How to make breadcrumbs_on_rails render a Bootstrap compatible breadcrumb navigation

bootstrap_breadcrumbs_builder.rb

# The BootstrapBreadcrumbsBuilder is a Bootstrap compatible breadcrumb builder.
# It provides basic functionalities to render a breadcrumb navigation according to Bootstrap's conventions.
#
# BootstrapBreadcrumbsBuilder accepts a limited set of options:
# * separator: what should be displayed as a separator between elements
#
# You can use it with the :builder option on render_breadcrumbs:
#     <%= render_breadcrumbs :builder => ::BootstrapBreadcrumbsBuilder, :separator => "&raquo;" %>
#
# Note: You may need to adjust the autoload_paths in your config/application.rb file for rails to load this class:
#     config.autoload_paths += Dir["#{config.root}/lib/"]
#
class BootstrapBreadcrumbsBuilder < BreadcrumbsOnRails::Breadcrumbs::Builder
  def render
    @context.content_tag(:ul, class: 'breadcrumb') do
      @elements.collect do |element|
        render_element(element)
      end.join.html_safe
    end
  end

  def render_element(element)
    current = @context.current_page?(compute_path(element))

    @context.content_tag(:li, :class => ('active' if current)) do
      link_or_text = @context.link_to_unless_current(compute_name(element), compute_path(element), element.options)
      divider = @context.content_tag(:span, (@options[:separator]  || '/').html_safe, :class => 'divider') unless current

      link_or_text + (divider || '')
    end
  end
end

Also FWIW, I had to do some debugging to figure it out, if you want to check if any breadcrumbs exist, i.e. only render if they do... the breadcrumbs are in the instance variable @breadcrumbs.

Updated for Bootstrap 4 (alpha6): https://gist.github.com/SaladFork/270a4cb3ac20be9715b7117551c31ec7

Of note:

• No longer has :separator option (Bootstrap 4 accomplishes this with CSS)
• Now has :container_tag option in addition to :tag option (default to :ol and :li, respectively)
• Now has :show_empty option which defaults to false to prevent rendering when there are no crumbs (h/t @mrfoto's fork)

Bootstrap 5

# The BootstrapBreadcrumbsBuilder is a Bootstrap compatible breadcrumb builder.
# It provides basic functionalities to render a breadcrumb navigation according to Bootstrap's conventions.
#
# You can use it with the :builder option on render_breadcrumbs:
#     <%= render_breadcrumbs :builder => ::BootstrapBreadcrumbsBuilder %>
#
# Note: you can put this file into app/utils/bootstrap_breadcrumbs_builder.rb and it will autoload.
#
class BootstrapBreadcrumbsBuilder < BreadcrumbsOnRails::Breadcrumbs::Builder
  def render
    @context.content_tag(:nav, 'aria-label' => "breadcrumb") do
      @context.content_tag(:ol, class: 'breadcrumb') do
        @elements.collect do |element|
          render_element(element)
        end.join.html_safe
      end
    end
  end

  def render_element(element)
    current = @context.current_page?(compute_path(element))

    classes = ['breadcrumb-item']
    classes.push('active') if current

    @context.content_tag(:li, :class => classes.join(" ") ) do
      link_or_text = @context.link_to_unless_current(compute_name(element), compute_path(element), element.options)
      link_or_text
    end
  end
end

Thanks @sadjow!