Introduce Parameter Object

This is a technique to reduce the number of input parameters to a method.

To introduce a Parameter Object:

• Pick a name for the object that represents the grouped parameters.
• Replace the method’s grouped parameters with the object.

Uses

• Removes long parameter lists.
• Groups parameters that naturally fit together.
• Encapsulates behavior between related parameters.

Example

Let’s take a look at the example from Long Parameter List and improve it by grouping the related parameters into an object:

# app/mailers/mailer.rb
class Mailer < ActionMailer::Base
  default from: "from@example.com"

  def completion_notification(first_name, last_name, email)
    @first_name = first_name
    @last_name = last_name

    mail(
      to: email,
      subject: 'Thank you for completing the survey'
    )
  end
end

# app/views/mailer/completion_notification.html.erb
<%= @first_name %> <%= @last_name %>

By introducing the new parameter object recipient we can naturally group the attributes first_name, last_name, and email together.

` app/models/recipient.rb

# app/mailers/mailer.rb
class Mailer < ActionMailer::Base
  default from: "from@example.com"

  def completion_notification(recipient)
    @recipient = recipient

    mail(
      to: recipient.email,
      subject: 'Thank you for completing the survey'
    )
  end
end

Notice that we’ve also created a new full_name method on the recipient object to encapsulate behavior between the first_name and last_name.

# app/views/mailer/completion_notification.html.erb
<%= @recipient.full_name %>

Next Steps

• Check to see if the same data clump exists elsewhere in the application and reuse the parameter object to group them together.
• Verify the methods using the parameter object don’t have feature envy.