simple schema that has the object type (or one of its superclasses) that you wish to store.

<B>Warning:B> the examples in this section have been quickly written, and there are probably some minor bugs. If you encounter problems, please send a message to the mailing list (see SUPPORT).

Creating this structure is a lot easier with the I’ll-release-it-RSN-ware web-based VT2 tool, but here is some example code to set up a schema file;



  my $schema = T2::Schema->new
      ( T2::Class->new( name => "ExampleClass",
                        attributes => [ T2::Attribute->new
                                        ( type => "perl_dump",
                                          name => "foo",
                                        ) ],
      );

  use Storable qw(freeze);
  open FOO, ">etc/myApp.t2";
  binmode FOO;
  $Storable::Deparse = 1;
  print FOO freeze $schema;
  close FOO;



Yes, OK, that’s not exactly a one-liner. But it should give you an idea of what’s involved. Again, the web-based tool to manage the above schema structures makes things a lot quicker.

You’d then create a file etc/myApp.dsn, with something like the following contents:



  dsn dbi:mysql:database=myApp
  user myApp
  auth razamatazz
  schema myApp



(yes, this should be YAML. Patches welcome :))

Then, you need to create the user and set up the database. You can use the t2-migrate-db.pl utility script in the T2 distribution to do the latter;



  $ echo grant all privileges on myApp.* to myApp@localhost \
              identified by "razamatazz" | mysql -uroot -p
  Password:
  $ t2-migrate-db.pl --read --deploy myApp
  ...
  $



Still with me? Good. Then the script to actually use the code is simple;



  use T2::Storage;
  my $storage = T2::Storage->open("myApp");

  # this line makes the classes for you, if you didnt
  # already have ExampleClass written.
  $storage->schema->generator();

  # stick an object in the DB and return an object ID
  my $oid = $storage->insert(new ExampleClass(foo => "bar"));

  # anything in $exampleClass->foo can be stored (via
  # perl_dump mapping)
  use Whatever;

  # fetch the object back
  my $exampleClass = $storage->load($oid);
  $exampleClass->set_foo( { bar => new Whatever() } );
  $storage->update($exampleClass);

  $storage->insert(new ExampleClass
                        (foo => [ 1, 2, $exampleClass ]));



See Tangram::Tour for a more detailed example of how to manipulate objects in the database, and Class::Tangram for more information on how to use the generated constructors and accessors (eg, new ExampleClass and set_foo in the above example).

You don’t have to use Class::Tangram classes in a T2 store. But they are very well behaved OO classes that communicate via message passing exclusively, and export the information needed by T2 to store them in a package global. So they’re convenient.

<B>How do I just quickly get an object out?B> If you still have the object ID, you can fetch it by that;



  my $user = $storage->load($user_oid);



If that user has a reference to another storage object in it (what would normally be accomplished using an ID into another table in SQL), then you can just access the value; it is ‘demand paged’ / ‘lazy loaded’ via Data::Lazy or an equivalent. For example;



  my $details = $user->details;



If the referant object is already in memory, then the in-memory version is used instead of being explicitly loaded from the database.

Otherwise, you can use the OO query format explained in much more detail in Tangram::Tour;



  my $remote_session = $storage->remote("MySession::Class");
  my ($session) = $storage->select
                     ($remote_session,
                      $remote_session->{SID} eq $somesid);



The query format is flexible enough to represent virtually any SQL query. The $remote_session object is an object which represents an object in the database of the given type. So, the expression $remote_session-{SID} eq $somesid> returns an object that represents all MySession::Class objects that have a SID of $somesid.

Generally, people either think that this syntax is the cleanest method of expressing relationships in a non-SQL way possible, or, as one of the Pixie authors put it, you ’turn running and screaming’.

<B>OK, now how do I change the schema?B> Simply write a new etc/myApp.t2 schema file. Modify the script above and re-run it.
<B>How does Tangram compare to XXX?B> First, you should have a look at the POOP (that’s Perl Object Oriented Persistence :)) summary at <http://poop.sf.net/>.

In a nutshell, there are two types of persistence tools; SQL abstractors (eg, Alzabo, Class::DBI) and true Object Persistence tools (eg Pixie, Tangram).

The SQL abstractors tend to require that all of your objects derive from a common base class. This is encouraged with T2, but not required.

The Object Persistence tools will generally let you store pretty much anything, without ‘intruding’ on your objects. In theory, this means that they may easily be loaded from one storage class and thrown into another.

<B>What Query languages do I use?B> See Tangram::Tour for a run-down on the query format.
<B>How can I manage schemas?B> Using the web-based tool that manages these T2::Schema data structures in a Tangram store, and the supplied utility scripts. The web-based application is expected to be released by Q2 2004.
<B>Why bother with schema maintenance at all?B> At some level, your application has a schema governing it.

The idea with these modules is that you express it all in a standard way in one place, rather than keeping some in your head, some in the module source, some in the database tables...

<B>Wouldn’t it be faster to roll your own indexes?B> There is an argument that it is simpler to opt for *very* simple data persistence, and just use your own objects for the indexes.

The answer is, this really sucks for concurrency and transaction safety. Especially as the size of the index grows. This is one of the reasons you might want to use a database in the first place :-).

SUPPORT

Support for bugs is provided on a politely state your problem with a test case to demonstrate the bug, and if you’re lucky, someone will fix it for you basis by the Tangram community.

The current home page, including information on the mailing list, IRC channels, SVN servers, etc is at <http://tangram.utsl.gen.nz/>.

CREDITS / AUTHOR

Jean-Louis Leroy is the original author of Tangram 1 and 2.

Sam Vilain is the author of T2::* and Class::Tangram, and current maintainer of Tangram.

Changes are known to be contributed to the combined Tangram and T2 code base from:



  - Gabor Herr
  - Marian Kelc
  - Aaron Mackey
  - Kurt Stephens
  - Kate Pugh



LICENSE

Most of the code in this collection is dual GPL / Perl Artistic license. However, at least some of it is purely GPL. So, the combined license of this collection is currently the GPL.

For information on the GPL, please see <http://www.gnu.org/>.

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