testing for n+1 query regressions

[Paul Annesley]

I recently read a post about writing tests to catch n+1 query regressions, but I have no idea where it was.

I'm not sure it's a good idea (or possible to do in a non-fragile way), but it would be useful at this phase of an app I'm working on.

Does anybody remember the post, or have any thoughts on the matter?

— Paul

[Tim McEwan]

Could you set bullet* to run in your test env and assert that you don't see its log output?  It would slow your tests down a fair bit though.

Don't worry, I'm aware this is a terrible idea - but hopefully it gets the ball rolling.

* https://github.com/flyerhzm/bullet

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[Mark Wotton]

I think it's a pretty bad idea to mix up benchmarking and correctness tests.

mark

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[Paul Annesley]

Each sub-resources I add to an API representation introduces an n+1.
I'd like a test to fail until I adjust the eager-loading accordingly.
Seems pragmatic, despite the dangers of testing for performance optimisations.

I've gone with this for the moment:
https://gist.github.com/3606669

The QuerySpy helps assert that increasing the number of records fetched does not increase the number of queries:

it "should not exhibit n+1" do
add_thing
count1 = QuerySpy.count { get "/things" }
add_thing
count2 = QuerySpy.count { get "/things" }
count2.should <= count1
end

The API could be fancier, but it does the job.
I can break eager-loading and see it fail, fix it and see it pass.

— Paul

[Craig Read]

I don't remember seeing a post on this, but do remember watching the Railscast on Bullet recently.

http://railscasts.com/episodes/372-bullet

— Paul

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Craig Read

@Catharz

https://github.com/Catharz

http://stackoverflow.com/users/158893/catharz

[Malcolm Locke]

There's a gem for that:

https://github.com/flyerhzm/bullet

Never used it though so cannot testify to it's effectiveness.

Malc

[Paul Annesley]

Bullet does look cool, but as far as I know it doesn't provide assertions like I'm after.

The post I read was specifically about writing RSpec (or Test::Unit?) assertions based on number of queries.

The QuerySpy I ended up with works well for me.

— Paul

[Fred Wu]

I've used Bullet briefly many moons ago but haven't used it for assertion. However, looking at Bullet's source, I think you might be able to wrap its `NPlusOneQuery` class for assertion? https://github.com/flyerhzm/bullet/blob/master/spec/bullet/detector/n_plus_one_query_spec.rb