Log slow queries in Rails

query_tracer.rb

class QueryTracer < ActiveSupport::LogSubscriber

  ACCEPT = %r{^(app|config|lib)}.freeze
  FRAMES = 15
  THRESHOLD = 100 # In ms

  def sql(event)
    return unless event.duration > THRESHOLD
    callers = Rails.
      backtrace_cleaner.
      clean(caller).
      select { |f| f =~ ACCEPT }.
      take(FRAMES).
      map { |f| f.split(":").take(2).join(":") }.
      join(" | ")

    # Shamelessly stolen from ActiveRecord::LogSubscriber
    warning = color("SLOW QUERY", RED, true)
    name = '%s (%.1fms)' % [event.payload[:name], event.duration]
    sql  = event.payload[:sql].squeeze(' ')

    warn "  #{warning}"
    warn "    #{name} #{sql}"
    warn "    Trace: #{callers}"
  end

end

QueryTracer.attach_to :active_record