Tagger

Installing

Install it with gem install tagger
Generate the migration files with rails generate tagger:install.
Finally, run the migrations with rake db:migrate.

Usage

Add the method call taggable to your model.

class Link < ActiveRecord::Base
  taggable :scope => :category
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :links
end

@post = Post.first
@category = Category.first

# set tags using the chosen tag separator
@post.tagged_with = "rails, activerecord, models"

# get tags joined with the chosen tag separator
@post.tagged_with

# get all tags as objects
@post.tags

# check if an user tagged this object
@post.tag_owner?(current_user)

# generate a tag cloud
@tags = Tag.cloud(:link, :limit => 100, :scope => @category)

# at the view, call the helper tag_cloud
tag_cloud @tags do |tag_name, css|
  <%= link_to tag_name, tag_path(tag_name), :class => css
end