Ruby library for building hypermedia APIs

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The library that understands hypermedia.

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Yaks takes your data and transforms it into hypermedia formats such as HAL, JSON-API, or HTML. It allows you to build APIs that are discoverable and browsable. It is built from the ground up around linked resources, a concept central to the architecture of the web.

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
  • Collection+JSON
  • HTML
  • HALO
  • Transit

Table of Contents


State of Development

Recent focus has been on stabilizing the core classes, improving format support, and increasing test (mutation) coverage. We are committed to a stable public API and semantic version. On the 0.x line the minor version is bumped when non-backwards compatible changes are introduced. After 1.x regular semver conventions will be used.


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

yaks = do
  default_format :hal
  rel_template '{rel}'
  format_options(:hal, plural_links: [:copyright])
  mapper_namespace ::MyAPI
  json_serializer do |data|

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


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

Configure a Yaks instance and start serializing!

yaks =

or a bit more elaborate

yaks = do
  default_format :json_api
  rel_template '{rel}'
  format_options(:hal, plural_links: [:copyright])
end, mapper: ::PostMapper, format: :hal)


Use the attribute or attributes DSL methods 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)

The attribute method may also take a block that will be called with the context of the mapper instance. The default implementation will use the block if provided, otherwise it 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 :body, :date
  attribute :id do

  def date
    object.created_at.strftime("at %I:%M%p")


Mapper can contain form defintions, for formats that support them. The form DSL mimics the HTML5 field and attribute names.

class PostMapper < Yaks::Mapper
  attributes :id, :body, :date

  form :add_comment do
    action '/api/comments'
    method 'POST'
    media_type 'application/json'

    text :body
    hidden :post_id, value: -> { }

TODO: add more info on form element types, attributes, conditional rendering of forms, dynamic form sections, ...


You can override #attributes, or #associations.

class SongMapper < Yaks::Mapper
  attributes :title, :duration, :lyrics

  has_one :artist
  has_one :album

  def minimal?
    env['HTTP_PREFER'] =~ /minimal/

  # @return Array<Yaks::Mapper::Attribute>
  def attributes
    return super.reject {|attr| :lyrics } if minimal?

  # @return Array<Yaks::Mapper::Association>
  def associations
    return [] if minimal?


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 '', '/api/foo/{id}/comments'

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 '', '/api/line_items?product_id={product_id}&quantity={quantity}', expand: [:product_id]

# "_links": {
#    "": {
#      "href": "/api/line_items?product_id=273&quantity={quantity}",
#      "templated": true
#    }
# }

You can pass a proc instead of a template, in that case the proc will be resolved in the context of the mapper. What this means is that, if the proc takes no arguments, it will be evaluated with the mapper instance as the value of self. If the proc does take an argument, then it will receive the mapper instance, and will be evaluated as a closure, i.e. with access to the scope in which it was defined.

class FooMapper < Yaks::Mapper
  link '', -> { home_url }
  # by default calls object.home_url

  def home_url

To only include links based on certain conditions, add an :if option, passing it a block. The block will be resolved in the context of the mapper, as explained before.

For example, say you want to notify the consumer of your API that upon confirming an order, the previously held cart is no longer valid, you could use the IANA standard invalidates rel to communicate this.

class OrderMapper < Yaks::Mapper
  link :invalidates, '/api/cart', if: ->{ env['api.invalidate_cart'] }


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


  • :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
  • :if: Only render the association if a condition holds
  • :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 }


Yaks provides mixins to change how your mappers work. These need to be required separately, they are not loaded by default.


You may choose to not render associations by default, but to only do so when the client explicitly asks for them. This can be done by including Yaks::Behaviour::OptionalIncludes.

Which associations to load is specified with the the include query parameter. You can use dots to load nested associated.

require "yaks/behaviour/optional_includes"

class PostMapper < Yaks::Mapper
  include Yaks::Behaviour::OptionalIncludes

  has_one :author
  has_many :comments

class AuthorMapper < Yaks::Mapper
  include Yaks::Behaviour::OptionalIncludes

  has_one :profile
GET /post/42?include=comments,author.profile

Note that this will only work when Yaks has access to the Rack environment. When using an existing integration like yaks-sinatra this will be handled for you.

To force an association to always be included, override its if condition to always return true.

require "yaks/behaviour/optional_includes"

class PostMapper < Yaks::Mapper
  include Yaks::Behaviour::OptionalIncludes

  has_one :author
  has_many :comments, if: ->{ true }

Calling Yaks

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.

Rack env

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

class FooMapper < Yaks::Mapper
  attributes :bar

  def bar
    if env['something']

yaks =, env: my_env)

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 the policy section. If you have your mappers inside a module, use mapper_namespace.

module API
  module Mappers
    class PostMapper < Yaks::Mapper

yaks = do
  mapper_namespace API::Mappers

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, ''

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, ''
      attributes :total

      def total
        collection.inject(0) do |memo, line_item|
          memo + line_item.price * line_item.quantity

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

Custom attribute/link/subresource handling

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,

    case object
    when AllocationException
      attrs[:http_code] = 422
    when ActiveRecord::RecordNotFound
      attrs[:http_code] = 404
      attrs[:type] = "record_not_found"


Resources, Formatters, Serializers

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

Finally a serializer will take this document structure and turn it into a string. For JSON documents the intermediate format consists of Ruby primitives like arrays and hashes. HTML/XML based formats on the other hand return a Hexp::Node.

For JSON based format there's an extra step between format and serialize called primitivize, this way Ruby objects which don't have an equivalent in the JSON spec, like Symbol or Date, can be turned into objects that are representable in JSON. See Primitiver.


Below follows a brief overview of formats that are available in Yaks. The maturity of these formats varies, since we depend on people that use a certain format actively to contribute. Implementing formats is in generally straightforward, and consists mostly of deciding how the attributes, links, forms, of a Yaks::Resource should be represented. Depending on the format this might be a subject for debate. We welcome these discussions, and if your opinion differs from what ends up in Yaks, it should be trivial to change these representations for your use case.


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 = do
  format_options :hal, plural_links: ['']

CURIEs are not explicitly supported (yet), but it's possible to use them with some manual effort.

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.

Yaks also has a derived format called HALO, which is a non-standard extension to HAL which includes form elements.


The hypermedia format par excellence. Yaks can generate a version of your API, including links and forms, that is usable straight from a standard web browser. This allows API interactions to be developed and tested independent from any client application.

If you let Yaks handle your content type negotiation (i.e. pass it the rack env, and honour the content type it detects, see integration, simply opening a browser and pointing it at your API entry point should do the trick.

  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 < Yaks::Mapper
  type :pet

Or the policy can be overridden:

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

For optional includes, see Yaks::Behaviour::OptionalIncludes.


Collection+JSON has support for write templates. To use them, the :template option can be used. It will map the specified form to a CJ template. Please notice that CJ only allows one template per representation. do
  default_format :collection_json

  collection_json = do
    format_options :collection_json, template: :my_template_form

class PostMapper < Yaks::Mapper
  form :my_template_form do
    # This will be used for template

  form :not_my_template do
    # This won't be used for template

Subresources aren't mapped because Collection+JSON doesn't really have that concept.


There is experimental support for Transit. The transit gem handles serialization internally, so there is no intermediate document. The format step already returns the serialized string.


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 = do
  # Automatically give every resource a self link
  after :map, :add_self_link do |resource|
    resource.add_link(, "/#{resource.type}/#{resource.attributes[:id]}"))

  # Skip serialization, so Ruby primitives come back instead of JSON
  # This was the default before versions < 0.5.0
  skip :serialize

Policy over Configuration

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 = do
  mapper_for Post, SpecialMapper

  derive_mapper_from_object do |model|
    # ...

  derive_mapper_from_collection do |collection|
    # ...

  derive_mapper_from_item do |model|
    # ...

  derive_type_from_mapper_class do |mapper_class|
    # ...

  derive_mapper_from_association do |association|
    # ...

  derive_rel_from_association do |mapper, association|
    # ...

Note that within these blocks, you may call super() which would call the default implementation.

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

class MyPolicy < Yaks::DefaultPolicy

yaks = do
  policy_class MyPolicy


This is called when trying to serialize something and no explicit mapper is given. To recap, it's always possible to be explicit, e.g., mapper: WidgetMapper), mapper: MyCollectionMapper, item_mapper: WidgetMapper)

If the mapper is left unspecified, Yaks will inspect whatever you pass it. First it will test the given object against the mappings defined using mapper_for. If no mapper is found, it will call derive_mapper_from_item or derive_mapper_from_collection depending on whether the given object is a collection or not. If the object responds to to_ary it is considered a collection.


This method allows you to define a one-to-one mapping between a mapping rule and a mapper class. During the lookup, Yaks will check if any mapping rule matches the given object using the #=== operator.

Here are a few examples on how to use it:

yaks = do
  mapper_for(:home, HomeMapper)
  mapper_for(Post, SpecialMapper)
  mapper_for(->(author) { author.respond_to?(:name) && == 'doh' }, AuthorMapper)
end # would map using HomeMapper # would map using PostMapper 'doh')) # would map using AuthorMapper


This method will try various constant lookups based on naming. These all happen in the configured namespace, which defaults to the Ruby top level.

If the first object in the collection has a class of Widget, and the configured namespace is API, then these are tried in turn

  • API::WidgetCollectionMapper
  • API::CollectionMapper
  • Yaks::CollectionMapper

Note that Yaks can only find a specific collection mapper for a type if the collection passed to Yaks contains at least one element. If it's important that empty collections are handled by the right mapper (e.g. to set a specific self or profile link), then you have to be explicit.


When using this method, the lookup happens based on the class name, and will traverse up the class hierarchy in the configured namespace if no suitable mapper is found. Take the following code:

module Stuff
  class Thing ; end
  class Widget < Thing ; end

The lookup we'll be done as followed.

  • If the namespace option is set (to Mappers for example):

    • Mappers::Stuff::WidgetMapper
    • Mappers::Stuff::ThingMapper
    • Mappers::Stuff::ObjectMapper
    • Mappers::Stuff::BasicObjectMapper
    • Mappers::WidgetMapper
    • Mappers::ThingMapper
  • If the namespace option is not set:

    • Stuff::WidgetMapper
    • Stuff::ThingMapper
    • Stuff::ObjectMapper
    • Stuff::BasicObjectMapper
    • WidgetMapper
    • ThingMapper

If none of these are found an error is raised.


When no mapper is specified for an association, then this method is called to find the right mapper, based on the association name. In case of has_many collections this is the "item mapper", the collection mapper is resolved using derive_mapper_from_object.

By default the mapper class is derived from the name of the association, e.g.

has_many :widgets #=> WidgetMapper
has_one :widget   #=> WidgetMapper

It is always possible to explicitly set a mapper.

has_one :widget, mapper: FooMapper
has_many :widgets, collection_mapper: MyCollectionMapper, mapper: FooMapper


Associations have a "rel", an IANA registered identifier or fully qualified URI, that specifies how the object relates to the parent document.

When configuring Yaks one can set a rel_template, that will be used to generate these rels if not explicitly given. The rel placeholder in the template will be substituted with the association name.

yaks = do
  rel_template "{rel}"

class MyMapper < Yaks::Mapper
  # rel: ""
  has_many :widgets

  # rel: ""
  has_one :widget


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

Here's an example with a custom Currency class, which can be represented as an integer. do
  map_to_primitive Currency do |currency|

One notable use case is representing dates and times. The JSON specification does not define any syntax for these, so the only solution is to represent them either as numbers or strings. If you're not sure what to do with these then the ISO8601 standard is a safe bet. It defines a way to represent times and dates as strings, and is also adopted by the W3C in RFC3339.

An alternative representation that is sometimes used is "unix time", defined as the numbers of seconds passed since 1 January 1970.

Here's an example for a Rails app, so including ActiveSupport's TimeWithZone. do
  map_to_primitive Date, Time, DateTime, ActiveSupport::TimeWithZone, &:iso8601

map_to_primitive 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. do
  map_to_primitive Hamster::Vector, Hamster::List do |list| do |item|

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


It is recommended to let Yaks handle the negotiation of media types, so that consumer can request the format they prefer using an Accept: header. To do this requires two steps: first make sure you pass the rack env to Yaks, this way it will detect any Accept header and honor it. While this is enough to get the correct serialized output, it will likely be served up with the wrong Content-Type header by your web framework.

To fix this, ask Yaks first for the "runner" for a given input, then get the media type and serialized resource from the runner.

# Tell your web framework about the supported formats
Yaks::Format.all.each do |format|
  mime_type format.format_name, format.media_type

# one time Yaks configuration
yaks =

# on each request
runner = yaks.runner(post, env: rack_env)
format = runner.format_name
output =

Real World Usage

Yaks is used in production by

  • Ticketsolve. You can find an example API endpoint here.
  • Advertile Mobile for their product AppBounty (internal API)


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


See the cookbook for some usage examples taking from a real world app.

Standards Based

Yaks is based on internet standards, including

How to contribute

Run the tests, the examples, try it with your own stuff and leave your impressions in the issues.

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