title: activeadmin habtm better example for uniqueness case
body: <p>Am able to manage habtm as per follow, and I wanted a better way for this</p>
  
  <p>I have habtm between User and Tag on Rails 3, aa 0.5.1
  Tag name is uniq</p>
  
  <pre><code>  f.input :tags, :label =&gt; 'Assign existing tag'                             
    # this above allows to select from existing tags, but cannot allow to create one
  
    f.has_many :tags, :label =&gt; 'Add new tags, modify existings' do |ff|
      ff.input :name
      ff.input :_destroy, :as =&gt; :boolean
    end
    # this above allows to create new one but not allow to specify existing one
    # if we specify existing one, uniqueness wont let create this one, neither existing get used
    # and throws validation error
  </code></pre>
  
  <p>any hints?</p>
  
  <p>Adding my models</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_and_belongs_to_many :tags
    scope :tagged_with, lambda {|tags| joins(:tags).where("tags.name" =&gt; tags)}
    accepts_nested_attributes_for :tags, :allow_destroy =&gt; true
  end
  
  class Tag &lt; ActiveRecord::Base
    has_and_belongs_to_many :users
    validates :name, :uniqueness =&gt; { :case_sensitive =&gt; false }
  end
  </code></pre>