Meerkat::Tutorial - Getting started with Meerkat


version 0.015



If you don't already have MongoDB installed and running, see the installation guide.

Check that you can connect to the test database from the mongo shell:

    $ mongo test

Creating Your Document Model

A Meerkat document is just a Moose class with the Meerkat::Role::Document role applied. This tutorial uses a simplified version of the My::Model::Person class used for testing. It has a required name attribute, and also attributes for likes and tags:

    package My::Model::Person;

    use Moose;

    with 'Meerkat::Role::Document';

    has name => (
        is       => 'ro',
        isa      => 'Str',
        required => 1,

    has likes => (
        is      => 'ro',
        isa     => 'Num',
        default => 0,

    has tags => (
        is      => 'ro',
        isa     => 'ArrayRef',
        default => sub { [] },


Meerkat::Role::Document will also add _id, _collection and _removed attributes.

Note that all attributes are read-only. You don't want to modify these directly or you'll be out of sync with the database and bad things will happen. You will update attributes using the update methods, shown later.

Connecting to the Database

Once the document class is written, using it requires a Meerkat object to manage the connection to the database:

    use Meerkat;

    my $meerkat = Meerkat->new(
        model_namespace => "My::Model",
        database_name   => "test",

By specifying a model_namespace of "My::Model", the object will return collections for classes underneath that namespace. The example above will connect to the default MongoDB on localhost. If your MongoDB is running on a different host or port, you could pass client_options which will be passed through to the MongoDB::MongoClient constructor.

Actually working with the document class requires getting a Meerkat::Collection object from the Meerkat object. A Meerkat collection associates a Perl class name with a specific MongoDB collection in the database managed by the Meerkat object.

    my $person = $meerkat->collection("Person"); # My::Model::Person

The collection name is derived from the Perl class by replacing "::" with "_". So "My::Model::Person" objects are stored in the "My_Model_Person" collection. In this tutorial example, that collection is in the "test" database on the localhost MongoDB.

Creating New Documents

A Meerkat document needs to know what collection it came from, so the Meerkat::Collection is a factory for constructing objects.

    my $obj = $person->create( name => "Larry Wall" );

When the object is created, it is immediately inserted into the database. (If an error occurs, an exception is thrown.)

Don't create objects directly from the document class. Even if you provide a Meerkat::Collection, the documents won't be inserted and things will never get in sync. Always create objects from a Meerkat::Collection.

Updating, Synchronizing and Removing Documents

With Meerkat, objects reflect the state of a document in the database at a moment in time. Therefore, you never change the state of your object directly, because it might no longer reflect the true state of the document in the database. Instead, you issue database commands to modify the document atomically in the database and then Meerkat synchronizes your object with the result.

Conveniently, Meerkat::Role::Document gives your model class some methods to make that easy:

    $obj->update_set ( name  => "Warry Lall"   ); # change a field
    $obj->update_inc ( likes => 1              ); # increment a counter
    $obj->update_push( tags  => qw/hot trendy/ ); # push to an array

If you aren't changing data, but want to update your object's snapshot of the document in the database, call the sync method:


And should you need to get rid of a document, the remove method will take care of it.


Afterwards, is_removed will be true and update and sync calls will do nothing and return a false value.


Generally, Meerkat methods return true if they executed successfully and false if they could not.

For example, if a document was removed in the database by another process and an update or sync is called, it will return false. (It will flag the object as having been removed from the database, but will not otherwise modify its data.)

Should any major error occur, an exception will get thrown.

Searching the Database

To retrieve a document from the database, the collection provides search methods similar to MongoDB::Collection, but which return objects from your model class.

For single objects, there are the find_id and find_one methods:

    # find a single object
    my $obj1 = $person->find_id( $id );
    my $obj2 = $person->find_one( { name => 'John' } );

If you have a MongoDB query for multiple objects, you pass it to find and get a Meerkat::Cursor object back. It proxies for MongoDB::Cursor but returns objects when iterated.

    my $cursor = $person->find( $query_hashref );

    while ( my $obj = $cursor->next ) { ... }

Next steps

Try out the example class above using the 'test' database on your machine.

Afterwards, check out the Meerkat::Cookbook for more on working with Meerkat.


David Golden <>


This software is Copyright (c) 2013 by David Golden.

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004