title: Rails. Check if ANY of the attributes in a collection is blank
body: <p>I have a collection of invoices. One of the attributes is exchange_rate (is used to calculate the currency from US dollars for Mexican pesos). I need to create a warning if even ONE of the records doesn't have an exchange_rate set.</p>
  
  <p>I could check if the exchange_rate of a record in a collection is blank like this...</p>
  
  <pre><code>&lt;% is_blank = false %&gt;
  
  &lt;% @invoices.each do |invoice| %&gt;
    &lt;% if invoice.exchange_rate.blank? %&gt;
      &lt;% is_blank = true %&gt;
    &lt;% end %&gt;
  &lt;% end %&gt;
  
  &lt;% if is_blank %&gt;
    shoot warning: all of the invoices must have an exchange rate in order
    to calculate pesos total
  &lt;% end %&gt;
  </code></pre>
  
  <p>What is a more Railsy way of writing the above?</p>