Use form_for without the enclosing form tag
In rare cases you might need something like
form_for (for using form builder methods on the resulting block element) but without the surrounding form. One such case would be updating some of a form's fields via XHR.
You can simply use Rails'
fields_for to do things like this in your views (HAML here):
- fields_for @user do |form|
= form.label :email, 'E-Mail'
= form.text_field :email
You will only receive the form content you gave, no hidden inputs including the authenticity token etc.