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Poncho

Poncho is an API to build APIs or, in other words, a DSL to build REST interfaces.

It'll validate input and output, coerce values and is easily extendable with custom data types.

It's compatible with any rack-based framework, such as Rails or Sinatra.

Installation

Add this line to your application's Gemfile:

gem 'poncho'

And then execute:

$ bundle

Or install it yourself as:

$ gem install poncho

TLDR Usage

class ChargeResource < Poncho::Resource
  param :amount, :type => :integer
  param :currency

  def currency
    super || 'USD'
  end
end

class ChargeCreateMethod < Poncho::JSONMethod
  param :amount, :type => :integer, :required => true
  param :currency, :in => ['USD', 'GBP']

  def invoke
    charge = Charge.new
    charge.amount = param(:amount)
    charge.currency = param(:currency)
    charge.save

    ChargeResource.new(charge)
  end
end

post '/charges', &ChargeCreateMethod

Getting started with Methods

Methods inherit from Poncho::Method and override invoke, where they perform any necessary logic.

In a similar vein to Sinatra, anything returned from invoke is sent right back to the user. You can return a http status code, a body string, or even a Rack response array.

class ChargeListMethod < Poncho::Method
  def invoke
    # Some DB shizzle

    200
  end
end

To invoke the method just add it to your routes.

Using Rails:

match '/users' => UsersListMethod, :via => :get

Using Sinatra:

get '/users', &UsersListMethod

Or invoke manually:

UsersListMethod.call(rack_env)

If you're writing a JSON API, you'll probably want to inherit the Method from Poncho::JSONMethod instead of Poncho::Method, but we'll cover that later.

Params

You can get access to the request params, via the params or param(name) methods.

Before you can use a param though, you need to define it:

param :param_name

By default, param are of type 'string'. you can choose a different type via the :type option:

param :amount, :type => :integer

There are a bunch of predefined types, such as :integer, :array, :boolean_string etc, but you can also easily define your own custom ones (covered later).

Poncho will automatically validate that if a paramter is provided it is in a valid format. Poncho will also handle type conversion for you.

So for example, in the case above, Poncho will automatically validate that the amount param is indeed an Integer or an Integer string, and will coerce the parameter into an integer when you try to access it.

Validation

As well as the default type validation, Poncho lets you validate presence, format, length and much more!

For example, to validate that a :currency parameter is provided, pass in the `:presence' option:

param :currency, :presence => true

To validate that a currency is either 'USD' or 'GBP', use the :in option.

param :currency, :in => ['USD', 'GBP']

The other supported validations out of the box are :format, :not_in, and :length:

param :email, :format => /@/
param :password, :length => 5..20

Custom Validation

You can use a custom validator via the validate method, passing in a block:

validate do
  unless param(:customer_id) ~= /\Acus_/
    errors.add(:customer_id, :invalid_customer)
  end
end

# Or

validates :customer_id, :customer_validate

Alternatively, if your validation is being used in multiple places, you can wrap it up in a class and pass it to the validates_with method.

validates_with CustomValidator

For a good example of how to build validations, see the existing ones.

Custom Params

As your API grows you'll probably start to need custom parameter types. These can be useful to ensure parameters are both valid and converted into suitable values.

To define a custom parameter, simply inherit from Poncho::Param. For example, let's define a new param called CardHashParam. It needs to validate input via overriding the validate_each method, and convert input via overriding the convert method.

module Poncho
  module Params
    class CardHashParam < Param
      def validate_each(method, attribute, value)
        value = convert(value)

        unless value.is_a?(Hash) && value.keys == [:number, :exp_month, :exp_year, :cvc]
          method.errors.add(attribute, :invalid_card_hash, options.merge(:value => value))
        end
      end

      def convert(value)
        value && value.symbolize_keys
      end
    end
  end
end

You can use custom parameters via the :type option.

param :card, :type => Poncho::Params::CardHashParam

# Or the shortcut
param :card, :type => :card_hash

Request & Response

You can gain access to the rack request via the request method, for example:

def invoke
 accept = request.headers['Accept']
 200
end

The same goes for the response object:

def invoke
  response.body = ['Fee-fi-fo-fum']
  200
end

There are some helper methods to set such things as the HTTP status response codes and body.

def invoke
  status 201
  body 'Created!'
end

Method filters

There are various filters you can apply to the request, for example:

class MyMethod < Poncho::Method
  before_validation do
    # Before validation
  end

  before do
    # Before invoke
    p params
  end

  after do
    # After invocation
  end
end

Error responses

You can provide custom responses to exceptions via the error class method.

Pass error a exception type or status code.

class MyMethod < Poncho::Method
  error MyCustomClass do
    'Sorry, something went wrong.'
  end

  error 403 do
    'Not authorized.'
  end
end

JSON APIs

If your API only returns JSON then Poncho has a convenient JSONMethod class which will ensure that all response bodies are converted into JSON and that the correct content type header is set.

class TokenCreateMethod < Poncho::JSONMethod
  param :number, :required => true

  def invoke
    {:token => '123'}
  end
end

JSONMethod also ensures that there's valid JSON error responses to 404s and 500s, as well as returning a JSON error hash for validation errors.

$ curl http://localhost:4567/tokens -d number=
  {"error":{"param":"number","type":"presence"}}

Resources

Resources are wrappers around other classes, such as models, providing a view representation of them.

You can specify attributes to be returned to the client using the same param syntax as documented above.

class Card
  attr_reader :number

  def initialize(number)
    @number = number
  end
end

class CardResource < Poncho::Resource
  param :number
  param :description

  def number
    super[-4..-1]
  end
end

As you can see in the example above, you can override params and return a custom response.

When the Resource instance is converted into JSON the appropriate params will be used and serialized.

class ChargeResource < Poncho::Resource
  param :amount, :type => :integer
  param :currency
  param :card, :resource => CardResource

  def currency
    super || 'USD'
  end
end

class ChargeListMethod < Poncho::JSONMethod
  def invoke
    [
      ChargeResource.new(Charge.new(1000, 'USD')),
      ChargeResource.new(Charge.new(50, 'USD'))
    ]
  end
end

If a particular param points to another resource, you can use the :type => :resource option as demonstrated above.

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