How to simply validate a checkbox in rails

Adding

validates :terms_of_service, :acceptance => true

to your model should do it. Look here for more details and options.

However, if accepting the terms is not part of a form for your model, you should use client-side validations, i.e. JavaScript, like this (in jQuery):

function validateCheckbox()
{
    if( $('#checkbox').attr('checked')){
      alert("you have to accept the terms first");
    }
}

You can add a script file to your view like this:

<%= javascript_include_tag "my_javascipt_file" %>

and trigger the function on click:

<%= submit_tag "Submit", :onclick: "validateCheckbox();" %>

EDIT: you can assign an id to your checkbox like this: check_box_tag :checkbox. The HTML will look like this: <input id="checkbox" See these examples for more options.

I was able to skip the jQuery portion and get it validation to work with this questions help. My method is below, I'm on Rails 5.1.2 & Ruby 2.4.2.

Put this code in your slim, erb or haml; note syntax may differ slightly in each. The line below was specifically for slim.

f.check_box :terms_of_service, required: true 

I used a portion of kostja's code suggestion in the model.

validates :terms_of_service, :acceptance => true