Implement a derived attribute in Rails 7

I am trying to implement a derived field in Rails 7. I have a Tag model. A tag has a prefix, a loop number, and an optional suffix. (Yes, I'm an instrument engineer :-) I keep these attributes separately in the database, because it is required to sort and search on them individually, but most of the time the tag is referenced and displayed as the combination of these fields. I defined an attribute accessor, and a setter function in private, to join the fields, and I set the after_find helper to trigger the setter. But they don't seem to work. In the console, Tag.first.full_tag results in nil, and nothing is displayed in views.

I would have thought this requirement is relatively common, but haven't found anything useful in my searches. Of course, I could probably persist the derived attribute into the database, but that would contravene the basic rule of "one source of information". I would greatly appreciate if someone can point out where I am going wrong.

# tag.rb:
class Tag < ApplicationRecord
  belongs_to :project
  belongs_to :discipline, foreign_key: :discipline, primary_key: :code

  attr_reader :full_tag
  after_find :set_full_tag

...

 private

    def set_full_tag
      full_tag = prefix + "-" + loop.to_s.rjust(4, '0')
      if !suffix.empty?
        full_tag += "." + suffix
      end
    end
end

Solution

So, let's build an example here:

Proposed Code:

class Tag < ApplicationRecord
  # attr_accessor for the derived full_tag
  attr_accessor :full_tag
  
  # Set the full_tag after finding the record
  after_find :set_full_tag

  private

  def set_full_tag
    # Set the full_tag on the instance using self.full_tag
    self.full_tag = "#{prefix}-#{self.loop_number.to_s.rjust(4, '0')}"
    
    # Add suffix if it's present
    self.full_tag += ".#{suffix}" if suffix.present?
  end
end

The main issue (as I see it) with the original code is that it does not correctly set the full_tag value. To fix this, we first replace attr_reader with attr_accessor. Why? attr_reader only lets you read the value, while attr_accessor lets you both read and write it. As I understand it, you want to change the full_tag value after finding the record.

Next, full_tag = ... creates a temporary variable, which doesn't store the combined tag in the model. We can use self.full_tag = ..., to tell Rails to use the setter method generated by attr_accessor, so that the full_tag value is properly updated and saved in the object.

I hope this points you in the right direction.

Test Framework:

rails new tag_proj
cd tag_proj
rails generate model Tag prefix:string loop_number:integer suffix:string
rails db:migrate

echo "
class Tag < ApplicationRecord
  attr_accessor :full_tag
  after_find :set_full_tag

  private

  def set_full_tag
    self.full_tag = \"#{prefix}-#{self.loop_number.to_s.rjust(4, '0')}\"
    self.full_tag += \".#{suffix}\" if suffix.present?
  end
end
" > app/models/tag.rb

echo "
Tag.create(prefix: 'XX', loop_number: 123, suffix: 'A')
Tag.create(prefix: 'YY', loop_number: 45, suffix: 'B')
Tag.create(prefix: 'ZZ', loop_number: 67, suffix: nil)
" >> db/seeds.rb

rails db:seed

echo "
# lib/tasks/tag_full_tag.rake
namespace :tag do
  task full_tag: :environment do
    puts Tag.first.full_tag   
    puts Tag.second.full_tag
    puts Tag.third.full_tag
  end
end
" > lib/tasks/tag_full_tag.rake

rake tag:full_tag

Output

> rake tag:full_tag
XX-0123.A
YY-0045.B
ZZ-0067