The library that understands hypermedia.

Yaks consists of a resource representation that is independent of any output type. A Yaks mapper transforms an object into a resource, which can then be serialized into whichever output format the client requested. These formats are presently supported:

• HAL
• JSON API
• Collection+JSON
• HTML
• HALO

This library came into existence because we at Ticketsolve wanted to build a rich hypermedia API, and were dissatisfied with any of the existing Ruby solutions at the time (November 2013).

We are still actively expanding our API, and with that Yaks continues to grow and improve. The aim is to create the reference for supporting hypermedia formats in Ruby.

Yaks can be used in production today, as we do, but until 1.0 is released there will regularly be breaking changes, as we figure out the best way to do things. These are all documented clearly in the changelog. At this point we recommend locking to an exact version number.

Yaks is a processing pipeline, you create and configure the pipeline, then feed data through it.

yaks = Yaks.new do default_format :hal rel_template 'http://api.example.com/rels/{rel}' format_options(:hal, plural_links: [:copyright]) namespace ::MyAPI json_serializer do |data| MultiJson.dump(data) end end yaks.call(data) # => '{"foo": "bar", "_links": { ... }}'

Yaks performs this serialization in three steps

• It maps your data to a Yaks::Resource
• It formats the resource to a syntax tree representation
• It serializes to get the final output

For JSON types, the "syntax tree" is just a combination of Ruby primitives, nested arrays and hashes with strings, numbers, booleans, nils.

A Resource is an abstraction shared by all output formats. It can contain key-value attributes, RFC5988 style links, and embedded sub-resources.

To build an API you create a "mapper" for each type of object you want to represent. Yaks takes care of the rest.

For all configuration options see Yaks::Config::DSL.

See also the API Docs on rdoc.info

Say your app has a Post object for blog posts. To serve posts over your API, define a PostMapper

class PostMapper < Yaks::Mapper link :self, '/api/posts/{id}' attributes :id, :title has_one :author has_many :comments end

Configure a Yaks instance and start serializing!

yaks = Yaks.new yaks.call(post)

or a bit more elaborate

yaks = Yaks.new do default_format :json_api rel_template 'http://api.example.com/rels/{rel}' format_options(:hal, plural_links: [:copyright]) end yaks.call(post, mapper: PostMapper, format: :hal)

Use the attributes DSL method to specify which attributes of your model you want to expose, as in the example above. You can override the load_attribute method to change how attributes are fetched from the model.

For example, if you are representing data that is stored in a Hash, you could do

class PostHashMapper < Yaks::Mapper attributes :id, :body # @param name [Symbol] def load_attribute(name) object[name] end end

The default implementation will first try to find a matching method for an attribute on the mapper itself, and will then fall back to calling the actual model. So you can add extra 'virtual' attributes like so :

class CommentMapper < Yaks::Mapper attributes :id, :body, :date def date object.created_at.strftime("at %I:%M%p") end end

You can override #attributes, or #associations.

class SongMapper attributes :title, :duration, :lyrics has_one :artist has_one :album def minimal? env['HTTP_PREFER'] =~ /minimal/ end # @return Array<Yaks::Mapper::Attribute> def attributes return super.reject {|attr| attr.name.equal? :lyrics } if minimal? super end # @return Array<Yaks::Mapper::Association> def associations return [] if minimal? super end end

You can specify link templates that will be expanded with model attributes. The link relation name should be a registered IANA link relation or a URL. The template syntax follows RFC6570 URI templates.

class FooMapper < Yaks::Mapper link :self, '/api/foo/{id}' link 'http://api.foo.com/rels/comments', '/api/foo/{id}/comments' end

To prevent a link to be expanded, add expand: false as an option. Now the actual template will be rendered in the result, so clients can use it to generate links from.

To partially expand the template, pass an array with field names to expand. e.g.

class ProductMapper < Yaks::Mapper link 'http://api.foo.com/rels/line_item', '/api/line_items?product_id={product_id}&quantity={quantity}', expand: [:product_id] end # "_links": { # "http://api.foo.com/rels/line_item": { # "href": "/api/line_items?product_id=273&quantity={quantity}", # "templated": true # } # }

You can pass a symbol instead of a template, in that case the symbol will be used as a method name on the object to retrieve the link. You can override this behavior just like with attributes.

class FooMapper < Yaks::Mapper link 'http://api.foo.com/rels/go_home', :home_url # by default calls object.home_url def home_url object.setting('home_url') end end

Use has_one for an association that returns a single object, or has_many for embedding a collection.

Options

• :mapper : Use a specific for each instance, will be derived from the class name if omitted (see Policy vs Configuration)
• :collection_mapper : For mapping the collection as a whole, this defaults to Yaks::CollectionMapper, but you can subclass it for example to add links or attributes on the collection itself
• :rel : Set the relation (symbol or URI) this association has with the object. Will be derived from the association name and the configured rel_template if ommitted
• :link_if: Conditionally render the association as a link. A :href option is required

class ShowMapper < Yaks::Mapper has_many :events, href: '/show/{id}/events', link_if: ->{ events.count > 50 } end

Once you have a Yaks instance, you can call it with call (serialize also works but might be deprecated in the future.) Pass it the data to be serialized, plus options.

• :env a Rack environment, see next section
• :format the format to be used, e.g. :json_api. Note that if the Rack env contains an Accept header which resolves to a recognized format, then the header takes precedence
• :mapper the mapper to be used. Will be inferred if omitted
• :item_mapper When rendering a collection, the mapper to be used for each item in the collection. Will be inferred from the class of the first item in the collection if omitted.

When serializing, Yaks lets you pass in an env hash, which will be made available to all mappers.

yaks = Yaks.new yaks.call(foo, env: my_env) class FooMapper attributes :bar def bar if env['something'] #... end end end

The env hash will be available to all mappers, so you can use this to pass around context. In particular context related to the current HTTP request, e.g. the current logged in user, which is why the recommended use is to pass in the Rack environment.

If env contains a HTTP_ACCEPT key (Rack's way of representing the Accept header), Yaks will return the format that most closely matches what was requested.

Yaks by default will find your mappers for you if they follow the naming convention of appending 'Mapper' to the model class name. This (and all other "conventions") can be easily redefined though, see below. If you have your mappers inside a module, use namespace.

module API module Mappers class PostMapper < Yaks::Mapper #... end end end yaks = Yaks.new do namespace API::Mappers end

If your namespace contains a CollectionMapper, Yaks will use that instead of Yaks::CollectionMapper, e.g.

module API module Mappers class CollectionMapper < Yaks::CollectionMapper link :profile, 'http://api.example.com/profiles/collection' end end end

You can also have collection mappers based on the type of members the collection holds, e.g.

module API module Mappers class LineItemCollectionMapper < Yaks::CollectionMapper link :profile, 'http://api.example.com/profiles/line_items' attributes :total def total collection.inject(0) do |memo, line_item| memo + line_item.price * line_item.quantity end end end end end

Yaks will automatically detect and use this collection when serializing an array of LineItem objects.

When inheriting from Yaks::Mapper, you can override map_attributes, map_links and map_resources to skip (or augment) above methods, and instead implement your own custom mechanism. These methods take a Yaks::Resource instance, and should return an updated resource. They should not alter the resource instance in-place. For example

class ErrorMapper < Yaks::Mapper link :profile, '/api/error' def map_attributes(resource) attrs = { http_code: 500, message: object.to_s, type: object.class.name.underscore } case object when AllocationException attrs[:http_code] = 422 when ActiveRecord::RecordNotFound attrs[:http_code] = 404 attrs[:type] = "record_not_found" end resource.update_attributes(attrs) end end

Yaks uses an intermediate "Resource" representation to support multiple output formats. A mapper turns a domain model into a Yaks::Resource. A serializer (e.g. Yaks::Serializer::Hal) takes the resource and outputs the structure of the target format.

Since version 0.4 the recommended API is through Yaks.new {...}.serialize. This will give you back a composite value consisting of primitives that have a mapping to JSON, so you can use your favorite JSON encoder to turn this into a character stream.

my_yaks = Yaks.new hal = my_yaks.call(model) puts JSON.dump(hal)

There are at least a handful of JSON libraries and implementations for Ruby out there, with different trade-offs. Yaks does not impose an opinion on which one to use

This is the default. In HAL one decides when building an API which links can only be singular (e.g. self), and which are always represented as an array. Yaks defaults to singular as I've found it to be the most common case. If you want specific links to be plural, then configure their rel href as such.

hal = Yaks.new do format_options :hal, plural_links: ['http://api.example.com/rels/foo'] end

CURIEs are not explicitly supported (yet), but it's possible to use them with some effort, see examples/hal01.rb for an example.

The line between a singular resource and a collection is fuzzy in HAL. To stick close to the spec you're best to create your own singular types that represent collections, rather than rendering a top level CollectionResource.

default_format :json_api

JSON-API has no concept of outbound links, so these will not be rendered. Instead the key will be inferred from the mapper class name by default. This can be changed per mapper:

class AnimalMapper key :pet end

Or the policy can be overridden:

yaks = Yaks.new do derive_type_from_mapper_class do |mapper_class| piglatinize(mapper_class.to_s.sub(/Mapper$/, '')) end end

default_format :collection_json

Subresources aren't mapped because Collection+JSON doesn't really have that concept, and the other way around templates and queries don't exist (yet) in Yaks.

It is possible to hook into the Yaks pipeline to perform extra processing steps before, after, or around each step. It also possible to skip a step.

yaks = Yaks.new do # Automatically give every resource a self link after :map, :add_self_link do |resource| resource.add_link(Yaks::Resource::Link.new(:self, "/#{resource.type}/#{resource.attributes[:id]}")) end # Skip serialization, so Ruby primitives come back instead of JSON # This was the default before versions < 0.5.0 skip :serialize end

It's an old adage in the Ruby/Rails world to have "Convention over Configuration", mostly to derive values that were not given explicitly. Typically based on things having similar names and a 1-1 derivable relationship.

This saves a lot of typing, but for the uninitiated it can also create confusion, the implicitness makes it hard to follow what's going on.

What's worse, is that often the Configuration part is skipped entirely, making it very hard to deviate from the Golden Standard.

There is another old adage, "Policy vs Mechanism". Implement the mechanisms, but don't dictate the policy.

In Yaks whenever missing values need to be inferred, like finding an unspecified mapper for a relation, this is handled by a policy object. The default is Yaks::DefaultPolicy, you can go there to find all the rules of inference. Single rules of inference can be redefined directly in the Yaks configuration:

yaks = Yaks.new do derive_mapper_from_object do |model| # ... end derive_type_from_mapper_class do |mapper_class| # ... end derive_mapper_from_association do |association| # ... end derive_rel_from_association do |mapper, association| # ... end end

You can also subclass or create from scratch your own policy class

class MyPolicy < DefaultPolicy #... end yaks = Yaks.new do policy MyPolicy end

For JSON based formats, the "syntax tree" is merely a structure of Ruby primitives that have a JSON equivalent. If your mappers return non-primitive attribute values, you can define how they should be converted. For example, JSON has no notion of dates. If your mappers return these types as attributes, then Yaks needs to know how to turn these into primitives. To add extra types, use map_to_primitive

Yaks.new do map_to_primitive Date, Time, DateTime do |date| date.iso8601 end end

This can also be used to transform alternative data structures, like those from Hamster, into Ruby arrays and hashes. Use call() to recursively turn things into primitives.

Yaks.new do map_to_primitive Hamster::Vector, Hamster::List do |list| list.map do |item| call(item) end end end

Yaks by default "primitivizes" symbols (as strings), and classes that include Enumerable (as arrays).

Yaks is used in production by Ticketsolve. You can find an example API endpoint here.

You can find an outdated example app at Yakports, or browse the HAL api directly using the HAL browser.

Yaks is based on internet standards, including

• RFC4288 Media types
• RFC5988 Web Linking
• RFC6906 The "profile" link relation
• RFC6570 URI Templates
• RFC4229 HTTP Header Field Registrations.

Run the tests, the examples, try it with your own stuff and leave your impressions in the issues. Or discuss on API-craft.

To fix a bug

1. Fork the repo
2. Fix the bug, add tests for it
3. Push it to a named branch
4. Add a PR

To add a feature

1. Open an issue as soon as possible to gather feedback
2. Same as above, fork, push to named branch, make a pull-request

Yaks uses Mutation Testing. Run rake mutant and look for percentage coverage. In general this should only go up.

MIT License (Expat License), see LICENSE