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Daniel Doubrovkine

aka dB., CTO at artsy.net, fun at playplay.io, NYC

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I’ve been co-maintaining the Ruby library hashie for a few years now. It’s a wonderful set of extensions written by some of the best Ruby developers. But sometimes, a piece of code turns rogue and starts hurting people. The devil takes over. That pure evil is Hashie::Mash. At the very least it’s the garbage can of recurrent problems that range from educational to psychotic.

The story of the Hashie::Mash began like all useful classes, when we decided that writing hash[:key] was annoying and hash['key'] was confusing. It was obvious that hash.key was better, that we wanted hash.key? and even hash.key = value. We got it with some help from method_missing.

The invariant key access was borrowed from Merb’s Mash, which has the following comment on top of the file.

This class has dubious semantics and we only have it so that people can write params[:key] instead of params['key'].

The comment was the first thing to be removed from Hashie::Mash, but a lot of new features, such as method access, were added.

def method_missing(method_name, *args)
  if (match = method_name.to_s.match(/(.*)=$/)) && args.size == 1
    self[match[1]] = args.first
  elsif (match = method_name.to_s.match(/(.*)\?$/)) && args.size == 0
  elsif (match = method_name.to_s.match(/(.*)!$/)) && args.size == 0
  elsif key?(method_name)
  elsif match = method_name.to_s.match(/^([a-z][a-z0-9A-Z_]+)$/)

def default(key = nil)
  if key.is_a?(Symbol) && key?(key.to_s)
    key ? super : super()

The Hashie::Mash was born. It was a pure example of Ruby metaprogramming. And some really smart people pitched in on Mash, including @mbleigh and @hassox, authors of Grape and Warden respectively. We were winning!

There were only small bumps in the beginning of this road, such as overwriting nil values, having to figure out how to convert a Mash to Hash, preserving sub-mashes in conversion, or making to work with to_json. Nothing that couldn’t be easily fixed with a bit of code. More features were added, including shallow updates by @mislav, author of rbenv and will_paginate. Everybody who is who in the Ruby world wanted a bit of Hashie::Mash action!

But what could go wrong when you override id? Ruby 1.8 has an Object#id method that is called when the Mash doesn’t have an id key. This causes unexpected failures since there is no id key-value, but Mash is still returning a value. Same goes for type. So it gets undone in #77 with a nice warning in the documentation (that everybody naturally reads).

Do not use the following keys: type, object_id, and id. These are Ruby Object methods. Overriding them means you’re going to have a Bad Time™.

At the same time, everyone has their favorite way of fetching values. Some want to write hash[:key], others hash.key or even hash.fetch(:key, 123). It doesn’t always work as you might expect.

1.9.3p392 :001 > require 'hashie'
1.9.3p392 :002 > h = Hashie::Mash.new
1.9.3p392 :003 > h[:key] = nil
1.9.3p392 :004 > h.fetch(:key, 123)

Whoops. Fixed in #93.

Have we dealt with respond_to?? Probably not. You’re in for a treat.

def respond_to?(method_name, include_private=false)
  return true if key?(method_name) || method_name.to_s.slice(/[=?!_]\Z/)

Unfortunately this breaks Rails 4 strong_parameters. Can you see why? This code will always respond true to permit?. Have a look at the following example.

settings = { foo: "1", attributes: { title: "Value" } }
record.attributes = settings.attributes # raises ActiveModel::ForbiddenAttributes

Whoops. When a key is a Hash, it’s automatically converted into a Mash. The Mash instance responds to :permit?, thus it triggers the ActiveModel forbidden attribute check.

module ForbiddenAttributesProtection
  def sanitize_for_mass_assignment(*options)
    new_attributes = options.first
    if !new_attributes.respond_to?(:permitted?) || new_attributes.permitted?
      raise ActiveModel::ForbiddenAttributes

Well, you can’t have permit? work both ways. A workaround that broke everything was carelessly applied in #104, reverted, and an extension added in #147 by creating Hashie::Extensions::Mash::ActiveModel. For Rails users, @MaximFilimonov even made hashie_rails to inject the extension for scenarios in which Rails cannot be auto-detected. We also have to worry about nested hashes, and make sure that works in #219.

As another proof that you cannot have nice things, Mash#deep_merge was being really slow, unnecessarily converting values. With a fix in #107, a Hashie::Mash.new on a big hash went form 1200 seconds to to less than 2 seconds. This won’t be the last time we face serious performance problems. We’re just warming up.

One day someone has a great idea to profile an application that relies on Hashie::Mash and discovers that we allocate a string, (.*?)([?!=_]?)$, 50,000 times. Is that string familiar? Fixed in #221.

At the same time Hashie was being refactored into separate extensions. Unfortunately they don’t play well with a backwards compatible Mash.

require 'hashie'

class TestHash < Hashie::Trash
  include Hashie::Extensions::Coercion
  include Hashie::Extensions::MergeInitializer
  include Hashie::Extensions::Dash::IndifferentAccess

class Bar < TestHash
  property :a

class Foo < TestHash
  property :bar
  coerce_key :bar, Bar

test = Foo.new Hashie::Mash.new(bar: Hashie::Mash.new(a: 42))
p test

Yes, this is actual code. It uses Hashie::Mash, three extensions and Hashie::Trash. Obviously, this causes a stack overflow. And who comes up with these names? Fixed in #164.

Furthermore, when using an ActiveRecord::HashWithIndifferentAccess that is nested (i.e. {a: {b: 1}}) to create a Hashie::Mash with the IndifferentAccess extension, an ArgumentException is raised. This is because the IndifferentAccess method convert_value overrides ActiveRecord::HashWithIndifferentAccess’s method of the same name. Changing the method name in IndifferentAccess solves the issue in #277, until next time.

I once was using Hashie for a throwaway project, and noticed this incredibly unexpected and annoying behavior. With a normal Hash, you can specify a default value for newly-accessed keys.

hsh = Hash.new{|h, k| h[k] = [] }
hsh[:hello] << 100
# => { :hello => [100] }

This is very useful for quick inject operations.

collection.inject(Hash.new{|h,k| h[k] = [] }) do |h, x|
  h[x] << somefoo(x) if x > foo(x)

Want to do this with Hashie::Mash? Sorry.

hsh = Hashie::Mash.new{|h, k| h[k] = [] }
hsh[:hello] << 100
# => {"hello"=>[]}

Fixed in #259.

Did you know that Hashie::Mash translated all keys into strings? A brave man, @michaelherold fixed this in #296. We will see him a lot more, as he’s now the de-factor primary maintainer of the library and the most active contributor to this craziness. In that PR he left this note.

There is one interesting thing that pops out from this. Checking for equality between Mashes (and Hashes with Mashes) now fails unless the keys are equivalent. For example: Hashie::Mash.new(p: 'test') == Hashie::Mash.new('p' => 'test') #=> false.

This is pretty awesome, as both instances will behave the exact same in every possible way, but aren’t equal.

Also, a surprisingly high number of libraries out there require 'hashie/mash' and not hashie. What could possible go wrong?

There was an error while trying to load the gem 'omniauth-oauth2'.
Gem Load Error is: uninitialized constant

This happened to omniauth via #391, #392 fixed this and was released as 3.5.1. It happened again in #401, fixed in #402, this time with rather complicated integration tests.

One of the most common issues we have reported is when a Hashie::Mash attempts to override a built-in method and fails to do so. To prevent this from being an issue in the future, @michaelherold added a logging layer accessible at Hashie.logger that was user-configurable. This should have helped report the error to the user before it becomes a problem in #381.

What could possibly go wrong? In elasticsearch-ruby#398, we find this.

I updated our bundle today which included hashie@3.5.1, and found our logs are now completely overwhelmed with warnings. I believe this is being caused by the sort key that comes back in the response body.

That’s nothing.

Our log rotation was not aggressive enough for the flood of this warning, ultimately taking our servers down.

Yes, sort is a function. And Hashie::Mash is used as a “freeform” wrapper around deeply structured Hashes coming from Elasticsearch, enabling method access to the structures.

Connecting the logger to Rails.logger and adding Hashie::Mash.disable_warnings was surprisingly tedious in #400, and was released as 3.5.2.

What could possibly go wrong? In #407 we can read the following.

bundler/runtime.rb:94:in `rescue in block (2 levels) in require':
There was an error while trying to load the gem 'omniauth-twitter'.
Gem Load Error is: private method `warn' called for nil:NilClass
Backtrace for gem load error is:
hashie/mash.rb:334:in `log_built_in_message'

So we broke omniauth adapters in two different ways. It’s a record. Fixed in #406 by making sure Hashie::Mash.disable_warnings carries into subclasses, and released as 3.5.3.

But wait. Omniauth has not finished suffering. In #410 we find out that there’s a performance regression on Mash’s method accessors. We switch include? to respond_to? to lookup method collisions in #411 and the problem goes away. Released as 3.5.4.

What could possibly go wrong? In #413 we discover that we can no longer assign a value to a key more than once.

foo = Hashie::Mash.new

foo['foobar'] = Array.new
foo['foobar'] = Array.new

This is pretty trivial stuff, right? Well, that broke.

# hashie/mash.rb:344:in `method': undefined method `foobar' for class `Hashie::Mash' (NameError)
  from hashie/mash.rb:344:in `log_built_in_message'
  from hashie/mash.rb:151:in `custom_writer'
  from bug.rb:6:in `<main>'

The issue is that we changed the code that logs warnings to use respond_to? to check if the call to Hashie::Mash[]= would override a method that exists on Hashie::Mash, or the thing that’s subclassing it. The documentation for Object#respond_to? says that if the method is not defined, respond_to_missing? method is called and the result is returned. Unfortunately Hashie::Mash overrides respond_to_missing? to return true if a key with that name exists in the hash. In the previous version this worked because methods.include? excluded the “magical” methods that Hashie::Mash will respond to.

Again, @michaelherold fixes this in #415 by avoiding logging multiple times, released as 3.5.5.

I am sure this isn’t over. I can’t wait for 3.5.6!

The good news is that we now have an integration test suite for any bizarre regression. And a huge community of people to help. I also suggest you stop using Hashie::Mash and mix-in well-defined and clearly scoped extensions, one-by-one, such as Hashie::Extensions::MethodAccess or Hashie::Extensions::MergeInitializer on a need-by-need basis.