title: ActiveRecord delete_all method updating instead of deleting
body: <p>I'm using Rails polymorphic associations, that way some models have many <code>cash_histories</code> children, like this:</p>
  
  <pre><code>has_many :cash_histories, as: :cashable
  </code></pre>
  
  <p>But when I try to delete all cash histories from a parent <code>@resource</code>, like this:</p>
  
  <pre><code>@resource.cash_histories.delete_all
  </code></pre>
  
  <p>I get the following query:</p>
  
  <pre><code>UPDATE "cash_histories" SET "cashable_id" = NULL WHERE "cash_histories"."cashable_id" = $1 AND "cash_histories"."cashable_type" = $2  [["cashable_id", 1], ["cashable_type", "ServiceOrder"]]
  </code></pre>
  
  <p>I can't understand this behavior, setting relationship id to null instead of removing, that will result in dead rows in my table. Why is that happening?</p>
  
  <p>I'm using Rails 4.1.</p>