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

ActiveSupport already provides a String#snakecase method. All you have to do is install a filter that does a deep iteration through the params hash and replaces the keys with key.snakecase.

before_filter :deep_snake_case_params!

def deep_snake_case_params!(val = params)
  case val
  when Array
    val.map {|v| deep_snake_case_params! v }
  when Hash
    val.keys.each do |k, v = val[k]|
      val.delete k
      val[k.snakecase] = deep_snake_case_params!(v)
    end
    val
  else
    val
  end
end

Answer 2

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.

Answer 3

You can create a filter that runs before any controller call and apply the following instructions to it:

# transform camel case string into snake case
snake_string  = Proc.new {|s| s.gsub(/([a-z])([A-Z])/) {|t| "#{$1}_#{$2.downcase}"}} 

# transform all hash keys into snake case
snake_hash    = Proc.new do |hash| 
  hash.inject({}) do |memo, item|
    key, value = item

    key = case key
          when String
            snake_string.call(key)
          when Symbol
            snake_string.call(key.to_s).to_sym
          else 
            key
          end    

    memo[key] = value.instance_of?(Hash) ? snake_hash.call(value) : value
    memo
  end
end

params = snake_hash.call(params)

You must have to consider the above procedure will impose a small overhead on every Rails call.

I am not convinced this is necessary, if it is just to fit in a convention.

Answer 4

Riffing on Eliot Sykes's answer above, I think we can do a bit better in the Rails5 case. I don't love overwriting that function entirely, since that code could change. So instead I suggest using function composition:

# 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] = (
  # Compose the original parser with a transformation
  ActionDispatch::Request.parameter_parsers[:json] >>
    # Transform camelCase param keys to snake_case
    ->(data) {
      data.deep_transform_keys(&:underscore)
    }
)

Answer 5

In Rails 6.1 will be added deep_transform_keys to ActionController::Parameters so it enables you to make it as simple as:

class ApplicationController < ActionController::Base
  before_action :underscore_params!

  private

  def underscore_params!
    params.deep_transform_keys!(&:underscore)
  end
end

Edit

At the moment you can backport as follows:

module DeepTransformKeys
  def deep_transform_keys!(&block)
    @parameters.deep_transform_keys!(&block)
    self
  end
end

ActionController::Parameters.include(DeepTransformKeys)

Answer 6

tlewin's answer didn't work for me in Rails 3. it seems the params' = operator renders future operations on it void. very strange. anyways the following works for me, as it only uses the []= and delete operators:

before_filter :underscore_param_keys
def underscore_param_keys
  snake_hash = ->(hash) {
    # copying the hash for iteration so we are not altering and iterating over the same object
    hash.to_a.each do |key, value|
      hash.delete key
      hash[key.to_s.underscore] = value
      snake_hash.call(value) if value.is_a? Hash
      value.each { |item| snake_hash.call(item) if item.is_a? Hash } if value.is_a? Array
    end
  }
  snake_hash.call(params)
end