Imperator is a small gem to help with command objects. The command pattern is a design pattern used to encapsulate all of the information needed to execute a method or process at a point in time. In a web application, commands are typically used to delay execution of a method from the request cycle to a background processor.
Why use commands?
The problem with controllers in Rails is that they're a part of the web domain—their job is to respond to requests, and ONLY to respond to requests. Anything that happens between the receipt of a request and sending a response is Somebody Else's Job™. Commands are that Somebody Else™. Commands are also very commonly utilized to put work into the background.
Why are commands an appropriate place to handle that logic? Commands give you the opportunity to encapsulate all of the logic required for an interaction in one spot. Sometimes that interaction is as simple as a method call—more often there are several method calls involved, not all of which deal with domain logic (and thus, are inappropriate for inclusion on the models). Commands give you a place to bring all of these together in one spot without increasing coupling in your controllers or models.
Commands can also be regarded as the contexts from DCI.
Commands also give you an appropriate place to handle interaction validation. Validation is most often regarded as a responsibility of the data model. This is a poor fit, because the idea of what's valid for data is very temporally tied to the understanding of the business domain at the time the data was created. Data that's valid today may well be invalid tomorrow, and when that happens you're going to run into a situation where your ActiveRecord models will refuse to work with old data that is no longer valid. Commands don't absolve you of the need to migrate your data when business requirements change, but they do let you move validation to the interaction where it belongs.
Commands can also be used on forms in place of ActiveRecord models, when the ActiveModel::Naming interface is included in the command class. Imperator doesn't have a dependency on ActiveRecord/ActiveModel/ActiveSupport, but readily supports most of the AM interfaces like validations. These are trivial to include so support is not built into Imperator intentionally.
- test coverage—Imperator was extracted out of some other work, and coverage was a part of those test suites.
- Ensure compatibility with DJ, Resque and Sidekiq
- Virtus (https://github.com/solnic/virtus)
In your Gemfile:
Creating the command:
class DoSomethingCommand < Imperator::Command include ActiveModel::Validations attribute :some_object_id attribute :some_value validates_presence_of :some_object_id action do obj = SomeObject.find(self.some_object_id) obj.do_something(self.some_value) end end
Using a command on a form builder
First, you'll need to make your commands adhere to ActiveModel's naming interface:
In your Gemfile:
Your Command class:
class DoSomethingCommand < Imperator::Command include ActiveModel::Naming #... end
Then you can use them on a form just as you would a model:
<%= form_for(@command, :as => :do_something, :url => some_resource_path(@command.some_object_id), :method => :put) do |f| %> ... <% end %>
Using a command
class SomeController < ApplicationController def update command = DoSomethingCommand.new(params[:do_something]) if command.valid? command.perform redirect_to root_url else render edit_some_resource_path(command.some_object_id) end end end
Using a command in the background (Delayed::Job)
Using a background processor
You can create a custom background processor very easily. It merely
needs to implement the class method
class Imperator::NullBackgroundProcessor def self.commit(command, options = nil) command.perform end end
You can also pass options to the background processor either by the command or instance
class CompletePurchase < Imperator::Command background :queue => "high" def action end end command = CompletePurchase.new command.commit(:queue => "low")
Many thanks to the following contributors for bugfixes, testing, and additional functionality
- Jason Staten (@statianzo)
- Jay Adkisson (@jayferd)
- Nick Maloney (@nmaloney)
- Grégory Horion (@gregory)