application_controller.rb

# Consider the following
class ApplicationController < ActionController::Base
  helper_method :current_filter
  
  private

  def current_filter
    ActiveSupport::StringInquirer.new(params[:filter] || '')
  end
end

# Then in your controllers and/or views you can do the following:
current_filter # Get the current value
current_filter.blank? # Check if a filter is present
current_filter.unpaid? # Check if the current filter has the value unpaid.

index.html.erb

<% # Before %>
<%= link_to 'All',         invoices_path(filter: nil),      class: ("current" if params[:filter].blank?) %>
<%= link_to 'Only unpaid', invoices_path(filter: "unpaid"), class: ("current" if params[:filter] == "unpaid") %>

<%= render @invoices %> 

<% # After %>
<%= link_to_if current_filter.blank?,  "All",         invoices_path(filter: current_filter), class: 'current' %>
<%= link_to_if current_filter.unpaid?, "Only unpaid", invoices_path(filter: current_filter), class: 'current' %>

<%= render @invoices %>

invoices_controller.rb

# Before
class InvoicesController < ActiveRecord::Base
  def index
    if params[:filter] == "unpaid"
      @invoices = Invoice.unpaid
    else
      @invoices = Invoice.all
    end
  end
end

# After
class InvoicesController < ActiveRecord::Base
  def index
    @invoices   = Invoice.unpaid if current_filter.unpaid?
    @invoices ||= Invoice.all
  end
end