Question

I have written a task that will run periodically to recalculate and update the information in a particular column of an Active Record model. Frequently, the newly calculated object (it’s a json-encoded hash, if that matters) will be the same as the existing objects. In such cases, I suspect that it may be more efficient to check for this identity and abort the save rather than saving. Thus, I am considering using self.save! if changed? rather than merely self.save!. My questions are:

1. Does Active Record do this automatically, so that these two commands are actually equivalent?
2. If, as I suspect, it does not, am I likely to save CPU time by adding the if changed? condition?

(I apologize if I’ve misused some terminology here. I’m new to the game.)

Answer 1

No db query is made, here is a simple example using Rails 3.1.1 in console:

> s = Shop.first   
 => #<Shop id: 1, name: "Ben", address: "paris", latitude:
> 48.856614, longitude: 2.3522219, gmaps: true, created_at: "2011-10-13 13:17:24", updated_at: "2011-10-13 13:17:24">  ruby-1.9.2-p290 :003 >

> s.name = "Ben"  => "Ben"  
> s.save  
=> true 
> s.updated_at
=> Thu, 13 Oct 2011 13:17:24 UTC +00:00  

> s.update_attribute(:name, "Ben")
=> true 
> s.updated_at
=> Thu, 13 Oct 2011 13:17:24 UTC +00:00 

> s.update_attributes({:name => "Ben"})
=> true 
> s.updated_at
=> Thu, 13 Oct 2011 13:17:24 UTC +00:00 

> s.name = "Ben"  
=> "Ben"
> s.save!
> s.updated_at
=> Thu, 13 Oct 2011 13:17:24 UTC +00:00

---

Edit:

rails profiler 's = Shop.first; s.name = "Ben"; s.save;' 's = Shop.first; s.name =     
Loaded suite script/rails
Started
ProfilerTest#test_s_shop_first_s_name_ben_s_save (78 ms warmup)
    process_time: 5 ms
ProfilerTest#test_s_shop_first_s_name_ben_s_save_if_s_changed (1 ms warmup)
    process_time: 2 ms
Finished in 2.597531 seconds.

So it’s faster with if changed?.