I have this idea to create workflows for actions that need their data validated before acting and I don't want to shove it all into the model.

workflow-example.rb

module Workflow
  extend ActiveSupport::Concern
  
  included do
    extend ActiveModel::Naming
    include ActiveModel::Validations
  end
  
  def call
    if valid?
      process
    end
  end
  
  class WorkflowError < StandardError
    def initialize(workflow)
      @workflow = workflow
      super
    end
  end
  
  def call!
    if valid?
      unless process
        raise WorkflowError, self
      end
    end
  end
end

class ApproveAdjustmentWorkflow
  include Workflow

  def initialize(adjustment, approver)
    @adjustment, @approver = adjustment, approver
  end
  
  validates_with :approver_must_be_supervisor
  
  def process  
    @adjustment.update signed_off_by: @approver, state: "approved"
  end

  def approver_must_be_supervisor
    if @adjustment.user.supervisor == @approver
      errors.add(:approver, "is not supervisor")
    end
  end
end

class AdjustmentController < ApplicationController
  def approve
    @adjustment = Adjustment.find(params[:id])
    approver = User.find(params.require(:signed_off_by_id))
    workflow = ApproveAdjustmentWorkflow.new(adjustment, approver)
    
    if workflow.call
      flash.notice = "Approved!"
      redirect_to adjustment_path(adjustment)
    else
      @errors = workflow.errors
      render :show
    end
  end
end