Conditionally add a class to link_to in Rails

You can do it outside the link_to:

<% css_class = accepted ? "hidden" : "" %>
<%= link_to "Accept Friend Request", 
  "#", 
  class: "btn btn-success btn-sm btn-block requestSent #{css_class}",
  disabled: true %>

I posted a similar answer to this question.

A cleaner solution

The standard approach requires putting logic into the views and using string interpolation or moving things into a separate helper.

Here's an updated approach that avoids any of that:

<%= link_to "Accept Friend Request", 
    "#", 
    class: class_string("btn btn-success btn-sm ban-block requestSent" => true, hidden: accepted),
    disabled: true %>

class_string method

The class_string helper takes a hash with key/value pairs consisting of CSS class name strings and boolean values. The result of the method is a string of classes where the boolean value evaluated to true.

Sample Usage

class_names("foo bar" => true, baz: false, buzz: some_truthy_variable)
# => "foo bar baz"

Inspired by React

This technique is inspired by an add-on called classNames (formerly known as classSet) from Facebook’s React front-end framework.

Using in your Rails projects

As of now, the class_names function does not exist in Rails, but this article shows you how to add or implement it into your projects.

If you use interpolation with #{}, anything you put between it is run as plain old Ruby code. In this example you could add a conditional class in the string like this:

<%= link_to "Accept Friend Request", 
    "#", 
    class: "btn btn-success btn-sm btn-block requestSent #{'hidden' if accepted}",
    disabled: true %>

Just note that you should use single quotes around the class name 'hidden'. Also note that when a variable represents a boolean value (true or false), you don't need to explicitly say if accepted == true. You can simply say if accepted.

You can use a helper to build up the link as well:

def accept_friend_request_link
  classes = [:btn, :and_friends]
  if accepted
    classes << :hidden
  end
  link_to 'Accept Friend Request', '#', class: classes, disabled: true
end