Mongoid Taggable provides some helpers to create taggable documents.

You can simply install from rubygems:

gem install mongoid_taggable

or in Gemfile:

gem 'mongoid_taggable'

or as a Rails Plugin:

script/plugin install git://github.com/wilkerlucio/mongoid_taggable.git

To make a document taggable you need to include Mongoid::Taggable into your document and call the taggable macro with optional arguments:

class Post
  include Mongoid::Document
  include Mongoid::Taggable
  
  field :title
  field :content

  taggable
end

Then in your form, for example:

<% form_for @post do |f| %>
  <p>
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </p>
  <p>
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </p>
  <p>
    <%= f.label :tags %><br />
    <%= text_field_tag 'post[tags]', (@post.tags.join(', ') if @post.tags) %>
  </p>
  <p>
    <button type="submit">Send</button>
  </p>
<% end %>

You can of course use helpers or a FormBuilder extension to express this in a prettier way. In this case, the text field for tags should receive the list of tags separated by comma (below in this document you will see how to change the separator).

Your document will have a custom tags= setter which can accept either an ordinary Array or this separator-delineated String.

This lib can automatically create an aggregate collection of tags and their counts for you, updated as documents are saved. This is useful for getting a list of all tags used in documents of this collection or to create a tag cloud. This is disabled by default for sake of performance where it is unneeded — see the following example to understand how to use it:

class Post
  include Mongoid::Document
  include Mongoid::Taggable

  field :title
  field :content

  taggable :aggregation => true
end

Post.create!(:tags => "food,ant,bee")
Post.create!(:tags => "juice,food,bee,zip")
Post.create!(:tags => "honey,strip,food")

Post.tags # will retrieve ["ant", "bee", "food", "honey", "juice", "strip", "zip"]
Post.tags_with_weight # will retrieve:
# [
#   ['ant', 1],
#   ['bee', 2],
#   ['food', 3],
#   ['honey', 1],
#   ['juice', 1],
#   ['strip', 1],
#   ['zip', 1]
# ]

To change the default separator you may pass a separator argument to the macro:

class Post
  include Mongoid::Document
  include Mongoid::Taggable

  field :title
  field :content

  taggable :separator => ' '    # tags will be delineated by spaces
end