Extends YAML to support file based inheritance.

That can be very handy to build a configuration hierachy.


Add this line to your application's Gemfile:

gem 'yaml_extend'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install yaml_extend

Common information

It is possible to build inheritance trees like:

  \        \        \                         
 dev.yml  int.yml  prod.yml                   

or like this:

default.yml   english.yml          default.yml   german.yml       
         \    /                             \    /                
          uk.yml                            de.yml

A file can inherit from as many as you want. Trees can be nested as deep as you want.

YAML files are deep merged, the latest specified child file overwrites the former ones. Array values are merged as well by default. You can specifiy this with the 3rd Parameter.

The files to inherit from are specified by the key 'extends:' in the YAML file. That key can be customized if you prefer another one. See the examples below.


yaml_extend adds the method YAML.ext_load_file to YAML.

This method works like the original YAML.load_file, by extending it with file inheritance.


Basic Inheritance

Given the following both files are defined:

# start.yml
extends: 'super.yml'
    name: 'Mr. Superman'
    age: 134    
        - 'Raspberrys'
# super.yml
    name: 'Unknown'
    power: 2000
        - 'Bananas'
        - 'Apples'

When you then call ext_load_file

config = YAML.ext_load_file 'start.yml'

the returned YAML value results in

    name: 'Mr. Superman'
    age: 134
    power: 2000
        - 'Bananas'
        - 'Apples'
        - 'Raspberrys'

Inherit from several files

If you want to inherit from several files, you can specify a list (Array) of files. They are merged from top to bottom, so the latest file "wins" - that means it overwrites duplicate values if they exist with the values in the latest file where they occur.

    - 'super_file.yml'
    - 'parent_file.yml'

Using custom extend key

If you don't want to use the default key 'extends:', you can specify your own custom key in two ways.

    - 'super_file.yml'
foo: 'bar'
1. Specify by parameter

You can specify the key by parameter, this is the way to go if you want to use the different key only once or you use the ext_load_file method only once in your application.

config = YAML.ext_load_file 'custom1.yml', 'inherit_from'
2. Global configuration of the key

You can specify the key by configuration globally. So you only need to set the key once and not as parameter anymore

YAML.ext_load_key = 'inherit_from'
config = YAML.ext_load_file 'custom1.yml'
Reset the global key

To reset the global inheritance key, you can either set it to nil or call the reset_load_key method.

YAML.reset_load_key # more readable
YAML.ext_load_key = nil # more explicit

Using custom nested extend key

    extend_file: 'super_file.yml'
    database: false
foo: 'bar'
config = YAML.ext_load_file 'custom2.yml', ['options','extend_file']


YAML.ext_load_file(yaml_path, inheritance_key='extends', extend_existing_arrays=true)

  • yaml_path relative or absolute path to yaml file to inherit from
  • inheritance_key you can overwrite the default key, if you use the default 'extends' already as part of your configuration. The inheritance_key is NOT included, that means it will be deleted, in the final merged file. Default: 'extends'
  • extend_existing_arrays Extends existing arrays in yaml structure instead of replacing them. Default: true


Bug reports and pull requests are welcome on GitHub at https://github.com/magynhard/yaml_extend. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.