What is the best way to convert all controller params from camelCase to snake_case in Rails?

Question

As you already know, JSON naming convention advocates the use of camelSpace and the Rails advocates the use of snake_case for parameter names.

What is the best way t

Answer 1

When you've completed the steps below, camelCase param names submitted via JSON requests will be changed to snake_case.

For example, a JSON request param named passwordConfirmation would be accessed in a controller as params[:password_confirmation]

Create an initializer at config/initializers/json_param_key_transform.rb. This file is going to change the parameter parsing behaviour for JSON requests only (JSON requests must have the request header Content-Type: application/json).

Find your Rails version and choose the appropriate section below (find your Rails version in Gemfile.lock):

For Rails 5 and 6

For Rails 5 and 6, to convert camel-case param keys to snake-case, put this in the initializer:

# File: config/initializers/json_param_key_transform.rb
# Transform JSON request param keys from JSON-conventional camelCase to
# Rails-conventional snake_case:
ActionDispatch::Request.parameter_parsers[:json] = lambda { |raw_post|
  # Modified from action_dispatch/http/parameters.rb
  data = ActiveSupport::JSON.decode(raw_post)

  # Transform camelCase param keys to snake_case
  if data.is_a?(Array)
    data.map { |item| item.deep_transform_keys!(&:underscore) }
  else
    data.deep_transform_keys!(&:underscore)
  end

  # Return data
  data.is_a?(Hash) ? data : { '_json': data }
}

For Rails 4.2 (and maybe earlier versions)

For Rails 4.2 (and maybe earlier versions), to convert camel-case param keys to snake-case, put this in the initializer:

# File: config/initializers/json_param_key_transform.rb
# Transform JSON request param keys from JSON-conventional camelCase to
# Rails-conventional snake_case:
Rails.application.config.middleware.swap(
  ::ActionDispatch::ParamsParser, ::ActionDispatch::ParamsParser,
  ::Mime::JSON => Proc.new { |raw_post|

    # Borrowed from action_dispatch/middleware/params_parser.rb except for
    # data.deep_transform_keys!(&:underscore) :
    data = ::ActiveSupport::JSON.decode(raw_post)
    data = {:_json => data} unless data.is_a?(::Hash)
    data = ::ActionDispatch::Request::Utils.deep_munge(data)

    # Transform camelCase param keys to snake_case:
    data.deep_transform_keys!(&:underscore)

    data.with_indifferent_access
  }
)

Final step for all Rails versions

Restart rails server.