model.rb

require 'rubygems'
require 'mongo'

module DatabaseConnector
  extend self

  def connection
    @@connection ||= Mongo::Connection.new
  end

  def database=(name)
    @@database = name
  end

  def database
    @@database ||= 'test'
  end

  def handle
    connection[self.database]
  end
end

class MongoModel
  def collection
    DatabaseConnector.handle[self.class.name.downcase]
  end
end

class Foo < MongoModel
end

class Bar < MongoModel
end

class Baz < MongoModel
end

foo = Foo.new
p foo.collection.save({:a => 1})
p foo.collection.find_one

baz = Baz.new
p baz.collection.save({:a => 1})
p baz.collection.find_one

bar = Bar.new
p bar.collection.save({:b => 1})
p bar.collection.find_one