First try with ternary associations with ActiveRecord

test.rb

require 'active_record'
require 'sqlite3'

ActiveRecord::Base.establish_connection(
  :adapter => 'sqlite3',
  :database => ':memory:'
)

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.column :name, :string
  end

  create_table :tags do |t|
    t.column :name, :string
  end

  create_table :favs do |t|
    t.column :url, :string
  end

  create_table :user_fav_tags do |t|
    t.column :user_id, :integer
    t.column :tag_id, :integer
    t.column :fav_id, :integer
  end
end

class UserFavTag < ActiveRecord::Base
  class << self
    def add_fav_with_tags(user, url, tag_names)
      fav = Fav.find_or_create_by_url(:url => url)
      tag_names.each do |name|
        tag = Tag.find_or_create_by_name(name)
        self.create(:user_id => user.id, :tag_id => tag.id, :fav_id => fav.id)
      end
    end
  end

  belongs_to :user
  belongs_to :fav
  belongs_to :tag
end

class User < ActiveRecord::Base
  has_many :user_fav_tags
  has_many :tags, :through => :user_fav_tags 
  has_many :favs, :through => :user_fav_tags 

  def add_fav_with_tags(url, tags)
    UserFavTag.add_fav_with_tags(self, url, tags)
  end
end

class Tag < ActiveRecord::Base
  has_many :user_fav_tags
  has_many :users, :through => :user_fav_tags 
  has_many :favs, :through => :user_fav_tags 
end

class Fav < ActiveRecord::Base
  has_many :user_fav_tags
  has_many :users, :through => :user_fav_tags 
  has_many :tags, :through => :user_fav_tags 
end

u = User.create(:name => 'pepito')
u.add_fav_with_tags('http://www.google.com', ['tag1', 'tag2'])
u.add_fav_with_tags('http://www.yahoo.com', ['tag2', 'tag3'])

# Output
#
# ruby-1.9.2-p290 :001 > load 'test.rb'
# ruby-1.9.2-p290 :002 > u = User.first
#  => #<User id: 1, name: "pepito"> 

# Duplicados! Los id's aparecen varias veces en user_fav_tags

# ruby-1.9.2-p290 :003 > u.tags
#  => [#<Tag id: 1, name: "tag1">, #<Tag id: 2, name: "tag2">, #<Tag id: 2, name: "tag2">, #<Tag id: 3, name: "tag3">] 
# ruby-1.9.2-p290 :004 > u.favs
#  => [#<Fav id: 1, url: "http://www.google.com">, #<Fav id: 1, url: "http://www.google.com">, #<Fav id: 2, url: "http://www.yahoo.com">, #<Fav id: 2, url: "http://www.yahoo.com">] 

# Para filtrarlos:

# ruby-1.9.2-p290 :006 > u.tags.find(:all, :group => 'id')
#  => [#<Tag id: 1, name: "tag1">, #<Tag id: 2, name: "tag2">, #<Tag id: 3, name: "tag3">] 
# ruby-1.9.2-p290 :007 > u.favs.find(:all, :group => 'id')
#  => [#<Fav id: 1, url: "http://www.google.com">, #<Fav id: 2, url: "http://www.yahoo.com">]

# O también (no se cuál será más rápido):

# ruby-1.9.2-p290 :008 > u.tags.uniq
#  => [#<Tag id: 1, name: "tag1">, #<Tag id: 2, name: "tag2">, #<Tag id: 3, name: "tag3">] 
# ruby-1.9.2-p290 :009 > u.favs.uniq
#  => [#<Fav id: 1, url: "http://www.google.com">, #<Fav id: 2, url: "http://www.yahoo.com">]