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Merge keys not working as expected #8

wr0ngway opened this Issue Jan 20, 2011 · 19 comments


7 participants

Psych doesn't seem to support merge keys ( http://yaml.org/type/merge.html ) with alias/anchor the same way that syck does. I'm not sure if this is intentional or not, but its a pretty useful behavior so I'm filing a bug. Basically, while psych does allow one to use a merge key to pull a aliased mapping into another mapping, it does not allow one to add other items to that mapping. Very frequently I have a map that is mostly in common amongst a number of keys, with a subkey that differs, and this bug makes it impossible to achieve this behavior with psych.

For the yaml snippet below:

foo: &foo
  hello: world
  << : *foo
  baz: boo

syck produces: {"foo"=>{"hello"=>"world"}, "bar"=>{"hello"=>"world", "baz"=>"boo"}}

psych produces: {"foo"=>{"hello"=>"world"}, "bar"=>{"hello"=>"world"}}

Note that in the yaml 1.1 spec ( http://yaml.org/spec/1.1/#id902561 ) it says "Note that an alias node must not specify any properties or content, as these were already specified at the first occurrence of the node.", however I think this should only apply when doing something like "bar: *foo". When using a merge key, you are creating a new node (which you don't add to), but merging it into a different mapping node - which you should be able to add to.


tenderlove commented Jan 20, 2011

Thanks for the bug report! Would you mind filing this bug on redmine:


I can fix this, but if it's not on redmine, I'm afraid it won't get backported to 1.9.2.


tenderlove commented Jan 20, 2011

Awesome, thank you!

Is there a way to upgrade Psych for 1.9.2-p136? p180 seems to use RubyGems 1.5 and that is not compatible with Ruby on Rails 2.3.

A follow-up: his patch isn't in 1.9.2-p180. The only way to get it for Ruby on Rails 2.3 is to use HEAD and downgrade RubyGems to 1.3.7.

Lets just say that virtually every Ruby (and especially Ruby on Rails) app I have seen since 2005 that runs in multiple environments, relies on this feature of YAML :(


tenderlove commented Feb 28, 2011

For now, I suggest you downgrade to use Syck. The way to do that is add this to your environment.rb:

YAML::ENGINE.yamler = 'syck'

I am thinking of releasing psych as a gem, but it must be 1.9 only. :-(

So it is possible to switch back to Syck. Great, thank you. I personally think that 1.9-only version of Psych is totally fine: on 1.8, people seem to be pretty happy with Syck.


tenderlove commented Feb 28, 2011

The annoying thing is that even if I release a gem, you would be forced to say:

gem 'psych'

in your environment.rb. Otherwise the stdlib one would be required. Either I can teach people how to downgrade to syck, or teach people to install a gem, then add a special command to require it. I'm not sure which is least annoying. :-(

I vote for the gem - the reason I want to try psych is that I frequently segfault in syck when running our test cases with complex VCR yaml fixtures.


tenderlove commented Apr 18, 2011

Fixed in ruby trunk, so I'm closing. Also, I've released a gem that is 1.9 only.

@tenderlove tenderlove closed this Apr 18, 2011

@tenderlove, are there any plans to backport this to the next point release of 1.9.2? I see people hitting this issue over and over, including well-known folks in the Ruby community. That's kinda sad.


tenderlove commented May 25, 2011

@michaelklishin there is a ticket filed to backport to 1.9.2. In the mean time (as I have mentioned in the previous comments) please use the gem or downgrade to syck.

actually, better add to application.rb, not environment.rb
otherwise rake tasks that do not load environment won't fire O_O (e.q. drop/create DB)

@tenderlove: What version of the Psych gem has this fixed? I get the problem with Psych 1.2.0 still.

This is still an issue, and I'm having to fix every Rails app I run on 1.9.2 with this code in config/boot.rb:

if RUBY_VERSION.to_f >= 1.9
  # The psych engine for YAML doesn't handle `<<: *defaults` properly.
  # See: https://github.com/tenderlove/psych/issues/8
  require 'yaml'
  YAML::ENGINE.yamler = 'syck'

It would be great if this hack was no longer necessary.


tenderlove commented Aug 6, 2011

@ndbroadbent please open a new ticket. In the new ticket, post code that parses your YAML file using 1.8 and 1.9 (with psych). Post the output for 1.8 and the output for 1.9, and we can get it fixed. Thanks.


tenderlove commented Aug 6, 2011

Also make sure to include the YAML file along with your sample program. Thanks!

Aha! I see that it has been fixed in 1.9.2-p290, but not 1.9.2-p180.
I shall bring all my Rails 3 apps up to p290.
Sorry for the false alarm.

firedev commented Jan 22, 2013

Sorry but it still not merging keys correctly as of Ruby 1.9.3p374 and Rails 3.2.11 rails/rails#9025

# Models
        title: "Russian title"

  errors: &errors
      format: ! '%{attribute} %{message}'
        accepted: нужно подтвердить

      <<: *errors

Start console:

=> "Title"
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