How to add a virtual attribute to a model in Ruby

问题:

I'm working on a RubyonRails/ActiveAdmin application. My RoR version is 4.2.5 and AA version is 1.0.0. I have a model Message as follows.

class Message < ActiveRecord::Base

  belongs_to :user
  validates :user, :content, presence: true

  def palindrome
    # return true/false
  end

end

As you see, I want to have a read-only attribute palindrome which only depends on the content of message. I want this attribute to be treated exactly like a normal attribute. By normal, I mean when I retrieve messages via rails console or request json format of messages, I want to see a palindrome attribute in the list. I would also like to have a filter for message by this attribute.

I'm not sure how could I achieve this.

回答1:

In your model, you can write attribute accessors (reader/writer) for your virtual attribute palindrome attribute this way:

# attr_reader
def palindrome
  self[:palindrome]
end

# attr_writer
def palindrome=(val)
  self[:palindrome] = val
end

# virtual attribute
def palindrome
  #return true/false
end

And, as you are using Rails 4, you have to whitelist palindrome attribute like any other model attribute in your strong param definition inside your controller in order to able to mass assign the value of palindrome. Something like this:

# your_controller.rb
private

def your_model_params
  params.require(:message).permit(:palindrome)
end

Take a look at this RailsCast on Virtual Attributes. Although, it's a bit old, but would be useful for concepts.

Note:

A virtual attribute will not show up in the param list automatically. But, you should be able to access it via Rails console like this: Message.new.palindrome. Also, you can expose this virtual attribute in your JSON API, for example if you are using Active Model Serializer, you can have: attribute palindrome in your MessageSerializer and then palindrome will be exposed to the JSON API.

回答2:

Ruby actually lets you create virtual attributes this way, which keeps you from having to manually create getter and setter methods:

attr_reader   :palindrome #getter
attr_writer   :palindrome #setter
attr_accessor :palindrome #both

You can also pass multiple arguments too:

attr_accessor :palindrome, :foo, :bar

The documentation for it isn't the greatest.