Rails: saving duration between "events"

Long story short, each time a user views a resource, an Event model object is created. Each event has a duration column. How can I best save the duration between each event?

class Event
  belongs_to :session
end

My gut reaction was to use an after_save callback each time a new event was created, calculate the time between "now" and the previous event, and save that 开发者_开发百科as the previous event's duration. However, this creates a "waterfall" effect, as the callback is then executed as the previous event is saved, and then the previous before that, and so on.

Any suggestions?

Thanks!

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You'd better use after_create callback, this will not trigger while updating previous element

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I'd probably keep the last event time in the user's session, because it'd be fast... Whether or not this makes sense may depend on when you need this info, though--is it for reporting, used during interaction, etc.