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Extends YAML to support file based inheritance
 Project Readme


Gem Gem License: MIT

Extends YAML to support file based inheritance.

That can be very handy to build a configuration hierarchy.

Basic support for ERB (embedded ruby) is included and automatically applied when config files are named *.erb or *.erb.*.


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:

fruits.yml   vegetables.yml          default.yml   extensions.yml       
         \    /                             \    /                
          food.yml                          merged.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', options = {})
  • yaml_path (String) relative or absolute path to yaml file to inherit from
  • inheritance_key (String) 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'
  • options (Hash) collection of optional options, including all options of the based deep_merge gem
    • :preserve_inheritance_key (Boolean) Preserve inheritance key(s) from resulting yaml, does most time not make sense especially in multiple inheritance - DEFAULT: false
    • The following options are deep merge options that can be passed by - but the defaults differ from original
    • :preserve_unmergeables (Boolean) Set to true to skip any unmergeable elements from source - DEFAULT: false
    • :knockout_prefix (String) Set to string value to signify prefix which deletes elements from existing element - DEFAULT: nil
    • :overwrite_arrays (Boolean) Set to true if you want to avoid merging arrays - DEFAULT: false
    • :sort_merged_arrays (Boolean) Set to true to sort all arrays that are merged together - DEFAULT: false
    • :unpack_arrays (String) Set to string value to run "Array::join" then "String::split" against all arrays - DEFAULT: nil
    • :merge_hash_arrays (Boolean) Set to true to merge hashes within arrays - DEFAULT: false
    • :extend_existing_arrays (Boolean) Set to true to extend existing arrays, instead of overwriting them - DEFAULT: true
    • :merge_nil_values (Boolean) Set to true to merge nil hash values, overwriting a possibly non-nil value - DEFAULT: false
    • :merge_debug (Boolean) Set to true to get console output of merge process for debugging - DEFAULT: false

See also rubydoc at


Bug reports and pull requests are welcome on GitHub at 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.