hash_styles.rb

module HashStyles
  def underscore_hash_keys(value)
    case value
    when Array
      value.map { |v| underscore_hash_keys(v) }
    when Hash
      Hash[value.map { |k, v| [underscore_key(k), underscore_hash_keys(v)] }]
    else
      value
    end
  end

  def camelize_hash_keys(value)
    case value
    when Array
      value.map { |v| camelize_hash_keys(v) }
    when Hash
      Hash[value.map { |k, v| [camelize_key(k), camelize_hash_keys(v)] }]
    else
      value
    end
  end

  def underscore_key(k)
    k.to_s.underscore
  end

  def camelize_key(k)
    k.to_s.camelize :lower
  end
end

param_styler.rb

class ParamsStyler
  include HashStyles

  JSON_MEDIA_TYPE = /^application\/json/

  def initialize app
    @app = app
  end

  def call env
    if applicable_to_env?(env) && json_media_type?(env['CONTENT_TYPE'])
      env['action_dispatch.request.request_parameters'] = underscore_hash_keys env['action_dispatch.request.request_parameters']
    end
    status, headers, response = @app.call env
    if applicable_to_env?(env) && json_media_type?(headers['Content-Type'])
      response.body = camelize_hash_keys(JSON.parse(response.body)).to_json
    end
    [status, headers, response]
  end

  def applicable_to_env? env
    env['PATH_INFO'].match /\A\/api\/.*/
  end

  def json_media_type? media_type
    media_type =~ JSON_MEDIA_TYPE
  end
end