Rails3: Log slow queries.

log_slow_queries.rb

# see: http://weblog.therealadam.com/2011/02/12/simple-ruby-pleasures/comment-page-1/#comment-315

class QueryTracer < ActiveSupport::LogSubscriber
 
  ACCEPT = %r{^(app|config|lib)}.freeze
  FRAMES = 5
  THRESHOLD = 300 # 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