title: how to avoid overwriting nested attributes?
body: <p>I have:</p>
  
  <pre><code>class Activity &lt; ActiveRecord::Base
    has_and_belongs_to_many :balance_sheets
  end
  </code></pre>
  
  <p>and </p>
  
  <pre><code>class BalanceSheet &lt; ActiveRecord::Base
    has_and_belongs_to_many :activities
    accepts_nested_attributes_for :activities
  end
  </code></pre>
  
  <p>When I perform an UPDATE sending <code>"balance_sheet"=&gt;{"activity_ids"=&gt;["10", "20"]}</code> if I have previous activities loaded on the balance_sheet object, the activities collection is replaced. <strong>I don't want override the activities old values, I want to add new ones</strong>. How can I do this?</p>