Restrict the values assignable to ActiveRecord attributes or associations.
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assignable_values - Enums on vitamins

assignable_values lets you restrict the values that can be assigned to attributes or associations of ActiveRecord models. You can think of it as enums where the list of allowed values is generated at runtime and the value is checked during validation.

We carefully enhanced the cure enum functionality with small tweaks that are useful for web forms, internationalized applications and common authorization patterns.

Restricting scalar attributes

The basic usage to restrict the values assignable to strings, integers, etc. is this:

class Song < ActiveRecord::Base
  assignable_values_for :genre do
    ['pop', 'rock', 'electronic']

The assigned value is checked during validation: => 'rock').valid?     # => true => 'elephant').valid? # => false

The validation error message is the same as the one from validates_inclusion_of (errors.messages.inclusion in your I18n dictionary).

Listing assignable values

You can ask a record for a list of values that can be assigned to an attribute:

song.assignable_genres # => ['pop', 'rock', 'electronic']

This is useful for populating <select> tags in web forms: :genre, form.object.assignable_genres

Humanized labels

You will often want to present internal values in a humanized form. E.g. "pop" should be presented as "Pop music".

You can define human labels in your I18n dictionary:

        pop: 'Pop music'
        rock: 'Rock music'
        electronic: 'Electronic music'

You can access the humanized version for the current value like this:

song = => 'pop')
song.humanized_genre # => 'Pop music'

When obtaining a list of assignable values, each value will have a method #humanized that returns the translation:

song.assignable_genres.first           # => "pop"
song.assignable_genres.first.humanized # => "Pop music"

You can populate a <select> tag with pairs of internal values and human labels like this:

form.collection_select :genre, form.object.assignable_genres, :to_s, :humanized

Defining default values

You can define a default value by using the :default option:

class Song < ActiveRecord::Base
  assignable_values_for :genre, :default => 'rock' do
    ['pop', 'rock', 'electronic']

The default is applied to new records: # => 'rock'

Defaults can be lambdas:

class Song < ActiveRecord::Base
  assignable_values_for :genre, :default => lambda { } do
    1980 .. 2011

The lambda will be evaluated in the context of the record instance.

Values are only validated when they change

Values are only validated when they change. This is useful when the list of assignable values can change during runtime:

class Song < ActiveRecord::Base
  assignable_values_for :year do
    ( - 2) ..

If a value has been saved before, it will remain valid, even if it is no longer assignable:

Song.update_all(:year => 1985) # update all records with a value that is no longer valid
song = Song.last
song.year # => 1985
song.valid?  # => true

It will also be returned when obtaining the list of assignable values:

song.assignable_genres # => [2010, 2011, 2012, 1985]

Once a changed value has been saved, the previous value disappears from the list of assignable values:

song.genre = 'pop'!
song.assignable_years # => [2010, 2011, 2012]
song.year = 1985
song.valid? # => false

This is to prevent records from becoming invalid as the list of assignable values evolves. This also prevents <select> menus with blank selections when opening an old record in a web form.

Restricting belongs_to associations

You can restrict belongs_to associations in the same manner as scalar attributes:

class Song

  belongs_to :artist

  assignable_values_for :artist do
    Artist.where(:signed => true)


Listing and validating als works the same:

chicane = Artist.create!(:name => 'Chicane', :signed => true)
lt2 = Artist.create!(:name => 'LT2', :signed => false)

song =

song.assignable_artists # => [#<Artist id: 1, name: "Chicane">]

song.artist = chicane
song.valid? # => true

song.artist = lt2
song.valid? # => false

Similiar to scalar attributes, associations are only validated when the foreign key (artist_id in the example) changes. Previously saved values will remain assignable until another association has been saved.

How assignable values are evaluated

The list of assignable values is generated at runtime. Since the given block is evaluated on the record instance, so you can refer to other methods:

class Song < ActiveRecord::Base

  validates_numericality_of :year

  assignable_values_for :genre do
    genres = []
    genres << 'jazz' if year > 1900
    genres << 'rock' if year > 1960


Obtaining assignable values from another source

The list of assignable values can be provided by any object that is accessible from your model. This is useful for authorization scenarios like Consul or CanCan, where permissions are defined in a single class.

You can define the source of assignable values by setting the :through option to a lambda:

class Story < ActiveRecord::Base
  assignable_values_for :state, :through => lambda { Power.current }

Power.current must now respond to a method assignable_story_states or assignable_story_states(story) which returns an Enumerable of state strings:

class Power

  cattr_accessor :current

  def initialize(role)
    @role = role

  def assignable_story_states(story)
    states = ['draft', 'pending']
    states << 'accepted' if @role == :admin


Listing and validating works the same with delegation:

story = => 'accepted')

Power.current =
story.assignable_states # => ['draft', 'pending']
story.valid? # => false

Power.current =
story.assignable_states # => ['draft', 'pending', 'accepted']
story.valid? # => true

Note that delegated validation is skipped when the delegate is nil. This way your model remains usable when there is no authorization context, like in batch processes or the console:

story = => 'foo')
Power.current = nil
story.valid? # => true

Think of this as enabling an optional authorization layer on top of your model validations, which can be switched on or off depending on the current context.

Instead of a lambda you can also use the :through option to name an instance method:

class Story < ActiveRecord::Base
  attr_accessor :power
  assignable_values_for :state, :through => :power

Obtaining assignable values from a Consul power

A common use case for the :through option is when there is some globally accessible object that knows about permissions for the current request. In practice you will find that it requires some effort to make sure such an object is properly instantiated and accessible.

If you are using Consul, you will get a lot of this plumbing for free. Consul gives you a macro current_power to instantiate a so called "power", which describes what the current user may access:

class ApplicationController < ActionController::Base
  include Consul::Controller

  current_power do


The code above will provide you with a helper method current_power for your controller and views. Everywhere else, you can simply access it from Power.current.

You can now delegate validation of assignable values to the current power by saying:

 class Story < ActiveRecord::Base
   authorize_values_for :state

This is a shortcut for saying:

 class Story < ActiveRecord::Base
   assignable_values_for :state, :through => lambda { Power.current }

Head over to the Consul README for details.


Put this into your Gemfile:

gem 'assignable_values'

Now run bundle install and restart your server. Done.


  • Fork the repository.
  • Push your changes with specs. There is a Rails 3 test application in spec/app_root if you need to test integration with a live Rails app.
  • Send me a pull request.

I'm very eager to keep this gem leightweight and on topic. If you're unsure whether a change would make it into the gem, talk to me beforehand.


Henning Koch from makandra.