Validations

Sequent uses ActiveModel::Validations for validating things like Commands and ValueObjects.

For an in-depth explanation of all available ActiveModel validations, please check out the Active Record Validations Guide.

Command validations

Commands are executed to get things done in Sequent. In a typical web application they are bound to an HTML form. ActiveModel::Validations can be used to validate the input values in the Command.

For example:

<form action="/create" method="post">
  <input type="text" name="first_name">
  <input type="text" name="last_name">
  <button>Save</button>
</form>

class CreateUser < Sequent::Command
  attrs first_name: String, last_name: String
  validates :first_name, presence: true, length: {minimum: 3, maximum: 100}
  validates :last_name, presence: true
end

post '/create' do
  Sequent.command_service.execute_commands(
    CreateUser.new(
      params.slice(:first_name, :last_name).merge(
        aggregate_id: Sequent.new_uuid
      )
    )
  )
end

The CommandService validates all Commands that are executed by calling valid?. If a Command is not valid it raises a Sequent::Core::CommandNotValid error. The Sequent::Core::CommandNotValid has a reference to the Command which in turn, since it is an ActiveModel object, has a reference to the ActiveModel::Errors. You can use this error to, for instance, display the errors at their corresponding html form input fields. Unfortunately, at the time of writing, the Rails Form Helpers only support ActiveRecord objects and not ActiveModel objects. Therefore form binding should be done manually.

post '/create' do
  Sequent.command_service.execute_commands(
    CreateUser.new(
      params.slice(:first_name, :last_name).merge(
        aggregate_id: Sequent.new_uuid
      )
    )
  )
  redirect '/list'
rescue Sequent::Core::CommandNotValid => e
  @command = e.command
  erb :new
end

It is recommended to manually handle the Sequent::Core::CommandNotValid error, since any registered synchronous Workflow executing Commands can also raise a Sequent::Core::CommandNotValid.

More complex validations should be done in the AggregateRoot.

Aggregate Root validations

Some validations can not be performed in the Command since they are dependant on the current state of the AggregateRoot. Sequent does not provide this functionality out of the box, since it is highly dependant on your domain - Sequent is not a web framework trying to provide form binding.

Example:

class QueueInvoiceForSending < Sequent::Command; end
class InvoiceAlreadyQueued < My::DomainError; end

class Invoice < Sequent::AggregateRoot
  def queue_for_sending
    fail InvoiceAlreadyQueued if @queued
    apply InvoiceQueuedForSending 
  end
  
  on InvoiceQueuedForSending do
    @queued = true
  end
end

In the above example the Command QueueInvoiceForSending has no validations, so executing it twice will have no effect on the validity of the Command. Only the AggregateRoot can know if this operation is allowed or not. In your web application you can then rescue from all My::DomainErrors and show the user an appropriate error message.