Upgrading to Ruby 3.1 causes Psych::DisallowedClass exception when using YAML.load_file

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Symbol is also not allowed per default when loading YAML in Ruby. Therefore, you need to add Symbol to the permitted_classes in your case too when reading the YAML file:

hash = YAML.load_file(
  some_file_name, 
  permitted_classes: [Matrix, OpenStruct, Symbol]
)

See the list of default permitted_classes in Psych (the YAML parser used by Ruby).

Or, when using in Ruby on Rails, you can configure globally in your config/application.rb what classes your Ruby on Rails application should allow as permitted classes when reading a YAML files:

config.active_record.yaml_column_permitted_classes += [Matrix, OpenStruct, Symbol]

Note that for internal YAML parsing in Ruby on Rails Symbol is already the default for active_record.yaml_column_permitted_classes.

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