Read and write YAML files without destroying anchors and aliases? - ruby

Question

I need to open a YAML file with aliases used inside it:
defaults: &defaults
foo: bar
zip: button
node:
<<: *defaults
foo: other
This obviously expands out to an equivalent YAML document of:
defaults:
foo: bar
zip: button
node:
foo: other
zip: button
Which YAML::load reads it as.
I need to set new keys in this YAML document and then write it back out to disk, preserving the original structure as much as possible.
I have looked at YAML::Store, but this completely destroys the aliases and anchors.
Is there anything available that could something along the lines of:
thing = Thing.load("config.yml")
thing[:node][:foo] = "yet another"
Saving the document back as:
defaults: &defaults
foo: bar
zip: button
node:
<<: *defaults
foo: yet another
?
I opted to use YAML for this due to the fact it handles this aliasing well, but writing YAML that contains aliases appears to be a bit of a bleak-looking playing field in reality.

Answer 1

The use of << to indicate an aliased mapping should be merged in to the current mapping isn’t part of the core Yaml spec, but it is part of the tag repository.
The current Yaml library provided by Ruby – Psych – provides the dump and load methods which allow easy serialization and deserialization of Ruby objects and use the various implicit type conversion in the tag repository including << to merge hashes. It also provides tools to do more low level Yaml processing if you need it. Unfortunately it doesn’t easily allow selectively disabling or enabling specific parts of the tag repository – it’s an all or nothing affair. In particular the handling of << is pretty baked in to the handling of hashes.
One way to achieve what you want is to provide your own subclass of Psych’s ToRuby class and override this method, so that it just treats mapping keys of << as literals. This involves overriding a private method in Psych, so you need to be a little careful:
require 'psych'
class ToRubyNoMerge < Psych::Visitors::ToRuby
def revive_hash hash, o
#st[o.anchor] = hash if o.anchor
o.children.each_slice(2) { |k,v|
key = accept(k)
hash[key] = accept(v)
}
hash
end
end
You would then use it like this:
tree = Psych.parse your_data
data = ToRubyNoMerge.new.accept tree
With the Yaml from your example, data would then look something like
{"defaults"=>{"foo"=>"bar", "zip"=>"button"},
"node"=>{"<<"=>{"foo"=>"bar", "zip"=>"button"}, "foo"=>"other"}}
Note the << as a literal key. Also the hash under the data["defaults"] key is the same hash as the one under the data["node"]["<<"] key, i.e. they have the same object_id. You can now manipulate the data as you want, and when you write it out as Yaml the anchors and aliases will still be in place, although the anchor names will have changed:
data['node']['foo'] = "yet another"
puts Yaml.dump data
produces (Psych uses the object_id of the hash to ensure unique anchor names (the current version of Psych now uses sequential numbers rather than object_id)):
---
defaults: &2151922820
foo: bar
zip: button
node:
<<: *2151922820
foo: yet another
If you want to have control over the anchor names, you can provide your own Psych::Visitors::Emitter. Here’s a simple example based on your example and assuming there’s only the one anchor:
class MyEmitter < Psych::Visitors::Emitter
def visit_Psych_Nodes_Mapping o
o.anchor = 'defaults' if o.anchor
super
end
def visit_Psych_Nodes_Alias o
o.anchor = 'defaults' if o.anchor
super
end
end
When used with the modified data hash from above:
#create an AST based on the Ruby data structure
builder = Psych::Visitors::YAMLTree.new
builder << data
ast = builder.tree
# write out the tree using the custom emitter
MyEmitter.new($stdout).accept ast
the output is:
---
defaults: &defaults
foo: bar
zip: button
node:
<<: *defaults
foo: yet another
(Update: another question asked how to do this with more than one anchor, where I came up with a possibly better way to keep anchor names when serializing.)

Answer 2

YAML has aliases and they can round-trip, but you disable it by hash merging. << as a mapping key seems a non-standard extension to YAML (both in 1.8's syck and 1.9's psych).
require 'rubygems'
require 'yaml'
yaml = <<EOS
defaults: &defaults
foo: bar
zip: button
node: *defaults
EOS
data = YAML.load yaml
print data.to_yaml
prints
---
defaults: &id001
zip: button
foo: bar
node: *id001
but the << in your data merges the aliased hash into a new one which is no longer an alias.

Answer 3

Have you try Psych ? Another question with psych here.

Answer 4

I'm generating my CircleCI config file with a Ruby script and ERB templates. My script parses and regenerates the YAML, so I wanted to preserve all the anchors. The anchors in my config all have the same name as the key that defines them, e.g.
docker_images:
docker_auth: &docker_auth
username: '$DOCKERHUB_USERNAME'
password: '$DOCKERHUB_TOKEN'
cimg_base_image: &cimg_base_image
image: cimg/base:2022.09
auth: *docker_auth
jobs:
tests:
docker:
- *cimg_ruby_image
So I was able to solve this with regular expressions on the generated YAML string. It wrote a #restore_yaml_anchors method that converts &1 and *1 back into &docker_auth and *docker_auth.
# Ruby 3.1.2
require 'rubygems'
require 'yaml'
yaml = <<EOS
docker_images:
docker_auth: &docker_auth
username: '$DOCKERHUB_USERNAME'
password: '$DOCKERHUB_TOKEN'
cimg_base_image: &cimg_base_image
image: cimg/base:2022.09
auth: *docker_auth
jobs:
tests:
docker:
- *cimg_base_image
EOS
data = YAML.load yaml, aliases: true # needed for Ruby 3.x
def restore_yaml_anchors(yaml)
yaml.scan(/([A-Z0-9a-z_]+|<<): &(\d+)/).each do |anchor_name, anchor_id|
yaml.gsub!(/([:-]) (\*|&)#{anchor_id}/, "\\1 \\2#{anchor_name}")
end
yaml
end
puts [
"Original #to_yaml:",
data.to_yaml,
"-----------------------", '',
"With restored anchors:",
restore_yaml_anchors(data.to_yaml)
].join("\n")
Output:
Original #to_yaml:
---
docker_images:
docker_auth: &1
username: "$DOCKERHUB_USERNAME"
password: "$DOCKERHUB_TOKEN"
cimg_base_image: &2
image: cimg/base:2022.09
auth: *1
jobs:
tests:
docker:
- *2
-----------------------
With restored anchors:
---
docker_images:
docker_auth: &docker_auth
username: "$DOCKERHUB_USERNAME"
password: "$DOCKERHUB_TOKEN"
cimg_base_image: &cimg_base_image
image: cimg/base:2022.09
auth: *docker_auth
jobs:
tests:
docker:
- *cimg_base_image
It's working well for my CI config, but you may need to update it to handle some other cases in your own YAML.