task_completion.rb

class TaskCompletion
  include ActiveModel::Model
  attr_accessor :task, :complete
  
  validates :complete, inclusion: { in: [true, false] }

  def self.for_task(task, params={})
    new(params).tap do |completion|
      completion.task = task
    end
  end

  def save
    return true if task.complete?
    
    if valid?
      completion_detail.update_attributes({
        some_var: 'OK'  
      })
      task.update_attributes(complete: complete)
    else
      @errors = completion_detail.errors
      false
    end
  end
  
  def valid?
    completion_detail.valid?
  end
  
  private
  
  def completion_detail
    task.completion_detail
  end
end