Super Powers for ActiveRecord::Relation

May 31, 20241 minute readprogramming rails

Whenever you query your database with Rails, what you get in return is an An ActiveRecord::Relation; a chainable object that is lazily evaluated against the database only when the actual records are needed. Usually, you can chain other class methods and scopes to this ActiveRecord::Relation. For instance, let's say you have a Post model with a featured scope.

Post.where(topic: "animals").featured

This is good, but it only applies to the Post model, where the class method or scope is defined.

There is a way, however, to add other chainable methods that can be shared by all models with the clever use of a concern. Let's say you want to add a way to generate a CSV string out of an ActiveRecord::Relation. Then you could simply chain a method, such as to_csv to any query and get a CSV string representation of all the rows, using the name of the returned columns as headers.

Post.featured.select(:author_name, :topic).limit(2).to_csv 

  => "author_name,topic\nJohn Mountains,Nature\nAlma Aqua,Sea Levels\n"

Books.where(subject: "programming").select(:name, :publisher).limit(2).to_csv

  => "name,publisher\nSecure APIs,Manning\nEffective Go Recipes,Pragmatic Bookshelf\n"

To achieve this you could add a concern such as this one:

require "csv"

module CSVPrintable
  extend ActiveSupport::Concern

  class_methods do
    class CSVHeadersMismatchError < StandardError; end

    def to_csv(headers: nil)
      return "" unless all.any?

      attribs = all.first.attributes.reject { |k, v| k == "id" && v.nil? }
      column_names = attribs.keys

      raise CSVHeadersMismatchError, "Headers should match column numbers in query" if headers.present? && headers.length != column_names.length

      CSV.generate(headers: headers || column_names, write_headers: true) do |csv|
        all.each do |re|
          csv << re.attributes.values_at(*column_names)
        end
      end
    end
  end
end

Then, to make it available to all your models, simply add it to ApplicationRecord, from which all your models inherit from.

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  include CSVPrintable
end