docs/general/serializers

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Serializers

Given a serializer class:

class SomeSerializer < ActiveModel::Serializer
end

The following methods may be defined in it:

Attributes

::attributes

Serialization of the resource title and body

In Serializer #attributes
attributes :title, :body { title: 'Some Title', body: 'Some Body' }
attributes :title, :body
def body "Special #{object.body}" end		 { title: 'Some Title', body: 'Special Some Body' }

::attribute

Serialization of the resource title

In Serializer #attributes
attribute :title { title: 'Some Title' }
attribute :title, key: :name { name: 'Some Title' }
attribute :title { 'A Different Title'} { title: 'A Different Title' }
attribute :title
def title 'A Different Title' end		 { title: 'A Different Title' }

PR please for conditional attributes:)

Associations

::has_one

e.g.

has_one :bio
has_one :blog, key: :site
has_one :maker, virtual_value: { id: 1 }

has_one :blog do |serializer|
  serializer.cached_blog
end

def cached_blog
  cache_store.fetch("cached_blog:#{object.updated_at}") do
    Blog.find(object.blog_id)
  end
end

::has_many

e.g.

has_many :comments
has_many :comments, key: :reviews
has_many :comments, serializer: CommentPreviewSerializer
has_many :reviews, virtual_value: [{ id: 1 }, { id: 2 }]
has_many :comments, key: :last_comments do
  last(1)
end

::belongs_to

e.g.

belongs_to :author, serializer: AuthorPreviewSerializer
belongs_to :author, key: :writer
belongs_to :post
belongs_to :blog
def blog
  Blog.new(id: 999, name: 'Custom blog')
end

Polymorphic Relationships

Polymorphic relationships are serialized by specifying the relationship, like any other association. For example:

class PictureSerializer < ActiveModel::Serializer
  has_one :imageable
end

For more context, see the tests for each adapter.

Caching

::cache

e.g.

cache key: 'post', expires_in: 0.1, skip_digest: true
cache expires_in: 1.day, skip_digest: true
cache key: 'writer', skip_digest: true
cache only: [:name], skip_digest: true
cache except: [:content], skip_digest: true
cache key: 'blog'
cache only: [:id]

#cache_key

e.g.

# Uses a custom non-time-based cache key
def cache_key
  "#{self.class.name.downcase}/#{self.id}"
end

Other

::type

The ::type method defines the JSONAPI type that will be rendered for this serializer.
It either takes a String or Symbol as parameter.

Note: This method is useful only when using the :json_api adapter.

Examples:

class UserProfileSerializer < ActiveModel::Serializer
  type 'profile'
end
class AuthorProfileSerializer < ActiveModel::Serializer
  type :profile
end

With the :json_api adapter, the previous serializers would be rendered as:

{
  "data": {
    "id": "1",
    "type": "profile"
  }
}

::link

link :self do
  href "https://example.com/link_author/#{object.id}"
end
link :author { link_author_url(object) }
link :link_authors { link_authors_url }
link :other, 'https://example.com/resource'
link :posts { link_author_posts_url(object) }

#object

The object being serialized.

#root

PR please :)

#scope

Allows you to include in the serializer access to an external method.

It’s intended to provide an authorization context to the serializer, so that
you may e.g. show an admin all comments on a post, else only published comments.

  • scope is a method on the serializer instance that comes from options[:scope]. It may be nil.
  • scope_name is an option passed to the new serializer (options[:scope_name]). The serializer defines a method with that name that calls the scope, e.g. def current_user; scope; end. Note: it does not define the method if the serializer instance responds to it.

That’s a lot of words, so here’s some examples:

First, let’s assume the serializer is instantiated in the controller, since that’s the usual scenario.
We’ll refer to the serialization context as controller.

options Serializer#scope method definition
scope: current_user, scope_name: :current_user current_user Serializer#current_user calls controller.current_user
scope: view_context, scope_name: :view_context view_context Serializer#view_context calls controller.view_context

We can take advantage of the scope to customize the objects returned based
on the current user (scope).

For example, we can limit the posts the current user sees to those they created:

class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :body

  # scope comments to those created_by the current user
  has_many :comments do
    object.comments.where(created_by: current_user)
  end
end

Whether you write the method as above or as object.comments.where(created_by: scope)
is a matter of preference (assuming scope_name has been set).

Controller Authorization Context

In the controller, the scope/scope_name options are equal to
the serialization_scopemethod,
which is :current_user, by default.

Specfically, the scope_name is defaulted to :current_user, and may be set as
serialization_scope :view_context. The scope is set to send(scope_name) when scope_name is
present and the controller responds to scope_name.

Thus, in a serializer, the controller provides current_user as the
current authorization scope when you call render :json.

IMPORTANT: Since the scope is set at render, you may want to customize it so that current_user isn’t
called on every request. This was also a problem
in 0.9.

We can change the scope from current_user to view_context.

class SomeController < ActionController::Base
+  serialization_scope :view_context

  def current_user
    User.new(id: 2, name: 'Bob', admin: true)
  end

  def edit
    user = User.new(id: 1, name: 'Pete')
    render json: user, serializer: AdminUserSerializer, adapter: :json_api
  end
end

We could then use the controller method view_context in our serializer, like so:

class AdminUserSerializer < ActiveModel::Serializer
  attributes :id, :name, :can_edit

  def can_edit?
+    view_context.current_user.admin?
  end
end

So that when we render the #edit action, we’ll get

{"data":{"id":"1","type":"users","attributes":{"name":"Pete","can_edit":true}}}

Where can_edit is view_context.current_user.admin? (true).

#read_attribute_for_serialization(key)

The serialized value for a given key. e.g. read_attribute_for_serialization(:title) #=> 'Hello World'

#links

PR please :)

#json_key

PR please :)

Examples

Given two models, a Post(title: string, body: text) and a
Comment(name: string, body: text, post_id: integer), you will have two
serializers:

class PostSerializer < ActiveModel::Serializer
  cache key: 'posts', expires_in: 3.hours
  attributes :title, :body

  has_many :comments
end

and

class CommentSerializer < ActiveModel::Serializer
  attributes :name, :body

  belongs_to :post
end

Generally speaking, you, as a user of ActiveModelSerializers, will write (or generate) these
serializer classes.

More Info

For more information, see the Serializer class on GitHub

Overriding association methods

To override an association, call has_many, has_one or belongs_to with a block:

class PostSerializer < ActiveModel::Serializer
  has_many :comments do
    object.comments.active
  end
end

Overriding attribute methods

To override an attribute, call attribute with a block:

class PostSerializer < ActiveModel::Serializer
  attribute :body do
    object.body.downcase
  end
end