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.
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']
end
end
The assigned value is checked during validation:
Song.new(:genre => 'rock').valid? # => true
Song.new(:genre => 'elephant').valid? # => false
The validation error message is the same as the one from validates_inclusion_of (errors.messages.inclusion in your I18n dictionary).
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:
form.select :genre, form.object.assignable_genres
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:
en:
assignable_values:
song:
genre:
pop: 'Pop music'
rock: 'Rock music'
electronic: 'Electronic music'
You can access the humanized version for the current value like this:
song = Song.new(:genre => '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
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']
end
end
The default is applied to new records:
Song.new.genre # => 'rock'
Defaults can be lambdas:
class Song < ActiveRecord::Base
assignable_values_for :genre, :default => lambda { Date.today.year } do
1980 .. 2011
end
end
The lambda will be evaluated in the context of the record instance.
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
(Date.today.year - 2) .. Date.today.year
end
end
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.save!
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.
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)
end
end
Listing and validating als works the same:
chicane = Artist.create!(:name => 'Chicane', :signed => true)
lt2 = Artist.create!(:name => 'LT2', :signed => false)
song = Song.new
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.
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
genres
end
end
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 }
end
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
end
def assignable_story_states(story)
states = ['draft', 'pending']
states << 'accepted' if @role == :admin
states
end
end
Listing and validating works the same with delegation:
story = Story.new(:state => 'accepted')
Power.current = Power.new(:guest)
story.assignable_states # => ['draft', 'pending']
story.valid? # => false
Power.current = Power.new(:admin)
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 = Story.new(:state => '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
end
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
Power.new(current_user)
end
end
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
end
This is a shortcut for saying:
class Story < ActiveRecord::Base
assignable_values_for :state, :through => lambda { Power.current }
end
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_rootif 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.