title: Transmit params with form_tag
body: <p>In my views I have a form and trying to update quantity for an order line:</p>
  
  <pre><code>&lt;div class="quantity"&gt;Quantity&lt;/br&gt; 
      &lt;%= form_tag(order_line_path(line.id), method: "patch") do  %&gt;
          &lt;%= number_field_tag 'qty', '1', within: 1...line.book.stock %&gt;
          &lt;%= submit_tag "Update", class: "btn btn-primary" %&gt;
      &lt;% end %&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>The instance variable in the rest of my view is a collection of order lines, so I cannot use it.</p>
  
  <p>Then I have in my controller the update method:</p>
  
  <pre><code>def update
    @order = current_order
    @order_line = @order.order_lines.find(params[:id])
    @order_line.update_attributes(order_line_params)
  end
  </code></pre>
  
  <p>And my strong params definition:</p>
  
  <pre><code>def order_line_params
    params.require(:order_line).permit(:qty)
  end
  </code></pre>
  
  <p>I get this error :</p>
  
  <pre><code>param is missing or the value is empty: order_line
  </code></pre>
  
  <p>Could someone please have a look?</p>
  
  <p>Thanks!</p>