Use HAL APIs easily
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Latest commit 44ccc1b Dec 1, 2016 @pezra committed on GitHub Merge pull request #61 from pezra/refactor-links
Refactor links

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An easy to use client interface for REST APIs that use HAL.


The first step in using a HAL based API is getting a representation of one of its entry points. The simplest way to do this is using the get class method of HalClient.

blog = HalClient.get("")
# => #<Representation:>

HalClient::Representations expose a #property method to retrieve a single property from the HAL document.'title')
#=> "Some Person's Blog"

They also expose a #properties method to retrieve all the properties from the document, as a Hash.
#=> {"title"=>"Some Person's Blog", "description"=>"Some description"}

Link navigation

Once we have a representation we will want to navigate its links. This can be accomplished using the #related method.

articles = blog.related("item")
# => #<RepresentationSet:...>

In the example above item is the link rel. The #related method extracts embedded representations and link hrefs with the specified rel. The resulting representations are packaged into a HalClient::RepresentationSet. HalClient always returns RepresentationSets when following links, even when there is only one result. RepresentationSets are Enumerable so they expose all your favorite methods like #each, #map, #any?, etc. RepresentationSets expose a #related method which calls #related on each member of the set and then merges the results into a new representation set.

all_the_authors = blog.related("author").related("item")"name")
# => "Bob Smith"

Request evaluation order

If the author relationship was a regular link (that is, not embedded) in the above example the HTTP GET to retrieve Bob's representation from the server does not happen until the #property method is called. This lazy dereferencing allows for working more efficiently with larger relationship sets.


Links specified using a compact URI (or CURIE) as the rel are fully supported. They are accessed using the fully expanded version of the curie. For example, given a representations of an author:

{ "name": "Bob Smith,
  "_links": {
    "so:homeLocation": { "href": "" },
    "curies": [{ "name": "so", "href": "{rel}", "templated": true }]

Bob's home location can be retrieved with

# => #<Representation:>

Links are always accessed using the full link relation, rather than the CURIE, because the document producer can use any arbitrary string as the prefix. This means that clients must not make any assumptions regarding what prefix will be used because it might change over time or even between documents.

Templated links

The #related methods takes a Hash as its second argument which is used to expand any templated links that are involved in the navigation.

old_articles = blog.related("index", before: "2013-02-03T12:30:00Z")
# => #<RepresentationSet:...>

Assuming there is a templated link with a before variable this will result in a request being made to, the response parsed into a HalClient::Representation and that being wrapped in a representation set. Any options for which there is not a matching variable in the link's template will be ignored. Any links with that rel that are not templates will be dereferenced normally.


All HalClient::Representations exposed an #href attribute which is its identity. The value is extracted from the self link in the underlying HAL document.

blog.href # => ""

Hash like interface

Representations expose a Hash like interface. Properties, and related representations can be retrieved using the #[] and #fetch method.

blog['title'] # => "Some Person's Blog"
blog['item']  # =>  #<RepresentationSet:...>

Paged collections

HalClient provides a high level abstraction for paged collections encoded using standard item, next and prev link relations.

articles = blog.to_enum
articles.each do |an_article|
  # do something with each article representation

If the collection is paged this will navigate to the next page after yielding all the items on the current page. The return is an Enumerable so all your favorite collection methods are available.


HalClient supports PUT/POST/PATCH requests to remote resources via it's #put, #post and #patch methods, respectively.

#=> #<Representation:>
#=> #<Representation:>

#=> #<Representation:>

The first argument to #put, #post and #patch may be a String, a Hash or any object that responds to #to_hal. Additional options may be passed to change the content type of the post, etc.

PUT requests

HalClient supports PUT requests to remote resources via it's #put method.

#=> #<Representation:>

The argument to post may be String or any object that responds to #to_hal. Additional options may be passed to change the content type of the post, etc.

Editing representation

HalClient supports editing of representations. This is useful when creating resources from a template or updating resources. For example, consider a resource whose "author" relationship we want to update to point the author's new profile page.

doc = HalClient.get("")
improved_doc =                              # create an editor
    .reject_related("author") { |it|"name") == "John Doe"} # unlink Johe Doe's old page
    .add_link("author", "")                  # add link to his new page

doc.put(improved_doc)                                                   # save changes to server

This removes the obsolete link to "John Doe" from the documents list of authors and replaces it with the correct link then performs an HTTP PUT request with the updated document.

Custom media types

If the API uses one or more a custom mime types we can specify that they be included in the Accept header field of each request.

my_client = "application/vnd.myapp+hal+json")
# => #<Representation:>

Similarly we can set the default Content-Type for post requests.

my_client = "application/vnd.myapp+hal+json",
                          content_type: "application/vnd.myapp+hal+json")

Parsing representations on the server side

HalClient can be used by servers of HAL APIs to interpret the bodies of requests. For example,

new_post_repr = JSON.load(request.raw_post))
author = Author.by_href(new_post_repr.related('author').first.href)
new_post = title: new_post_repr['title'], author: author, #...

Created this way the representation will not dereference any links (because it doesn't have a HalClient) but it will provide HalClient::Representations of both embedded and linked resources.


Add this line to your application's Gemfile:

gem 'hal-client'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hal-client

Upgrading from 3.x to 4.x

Support for ruby 2.0 has been removed. Please upgrade to ruby 2.1 or later. No other breaking changes were made.

Upgrading from 2.x to 3.x

For most uses no change to client code is required. At 3.0 the underlying HTTP library changed to to better support our parallelism needs. This changes the interface of #get and #post on HalClient and HalClient::Representation in the situation where the response is not a valid HAL document. In those situations the return value is now a HTTP::Response object, rather than a RestClient::Response.

Upgrading from 1.x to 2.x

The signature of HalClient::Representation#new changed such that keyword arguments are required. Any direct uses of that method must be changed. This is the only breaking change.


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Implement your improvement
  4. Update lib/hal_client/version.rb following semantic versioning rules
  5. Commit your changes (git commit -am 'Add some feature')
  6. Push to the branch (git push origin my-new-feature)
  7. Create new Pull Request