routes_with_constraints.rake

namespace :routes do
  desc 'Print out all defined routes in match order, with names, per constraint class. Target specific constraint class with CONSTRAINT=x. Target specific controller with CONTROLLER=x.'
  task constrained: :environment do
    Rails.application.reload_routes!

    constraints_routes = Hash.new

    Rails.application.routes.routes.each do |route|
      group = (route.app.class == ActionDispatch::Routing::Mapper::Constraints ? route.app.send( :constraints ).first.to_s : 'No constraint class')
      constraints_routes[group] ||= []
      constraints_routes[group] << route
    end

    requested_constraint = ENV[ 'CONSTRAINT' ]

    constraints_routes.each do |group, all_routes|
      if requested_constraint.nil? or group == requested_constraint
        puts "\n\nConstraint class : #{group}\n\n"

        if ENV[ 'CONTROLLER' ]
          all_routes = all_routes.select { |route| route.defaults[ :controller ] == ENV[ 'CONTROLLER' ] }
        end

        routes = all_routes.collect do |route|
          reqs = route.requirements.dup
          reqs[ :to ] = route.app unless route.app.class.name.to_s =~ /^ActionDispatch::Routing/
          reqs = reqs.empty? ? '' : reqs.inspect

          {name: route.name.to_s, verb: route.verb.gsub( /[\^\$]/, '' ), path: route.path, reqs: reqs}
        end

        routes.reject! { |r| r[ :path ].spec.to_s =~ %r{/rails/info/properties|/assets} } # Skip the route if it's internal route

        name_width = routes.map { |r| r[ :name ].length }.max
        verb_width = routes.map { |r| r[ :verb ].length }.max
        path_width = routes.map { |r| r[ :path ].spec.to_s.length }.max

        routes.each do |r|
          puts "#{r[ :name] .rjust( name_width )} #{r[ :verb ].ljust( verb_width )} #{r[ :path ].spec.to_s.ljust( path_width )} #{r[ :reqs ]}"
        end
      end
    end
  end
end

Fixed to work with Rails 5.