title: Check for existing association 'belongs_to' in Rails 4
body: <p>I have the following model:</p>
  
  <pre><code>class Order &lt; ActiveRecord::Base
    validates :user, :order_status, :delivery_type, presence: true
    belongs_to :user
  end
  </code></pre>
  
  <p>Before creating a new order I need to check if User with user_id from params exists; I can get   any user_id, and it may be for nonexistent User. Is it possible to check existing through model validations or I must do it manually? Thanks. </p>