In Ruby, how to be warned of duplicate keys in hashes when loading a YAML document?

Question

In the following Ruby example, is there a mode to have YAML NOT silently ignore the duplicate key 'one'?

irb(main):001:0> require 'yaml'
=> true
irb(main):002:0> str = '{ one: 1, one: 2 }'
=> "{ one: 1, one: 2 }"
irb(main):003:0> YAML.load(str)
=> {"one"=>2}

Thanks!

Answer 1

Using Psych, you can traverse the AST tree to find duplicate keys. I'm using the following helper method in my test suite to validate that there are no duplicate keys in my i18n translations:

def duplicate_keys(file_or_content)
  yaml = file_or_content.is_a?(File) ? file_or_content.read : file_or_content
  duplicate_keys = []

  validator = ->(node, parent_path) do
    if node.is_a?(Psych::Nodes::Mapping)
      children = node.children.each_slice(2) # In a Mapping, every other child is the key node, the other is the value node.
      duplicates = children.map { |key_node, _value_node| key_node }.group_by(&:value).select { |_value, nodes| nodes.size > 1 }

      duplicates.each do |key, nodes|
        duplicate_key = {
          file: (file_or_content.path if file_or_content.is_a?(File)),
          key: parent_path + [key],
          occurrences: nodes.map { |occurrence| "line: #{occurrence.start_line + 1}" },
        }.compact

        duplicate_keys << duplicate_key
      end

      children.each { |key_node, value_node| validator.call(value_node, parent_path + [key_node.try(:value)].compact) }
    else
      node.children.to_a.each { |child| validator.call(child, parent_path) }
    end
  end

  ast = Psych.parse_stream(yaml)
  validator.call(ast, [])

  duplicate_keys
end