Think of acts_as_tree + acts_as_list, but doing what you want: root categories, subcategories, positions, individual inherited permissions and some view helpers.
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Note that there is a development branch for Rails 3. The Unit Tests pass at least, but it would still need some refactoring.

  Branch: 3.0-alpha

The Master is still for Rails > 2.3.5

  acts_as_category (Version 2.0 alpha)

Let me explain to you what I mean by acts_as_category, which is yet another acts_as plugin for Ruby on Rails ActiveRecord models. Copyright is 2009 by, released under the MIT/X11 license, which is free for all to do whatever you want with it.

acts_as_tree provides functionality for trees, but lacks some things:

  • It has no descendants method or things like ancestors_ids
  • It doesn’t validate parent_id whatsoever, which means that you can make a category a parent of itself, etc.
  • It has no caching for ancestors and descendants (you need that to output trees using <ul> and <li> efficiently)
  • It won’t help if you want certain users to see only certain nodes
  • There is no scoping, thus root.siblings will return all roots, instead of this users’ roots only.

acts_as_list is maybe not exactly what I want either:

  • It also has no validation or features to hide particular entries
  • It doesn’t support a sortable_list to reorder the tree
  • It has more than you might need, providing all these move_just_a_little_bit_higher methods
  • Last but not least, it won’t work together with acts_as_tree unless you hack around a lot with the scope code

So I came up with acts_as_category, and this is what it does:

  • It provides a structure for infinite categories and their subcategories (similar to acts_as_tree)
  • Each user can have his very own set of category trees using the :scope feature
  • It validates that no category will be the parent of its own descendant and all other variations of these foreign key things
  • You can define which hidden categories should still be permitted to the current user (through a simple class variable, thus it can easily be set per user)
  • There is a variety of instance methods such as ancestors, descendants, descendants_ids, root?, etc.
  • It has view helpers to create menus, select boxes, drag and drop ajax lists, etc. (they need refactorization, though)
  • It optionally provides sorting by a position column per hierarchy level, including administration methods that take parameters from the helpers
  • There are automatic cache columns for children, ancestors and descendants (good for fast menu output)
  • It is well commented and documented so that Rails beginners will learn from it or easily make changes
  • I18n localization for individual error messages
  • A full unit test comes along with it
  • As you can see in the test: All options (e.g. database field names) highly configurable via a simple hash

What can acts_as_category NOT do?

  • You can’t simply “turn off” the caching feature to speed up your application. If you really want to make this thing more efficient than it already is, memoize each critical function (it work’s fine, since I’m using it myself, but the unit tests will fail whenever I use memoize, that’s why it’s not published. Update: maybe I should double-check this again, maybe it works by now).
  • ActiveRecord’s “find” method won’t respect the hidden categories feature (but a somewhat alternative method called get is provided)
  • update and update_attributes must not be used to change the parent_id, because there is no validation callback
  • It can’t make you a coffee


Find a out-of-the-box demo application at (note that this demo is using version 1.0, but you get the idea).


  • Rails 2.3.5 or higher (maybe lower, as well :)


Just copy the acts_as_category directory into vendor/plugins in your Rails application.

To generate HTML documentation for all your plugins, run rake doc:plugins.
To generate it just for this plugin, go to vendor/plugins/acts_as_category and run rake rdoc.

To run the Unit Test that comes with this plugin, please read the instructions in vendor/plugins/acts_as_category/test/category_test.rb.


Including acts_as_category in your model

First of all you need a database table which looks something like this. Of course you can add arbitrary fields like name, description, etc.

  class CreateCategories < ActiveRecord::Migration
    def self.up
      create_table :categories, :force => true do |t|

        # Needed by acts_as_category
        t.integer :parent_id, :children_count, :ancestors_count, :descendants_count
        t.boolean :hidden
        # Optional
        t.string :name, :description
        t.integer :position, :pictures_count
    def self.down
      drop_table :categories

Notice that the mandatory table names above are needed by default (i.e. parent_id, children_count, ancestors_count, descendants_count, hidden). To make it work, you need to call acts_as_category in the corresponding ActiveRecord model:

  class Category < ActiveRecord::Base

If your database table has different column names, you can modify them. Note that :position and :order_by are optional. Using :order_by you can define any SQL ORDER BY statement. Default is position.

  class Category < ActiveRecord::Base
    acts_as_category :foreign_key => 'my_parent_id',
                     :hidden => 'my_hidden',
                     :children_count => 'my_children_count',
                     :ancestors_count => 'my_ancestors_count',
                     :descendants_count => 'my_descendants_count',
                     :position => 'my_position',
                     :order_by => 'title, id ASC'

You can also have associations with other models. If you have a belongs_to association, you must also provide a :scope. The scope can be a table column or even a full SQL condition.

  class Catalogue < ActiveRecord::Base
    has_many :scoped_categories
  class ScopedCategory < ActiveRecord::Base
    belongs_to :catalogue
    has_many   :pictures, :counter_cache => true
    acts_as_category :scope => :catalogue

Note that it is assumed that a tree is in the same scope by any means. I.e. Category.root.first.children will not respect the scope, but Category.roots.first.siblings will (because the roots may be in different scopes, whereas the children or a category will assumably have the same scope).

Including acts_as_category_content in your model

acts_as_category provides a function called .permitted? to find out whether a category is visible according to the current user permissions. However, you might want to have that feature for things that are inside your category, say pictures or articles. That way you could individually restrict access to these things. Just tell your content to acts_as_category_content and define the corresponding model (category is default if you leave it out). Like so:

  class Picture < ActiveRecord::Base
    acts_as_category_content, :category => 'my_category_model'

This will also validate the associations. However, it will currently not allow a category content to be in a category which has subcategories. It will be optional in future versions, just uncomment the validation in the vendor/plugins/acts_as_category/lib/active_record/acts/category_content.rb file to change this.


If everything is set up, you can actually use the plugin. Let’s say you have trees like this and your model is called Category.

  root1                   root2
   \_ child1               \_ child2
        \_ subchild1            \subchild3
        \_ subchild2                \subchild4

Then you can run the following methods. For more specific information about return values, please look at the HTML documentation generated by RDoc.

  Category.get(1)     # Returns category with id 1
  Category.get(1,5)   # Returns array of categories with ids 1 and 5
 Category.roots       # Returns an array with all permitted root categories [root1, root2]
 Category.roots!      # Same thing, but returns roots regardless of permissions (see further below)

(For the rest let’s assume, that root1 = Category.get(1), etc…)

  root1.root?         # Returns true, because root is a root category
  child1.root?        # Returns false
  child1.parent       # Returns root
  root.parent         # Returns nil, because root has no parent
  root.children       # Returns an array with [subchild1, subchild2].
  subchild1.ancestors       # Returns an array with [child1, root1]
  subchild1.ancestors_ids   # Returns the same array, but ids instead of categories [2,1]
  root1.ancestors           # Returns an empty array [], because root has none  
  root1.descendants         # Returns an array with [child1, subchild1, subchild2]
  root1.descendants_ids     # Returns the same array, but ids instead of categories [2,3,4]
  subchild1.descendants     # Returns an empty array [], because it has none
  root1.siblings                # Returns an array with all siblings [root2] 
  child1.siblings               # Returns an empty array [], because it has no siblings
  subchild1.self_and_siblings   # Returns an array [subchild1, subchild2], just like siblings, only with itself as well

Usage with permissions

Let’s bring permissions into the game. It let’s you show categories for certain users, even though the categories might be flagged “hidden”. If a category is hidden, it is practically invisible unless you have permissions.

  child1.hidden = true
  subchild1.hidden = true

Sets child1 and subchild1 to be hidden, they are now invisible to everyone

   \_ child1             (hidden)
        \_ subchild1     (hidden)
        \_ subchild2     (can't be found either, because child1 is hidden)

Your tree will look like this to the world:


Now we set permissions:

  Category.permissions = [2]    # i.e. []

Say child1 has the id 2. We just allowed the current user to see it though it’s hidden. (The idea is to set this class variable array whenever a user logs in).

Internally this is the structure of the tree:

   \_ child1            (still hidden, but you have permissions now)
        \_ subchild1    (still hidden to you)
        \_ subchild2

If you try to access it, it will look like this:

   \_ child1
        \_ subchild2
  root1.permitted?      # Returns true, because root1 is not hidden
  child1.permitted?     # Returns true, because it's hidden but you have permissions
  subchild1.permitted?  # Returns false, because it inherits "hiddenness" by child1 and you have no explicit rights for subchild1
  subchild2.permitted?  # Returns true, because it's not hidden and you have permissions for child1

Respectively, using acts_as_content you will be able to use the same function on a model which belongs_to a category:

  picture_of_child1.permitted?    # Returns the same thing as child1.permitted?

Note that you can still use Category.find(1) to override everything and get any category, regardless of it’s status. So you should never use it unless you really have to. Here is an alternative method to pick a permitted category directly:

  child1.children     # Returns only subchild1
  Category.get(4)     # Returns an empty array, trying to access forbidden subchild2

Please have a look at the comments for each function and the unit test to see, which method respects permissions and which one doesn’t (e.g. ancestors).


If you are using something, which has_many categories, like so:

  class ScopedCategory < ActiveRecord::Base
    belongs_to :catalogue
    acts_as_category :scope => :catalogue

You can easily use :scope to let acts_as_category respect that.

  ScopedCategory.roots.first.siblings        # Returns the siblings, which correspond to the same @Catalogue@.
  Catalogue.first.scoped_categories.roots    # Can be used to find all visible roots for this user.
  Catalogue.first.scoped_categories.create!  # Will create a category root in the scope of that Catalogue.

You get the idea. Please notice, that it is assumed that every tree is in one scope anyway! So children has nothing to do with scope, it simply returns the children.


Why is find not respecting hidden?

I didn’t feel comfortable overwriting the find method for Categories and it is not really needed.

Why are ancestors, ancestors_ids and self.parent not respecting hidden/permissions?

Because the whole idea of hidden is to exclude descendants of an hidden Category as well, thus the ancestors of a category you can access anyway are never going to be hidden.

Add AJAX positioning for ordering

WARNING: This is not tested on scopes yet! If you has_many :categories you might not be able to use this.

Let’s say you have a gallery and use acts_as_category on your categories. Then the categories will not be ordered by name (unless you want them to), but by a individual order. For this we have the position column. If the :position parameter refers to a non-existent column, this feature is simply disabled.

You can manually update these positions, but I strongly recommend to let this be done by the sortable_category helper and the Category.update_positions(params) method like so:

In your layout, make sure that you have all the JavaScripts included, that will allow drag and drop with, etc. For the beginning, let’s just add all:

  <%= javascript_include_tag :all %>

Then, in your view, you can call this little helper to generate a drag and drop list where you can re-sort the positions. Remember to provide the name of the model to use:

  <%= aac_sortable_tree Category %>

Finally, in your controller create an action method like this:

  def update_positions
    render :nothing => true

And you can already try it. You can change the URL to that action method like this:

  <%= aac_sortable_tree(Category, {:action => :update_positions}) %>
  <%= aac_sortable_tree(Category, {:controller => :mycontroller, :action => :update_positions}) %>

Ask questions and have fun!

Feel free to add your comments and don’t forget about the demo application.