Backend Pattern Delegation

[Pattern] Delegation

Description

Delegation is a pattern that reduces code repetition by allowing an object to pass calls for a method to another object, the delegate.

Location

N/A

Best Practices

There are a few common ways to delegate method calls that we use in Caseflow:

The delegate method

Rails provides a delegate method that allows you to delegate methods to a related object. This pattern is commonly used in our models to call methods on related objects and avoid declaring redundant methods.

class Hearing < ApplicationRecord
  ...
  delegate :available_hearing_locations, :closest_regional_office, :advanced_on_docket?, to: :appeal
  delegate :external_id, to: :appeal, prefix: true
  ...
end

The prefix parameter can be used to prefix the new method name to avoid method naming collisions. ¹

The SimpleDelegator class

The SimpleDelegator class is an easy way to delegate all methods of an object to a second object. Using SimpleDelegator is a good way to add additional functionality onto a class without adding to the class itself.

Tradeoffs

delegate is preferred to SimpleDelegator when you only need to delegate a subset of methods to the second object.

Resources

Examples in Caseflow

• PR to use SimpleDelegator for RequestIssueClosure class

Additional Reading

• 1: https://apidock.com/rails/Module/delegate
• https://ruby-doc.org/stdlib-2.5.1/libdoc/delegate/rdoc/Delegator.html
• https://ruby-doc.org/stdlib-2.5.1/libdoc/delegate/rdoc/SimpleDelegator.html

Related Patterns

Pattern	Description