How to define a named scope in a habtm relation

habtm_scope.rb

# First model: tag.rb
# note that pomodori is a custom plural for pomodoro

class Tag < ActiveRecord::Base
  has_and_belongs_to_many :pomodori
end

# Second model: pomodoro.rb
# here is how to define a Rails 3 scope through the join table:

class Pomodoro < ActiveRecord::Base
  has_and_belongs_to_many :tags

  scope :by_tag, lambda { |tag_text|
     joins("join pomodori_tags, tags").
     where('pomodori.id = pomodori_tags.pomodoro_id 
            AND pomodori_tags.tag_id = tags.id 
            AND tags.text = ?', tag_text) 
  }
end