I’ve moved over to Shine‘s Ruby on Rails team and, as such, have been exposed to a whole lot of Rails code over the past few weeks. Something I ran into a few weeks back was this bug, relating to a parsing bug in nested forms. I’ve submitted a series of patches, the last of which I hope to see in HEAD sometime over the next few days.

Anyway, the actual bug was caused by weirdo (and non-Hash-like) semantics in HashWithIndifferentAccess that causes certain types (Hash and Array) to be stored as a copy of the value passed in. The following code will give you an idea of the crazy logic that might result from such behavior:

def mktom(); {:name => 'Tom', :age => 23}; end

tom = mktom
people = {:tom => tom}
tom[:name] = 'Thomas'
puts people[:tom][:name] # displays 'Thomas' - cool!
puts "The same object" if tom.object_id == people[:tom].object_id # displays 'The same object'

tom = mktom
people = {:tom => tom}.with_indifferent_access
tom[:name] = 'Thomas'
puts people[:tom][:name] # displays 'Tom' - wtf!
puts "Not the same object!" if tom.object_id != people[:tom].object_id # displays 'Not the same object!'

The lesson learned from tracking down and fixing this bug?

  • Do not treat HashWithIndifferentAccess as though it were any old Hash. It has different semantics to Hash and may store copies of the keys and/or values instead of the original objects.
  • Do not nest HashWithIndifferentAccess instances in Hash instances unless you enjoy headaches. It’s so easy to think that HashWithIndifferentAccess is semantically identical to Hash (especially thanks to Hash#with_indifferent_access), but this assumption was the cause of two nasty, hard-to-find bugs in this instance.
  • When providing an API virtually identical to (and easily mistakable for) core Ruby classes, ensure that semantics are consistent with convention.

I’m not sure why they didn’t just override the reader for #[]. Maybe there’s more to that than I realize. Anyway, I’ve spent enough time thinking about this for tonight, time for bed. Sorry for the blog drought, I’m hoping to finalize a few more installments of the Ocaml series in along with the final part of my Scala/BCEL parser tutorial over the next few months. Stay tuned. :)

UPDATE Since I wrote this post, I’ve written a plugin that replaces Rails’ parameter parsing implementation to make dealing with complex nested forms easier still. Refer to Taking the Pain Out of Complex Forms in Rails.