How to change the default rails error div "field_with_errors"

Using Rails 6 with Bootstrap 4 you'll need to add the is-invalid class. Without getting too fancy I just did:

ActionView::Base.field_error_proc = proc do |html_tag, instance|
  html_tag.gsub("form-control", "form-control is-invalid").html_safe
end

For Bootstrap 3.2 you could use sth. like this (add nokogiri gem to your Gemfile):

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  html = %(<div class="field_with_errors">#{html_tag}</div>).html_safe
  # add nokogiri gem to Gemfile

  form_fields = [
    'textarea',
    'input',
    'select'
  ]

  elements = Nokogiri::HTML::DocumentFragment.parse(html_tag).css "label, " + form_fields.join(', ')

  elements.each do |e|
    if e.node_name.eql? 'label'
      html = %(<div class="control-group has-error">#{e}</div>).html_safe
    elsif form_fields.include? e.node_name
      if instance.error_message.kind_of?(Array)
        html = %(<div class="control-group has-error">#{html_tag}<span class="help-block">&nbsp;#{instance.error_message.uniq.join(', ')}</span></div>).html_safe
      else
        html = %(<div class="control-group has-error">#{html_tag}<span class="help-block">&nbsp;#{instance.error_message}</span></div>).html_safe
      end
    end
  end
  html
end

Place this code inside config/initializers/field_error_proc.rb file (create one if not exists)

This is slightly modified code from: Overriding ActionView::Base.field_error_proc in Rails

From the link above, if you put the following inside class Application < Rails::Application of config/application.rb

config.action_view.field_error_proc = Proc.new { |html_tag, instance| 
  "<div class=\"field_with_errors control-group error\">#{html_tag}</div>".html_safe
}

Your input tags will have a red marker around them whenever validation fails