title: Rails polymorphic association is inserting an extra field
body: <p>I created a migration to change my data model, in order to make my Conversation model polymorphic. </p>
  
  <p>Here are my files, slimmed down: </p>
  
  <pre><code># deal.rb
  class Deal &lt; ActiveRecord::Base
    has_many :conversations, as: :conversationable
  end
  
  # conversation.rb
  class Conversation &lt; ActiveRecord::Base
      belongs_to :conversationable, polymorphic: true
  end
  </code></pre>
  
  <p>In my seeds.rb, I am calling the line: </p>
  
  <pre><code>conversation = deal.conversations.create!(:seller_id =&gt; deal.user.id, :buyer_id =&gt; user.id)
  </code></pre>
  
  <p>Which generates the error:</p>
  
  <pre><code>SQLite3::SQLException: table conversations has no column named deal_id: INSERT INTO "conversations" ("buyer_id", "conversationable_id", "conversationable_type", "created_at", "deal_id", "seller_id", "updated_at") VALUES (?, ?, ?, ?, ?, ?, ?)
  </code></pre>
  
  <p>Why is rails still inserting deal_id? Are there remnants of the association elsewhere that I have to delete? </p>
  
  <p>When running the same code in rails console, the model is created with the correct associations.</p>
  
  <p>Thank you.</p>