Define your request and response JSON format with a Rack middleware!
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Little Sparrow by Apofiss

A Rack middleware for converting the params keys and JSON response keys of a Rack application.

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Add this line to your application's Gemfile:

gem 'cp-sparrow'

And then execute:

$ bundle

Or install it yourself as:

$ gem install cp-sparrow


If you're using Rails, that's it. You haven't to do anything more. If you're not using Rails, you will have to add to your

require 'sparrow'

use Sparrow::RequestMiddleware
use Sparrow::ResponseMiddleware


There are various configuration options as well as HTTP headers available to make the middleware act as you want it to.

in your configuration file (such as application.rb if you are using Rails)

Sparrow.configure do |config|
  config.excluded_routes = ['api/model/certificates')

Options may be:


Array(String|Regexp). Default: []

An Array of Strings and/or Regexps defining which paths should not be touched by the middleware. The entries should matchs paths for your application. They should not start with a leading slash.

Example: config.excluded_routes = ['api/model/certificates']


String. Default: underscore

Defines how the middleware should treat incoming parameters via Request. Which means how they get tranformed, i.e. defining underscore here means that incoming parameters get underscore. Possible values are underscore and camelize

Example: config.default_json_request_key_transformation_strategy = "underscore"


String. Default: camelize

Same as default_json_request_key_transformation_strategy, but for responses. I.e. this defines to which format the keys get transformed when the response gets sent.

Example: config.default_json_response_key_transformation_strategy = "camelize"


String. Default: request-json-format

Defines the HTTP Header key which sets the request transformation strategy as in default_json_request_key_transformation_strategy*. This definition has higher priority than the default definition. Any valid HTTP Header String value is possible. Defaults to request-json-format.


config.json_request_format_header = "request-json-format"


String. Default: response-json-format

Same as json_request_format_header, but for the response handling. Defaults to response-json-format

Example: config.json_response_format_header = "response-json-format"


Boolean. Default: true

A boolean that indicates to not camelize keys that are all Uppercase, like CountryCodes "EN", etc.

Example: config.camelize_ignore_uppercase_keys = true


Array(String). Default: %w[application/json application/x-www-form-urlencoded text/x-json]

A list of HTTP content types upon which the middleware shall trigger and possibly start conversion. Defaults to ['application/json', 'application/x-www-form-urlencoded', 'text/x-json']. If nil is present in the list, requests/responses with no Content-Type header will be processed as well. Possible values can also be the start of the content-type-header like application/ which matches everything that starts with application/ like application/json

Example: config.allowed_content_types = ['application/json', 'text/x-json']


Array(String). Default: ['application/json', 'application/x-www-form-urlencoded', 'text/x-json', nil]

Same as [allowed_content_types], but reacts to the HTTP Accept Header. Applies to the same possible options, behavior. Defaults to the same set of MIME types, but also includes nil by default, which ignores checking the Accept header in default configuration

Example: config.allowed_accepts = ['application/json', 'text/x-json']


Boolean. Default: false

Determines logging while the middleware is active. Primary output is which conversion strategy gets chosen upon which request/response.

Example: config.enable_logging = true


Symbol. Default: :lower

Defines which strategy to use when camelizing keys. Possible options are :lower and :upper, which tells the middleware to start camelized keys with an uppercased character or with a lowercased character.

Example: config.camelize_strategy = :lower


Just run rake and you are off to go. This runs the whole suite including specs for unit- & integrationtests for Rails and Rack including different versions of Rails.

If you want to test a specific version of Rails:

export RAILS_VERSION=3.2.17; bundle update; bundle exec rspec