NAME

Mongoose::Intro - an introduction

MOTIVATION

This module is an attempt to bring together the full power of Moose into the MongoDB database.

Before using this module you should take a little time to read on MongoDB.

REQUIREMENTS

To use this module, you need:

MongoDB installed somewhere in your network.

Grab a pre-built copy for your OS from here <http://www.mongodb.org/downloads>, or build it from sources <http://www.mongodb.org/display/DOCS/Source+Code>.

After intalling the software, start the Mongo daemon:

    mongod -dbpath /path/to/data

The MongoDB Perl driver

    cpan MongoDB
    cpan Mongoose

Set the "MONGOOSEDB" environment variable to your MongoDB connection in case it's not standard (localhost).

    make
    export MONGOOSEDB=host,mongodb://localhost,db_name,mytestdb
    make test
    make install

Moose classes

Create some Moose classes to start using Mongoose;

    package MyClass;
    use Moose;
    with 'Mongoose::Document';
    has 'yada' => ( is=>'rw', isa=>'Str' );

FEATURES

Some of Mongoose features:

It's fast. Not as fast as working with MongoDB documents directly though. But it's way faster than any other ORM and relational mapping modules out there.
It handles most object relationships, circular references included.
No persistency. It doesn't manage states for your object. If you save your object twice, it writes twice to the database. If you need partial updating you can do it as there is always access to the MongoDB layer.
Primary keys. This is quite a extraneuos concept for objects, and it's not mandatory. But it allows you to automatically control when new objects translate to new MongoDB documents, or just update them.
This is an experimental feature. There are other ways to do this anyway with the MongoDB built-in "_id" primary-key attribute.
Schema-less data. MongoDB does not need a schema. You can create new attributes for your object and delete old ones at your leasure.
No data-object binding means that you may reuse collections, and peruse inheritance to great extent.

CAVEATS

This module intrusively imports singleton based methods into your class. It's the price to pay for a simpler user interface and less keystrokes.
Object expansion from the database is done using plain bless most of the time. Which means your attribute triggers, etc. will not be fired during expansion. There are exceptions to this rule though.
After saving or loading objects from the database, your object will have an extra attribute, "_id". This is a unique identifier. The _id value can be overwritten if you wish.

GETTING STARTED

There are only two steps to start using Mongoose in your code:

1) Create at least one class that consumes a Mongoose::Document role.

2) Connect to a Mongo database in your main program.

MongoDB does not require you to previously create a database, a collection or a document schema for your collection. This is done on the fly for you.

To make your Moose classes "Mongoable", all they need is to consume either one of two roles: Mongoose::Document or Mongoose::EmbeddedDocument. Read on for details on the difference.

Turning your classes into Mongo Documents

There are two roles to make your Moose class a Mongoose document:

* Mongoose::Document

* Mongoose::EmbeddedDocument

The difference between these roles lies in the way objects of different classes will be joined and stored (collapsed) into the DB.

Read the MongoDB docs <http://http://www.mongodb.org/display/DOCS/Schema+Design#SchemaDesign-EmbeddingandLinking> if you don't understand the difference.

Document

Akin to a row in the relational model. Objects are stored into independent collections. Relationships are stored using references, MongoDB's foreign key system.

EmbbededDocument

Tells Mongoose to store your class as an embedded document, inside a parent document.

This is usually faster than using document-to-document reference joins. But it's not meant for object reuse by foreign documents.

Methods you get when using the Document roles

Both "Document" and "EmbeddedDocument" will import into your class the following methods:

save

Saves the current object to the database, inserting the document if needed.

    $person->save;

delete

Deletes the corresponding document from the database.

    $person->delete;

find

Wraps MongoDB's find method to return a cursor that expands data into objects.

    my $cursor = Person->find({ age => { '$lt' => 30 } });

find_one

Finds exactly one document.

    my $jack = Person->find_one({ first_name => 'Jack' });

collection

Returns the MongoDB::Collection object supporting this class. It's a way to switch quickly back to MongoDB hash documents.

    Person->find_one({ name=>'thyself' }); # isa Person

    # whereas

    Person->collection->find_one({ name=>'thyself' }); # ref = HASH

_id

Not really a method but an attribute used by Mongoose (and MongoDB). Contains a unique BSON::OID instance.

Deleting or modifying this attribute may cause your object to be re-inserted on the next "save", instead of being updated.

The Default Engine

Mongoose comes with a default engine, Mongoose::Engine that takes care of expanding and collapsing objects to and from the Mongo database.

Collapsing

Collapsing is the process of serializing classes.

Your Moose objects are collapsed by unblessing them until they become a hash. Relationships are mantained in the process.

Expanding

Expansion is the process of inflating Mongo documents (plain hashes) into Moose objects.

This is done by inspecting the class attribute metadata. The engine tries to do it's best identifying data types. The document is then "bless"ed into your class. This is faster than calling "new", but also means that no special class or attribute methods will be fired, such as default values, setters, triggers or coercion. You've been warned.

Naturally, there are many cases where this guesswork is not enough. These may be addressed in the future using attribute traits, but should be fine for most trivial classes.

CONFIGURATION

Mongoose roles are role parameterized for greater flexibility.

Collection naming

You can control the collection name for an individual class this way:

    package My::Mumbo::Jumbo::Class;
    use Moose;
    with 'Mongoose::Document' => { -collection_name => 'mumbo_jumbo' };

Global collection naming stategy

By default, Mongoose will turn package names into collections this way:

    Package name          | Collection name
    ----------------------+----------------------
    Person                | person
    Humpty::Dumpty        | humpty_dumpty
    HumptyDumpty          | humpty_dumpty
    MyApp::Schema::Jumbo  | my_app_schema_jumbo

You can change this standard anytime, by setting the "Mongoose::naming" anonymous sub to something of your liking:

    # remove prefix and return
    #  a lower case collection name

    Mongoose->naming( sub{
        my $pkg = shift;
        $pkg =~ s{^MyApp::Schema::}{}g;
        return lc $pkg;
    });

Primary keys

The standard way MongoDB deals with primary keys is by using the "_id" attribute. By default, a BSON::OID is assigned to each object you commit to the database with "save".

Checkout this Devel::REPL example:

    $ re.pl
    > use Person;

    > my $hurley = Person->new(name=>'Hurley');
    $Person1 = Person=HASH(0x102099d08);

    > $hurley->dump;
    $VAR1 = bless( { 'name' => 'Hurley' }, 'Person' );

    > $hurley->save;
    4c683525a74100a8df000000

    > $hurley->dump;
    $VAR1 = bless( {
        _id  => bless( { 'oid' => '4c683525a74100a8df000000' }, 'BSON::OID' ),
        name => 'Hurley'
    }, 'Person' );

This is pretty standard MongoDB stuff.

Now, for a more control over your primary key, use the role parameter "-pk".

    package BankAccount;
    use Moose;
    with 'Mongoose::Document' => {
        -pk => [qw/ drivers_license /]
    };
    has 'drivers_license' => (is=>'rw', isa=>'Int' );

That way, updates use the "drivers_license" field and inserts will fail if the primary key exists. (But be sure to set a unique index on your primary key.)

Schema Changes

If you first had a class definition as such:

    package Author;
    use Moose; with 'Mongoose::Document';

    has 'name' => ( is => 'rw', isa => 'Str' );

Saved some objects into the DB:

    Author->new( name => 'Mike Old Schema' )->save;

Then, later on, changed it to:

    has 'first_name' => ( is => 'rw', isa => 'Str' );

When reading from the database, at expansion time, since the MongoDB document is just blessed into your class the old attribute will be loaded as if nothing had happened:

    # load old schema document using the new schema:

    my $obj = Author->find_one({ name => 'Mike Old Schema' });
    print Dump $obj;

    # prints

    --- !!perl/hash:MyTestApp::Schema::Author
    _id: !!perl/hash:BSON::OID
          oid: 4c723348a741001455000000
    name: Mike Old Schema

At this time, no "BUILD" or "BUILDARGS" methods are called, which could be used to rearrange the object into the new schema.

On the meanwhile, you can always invoke an "alignment" method after loading the data, or do a bulk migration:

    Author->find->each( sub{
        my $obj = shift;
        $obj->first_name( delete $obj->{name} );
        $obj->save;
    });

You can also check the expanded() method where you can manipulate your just expanded object.