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Create the illusion that an object has specific attributes when those attributes actually belong to an associated object.

This is particularly useful for different classes within a single- table inheritance table to have access to separate fields in class-specific associations.


Add this line to your application's Gemfile:

gem "external_fields"

And then execute:

$ bundle

Or install it yourself as:

$ gem install external_fields


Include ExternalFields and define the external fields using the external_field method. For example, if grade_level, age and credits are defined in another class StudentData and you want to access them in Student you could do:

require "active_record"
require "active_support"

require "external_fields"

class Student < ActiveRecord::Base
  include ExternalFields

  has_one :data,
          class_name: StudentData

  external_field :grade_level,             # External attribute 1
                 :age,                     # External attribute 2
                 :credits,                 # External attribute 3
                 :data,                    # Name of the association
                 class_name: "StudentData" # Class name of association

where the external fields are defined in another associated class:

class StudentData < ActiveRecord::Base
  attr_accessor :grade_level, :age, :credits

Now you can directly call the accessors on the Student objects:

 > s = Student.create!
 > s.age
=> nil

 > s.age = 10
 > s.age
=> 10

 > s.grade_level = 4
 > s.grade_level
=> 4

Overriding default behavior using underscored accessors

You can also add underscored accessors using the underscore flag

  external_field :grade_level,             # External attribute 1
                 :age,                     # External attribute 2
                 :credits,                 # External attribute 3
                 :data,                    # Name of the association
                 class_name: "StudentData" # Class name of association
                 underscore: true          # Flag for underscored accessors

This will allow you to use the external fields using underscored methods:

s = Student.create!

This approach lets you override the default behavior cleanly. For example, you could override the grade level using this method:

def grade_level
  if _grade_level == 0

Accessing the original association

In some instances it's helpful to be able to use the original association without building an object on access. For instance, you might want to have a validation inspect a value without creating a new object on each save. In that case, you can use the use_original flag on the association like so:

validate :kindergarten_students_have_names

def kindergarten_students_have_names
  data_obj = data(use_original: true)

  if data_obj && grade_level == "Kindergarten" && name.blank?
    # Note that `name` is an attribute on `Student` but `grade_level`
    # is accessed through the `data` association as defined earlier
    # in the README.
    errors.add(:name, "must be present for kindergarten students")


We have documentation on RubyDoc.


  1. Fork it (
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request

Make sure your changes have appropriate tests (bundle exec rspec) and conform to the Rubocop style specified. We use overcommit to enforce good code.


ExternalFields is released under the MIT License.