Read and write YAML files without destroying anchors and aliases

Question

This question has been asked before: Read and write YAML files without destroying anchors and aliases?

I was wondering how to solve that problem with many anchors and aliases?

thanks

Answer 1

The problem here is that anchors and aliases in Yaml are a serialization detail, and so aren’t part of the data after it’s been parsed, so the original anchor name isn’t known when writing the data back out to Yaml. In order to keep the anchor names when round tripping you need to store them somewhere when parsing so that they are available later when serializing. In Ruby any object can have instance variables associated with it, so an easy way to achieve this would be to store the anchor name in an instance variable of the objet in question.

Continuing from the example in the earlier question, for hashes we can change our redifined revive_hash method so that if the hash is an anchor then as well as recording the anchor name in the @st variable so later alises can be recognised, we add the it as an instance variable on the hash.

class ToRubyNoMerge < Psych::Visitors::ToRuby
  def revive_hash hash, o
    if o.anchor
      @st[o.anchor] = hash
      hash.instance_variable_set "@_yaml_anchor_name", o.anchor
    end

    o.children.each_slice(2) { |k,v|
      key = accept(k)
      hash[key] = accept(v)
    }
    hash
  end
end

Note that this only affects yaml mappings that are anchors. If you want to have other types to keep their anchor name you’ll need to look at psych/visitors/to_ruby.rb and make sure the name is added in all cases. Most types can be included by overriding register but there are a couple of others; search for @st.

Now that the hash has the desired anchor name associated with it, you need to make Psych use it instead of the object id when serializing it. This can be done by subclassing YAMLTree. When YAMLTree processes an object, it first checks to see if that object has been seen already, and emits an alias for it if it has. For any new objects, it records that it has seen the object in case it needs to create an alias later. The object_id is used as the key in this, so you need to override those two methods to check for the instance variable, and use that instead if it exists:

class MyYAMLTree < Psych::Visitors::YAMLTree

  # check to see if this object has been seen before
  def accept target
    if anchor_name = target.instance_variable_get('@_yaml_anchor_name')
      if @st.key? anchor_name
        oid         = anchor_name
        node        = @st[oid]
        anchor      = oid.to_s
        node.anchor = anchor
        return @emitter.alias anchor
      end
    end

    # accept is a pretty big method, call super to avoid copying
    # it all here. super will handle the cases when it's an object
    # that's been seen but doesn't have '@_yaml_anchor_name' set
    super
  end

  # record object for future, using '@_yaml_anchor_name' rather
  # than object_id if it exists
  def register target, yaml_obj
    anchor_name = target.instance_variable_get('@_yaml_anchor_name') || target.object_id
    @st[anchor_name] = yaml_obj
    yaml_obj
  end
end

Now you can use it like this (unlike the previous question, you don’t need to create a custom emitter in this case):

builder = MyYAMLTree.new
builder << data

tree = builder.tree

puts tree.yaml # returns a string

# alternativelty write direct to file:
File.open('a_file.yml', 'r+') do |f|
  tree.yaml f
end