This code uses dm-core from snusnu's active_support branch and dm-mongo-adapter from master

mysql_and_mongodb_example.rb

require 'rubygems'
require 'dm-core'

DataMapper.setup(:default, "mysql://localhost/examples")
DataMapper.setup(:logs,    "mongo://localhost/examples")

class LoggedEvent
  include DataMapper::Mongo::Resource

  def self.default_repository_name; :logs; end

  property :id,     ObjectID
  property :action, String

  belongs_to :user

  all.destroy!
end

class User
  include DataMapper::Resource

  property :id,   Serial
  property :name, String

  has n, :logged_events, :repository => :logs

  auto_migrate!
end

user = User.create(:name => 'jola')
user.logged_events.create(:action => 'bazinga!')
user.logged_events.create(:action => 'foo')
user.logged_events.create(:action => 'bar')

puts '-' * 80

p user.logged_events(:action => /foo|bar/)
p LoggedEvent.all(:user => user, :action => ['bazinga!', 'foo'])

output.txt

--------------------------------------------------------------------------------
[#<LoggedEvent @id="4b8e441825de443108000002" @action="foo" @user_id=1>, #<LoggedEvent @id="4b8e441825de443108000003" @action="bar" @user_id=1>]
[#<LoggedEvent @id="4b8e441825de443108000001" @action="bazinga!" @user_id=1>, #<LoggedEvent @id="4b8e441825de443108000002" @action="foo" @user_id=1>]