title: active model - double belongs_to but just one at a time
body: <p>this is my scenario: One model called "callgroup" and a model called "payments"</p>
  
  <pre><code>class Callgroup &lt; ApplicationRecord
    has_many :payments
  end
  
  
  -------------------
  
  
  class Payment &lt; ApplicationRecord
    belongs_to :callgroup
  end
  </code></pre>
  
  <p>Now, i need to add another model called "Extragroup", and manage payments for this model with the same "payments" model.
  i tried to add relations in payments table with a relations migration end changing model in </p>
  
  <pre><code>class Payment &lt; ApplicationRecord
    belongs_to :callgroup
    belongs_to :extragroup
  end
  </code></pre>
  
  <p>The goal is to register payments in the table but refear to "Callgroup" OR to "extragroup" but without success: after changes payments are no long working.
  How can i solve?</p>