How do I add a class to a form in ruby on rails without setting / overriding the existing default helper class?

Adding a class for form_for through the html: {class: "form-horizontal"} option overrides the default new_model or edit_model class. How do I add my class while keeping the existing form_helper class?

I want:

<form class="edit_model form-horizontal"> or

<form class="new_model form-horizontal">

Instead of:

<form class="form-horizontal">