serializers.md

Back to Guides

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' }

An if or unless option can make an attribute conditional. It takes a symbol of a method name on the serializer, or a lambda literal.

e.g.

attribute :private_data, if: :is_current_user?
attribute :another_private_data, if: -> { scope.admin? }

def is_current_user?
  object.id == current_user.id
end

Associations

The interface for associations is, generically:

association_type(association_name, options, &block)

Where:

• association_type may be has_one, has_many, belongs_to.
• association_name is a method name the serializer calls.
• optional: options may be:
  • key: The name used for the serialized association.
  • serializer:
  • if:
  • unless:
  • virtual_value:
  • polymorphic: defines if polymorphic relation type should be nested in serialized association.
• optional: &block is a context that returns the association's attributes.
  • prevents association_name method from being called.
  • return value of block is used as the association value.
  • yields the serializer to the block.
  • include_data false prevents the data key from being rendered in the JSON API relationship.

::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_one :blog, if: :show_blog?
# you can also use a string or lambda
# has_one :blog, if: 'scope.admin?'
# has_one :blog, if: -> (serializer) { serializer.scope.admin? }
# has_one :blog, if: -> { scope.admin? }

def show_blog?
  scope.admin?
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

You can specify the serializers by overriding serializer_for. For more context about polymorphic relationships, 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

When using the :json_api adapter, the ::type method defines the JSONAPI type that will be rendered for this serializer.

When using the :json adapter, the ::type method defines the name of the root element.

It either takes a String or Symbol as parameter.

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

Examples:

class UserProfileSerializer < ActiveModel::Serializer
  type 'profile'

  attribute :name
end
class AuthorProfileSerializer < ActiveModel::Serializer
  type :profile

  attribute :name
end

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

{
  "data": {
    "id": "1",
    "type": "profile",
    "attributes": {
      "name": "Julia"
    }
  }
}

With the :json adapter, the previous serializer would be rendered as:

{
  "profile": {
    "name": "Julia"
  }
}

::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

Resource root which is included in JSON adapter. As you can see at Adapters Document, Attribute adapter (default) and JSON API adapter does not include root at top level. By default, the resource root comes from the model_name of the serialized object's class.

There are several ways to specify root:

• Overriding the root key
• Setting type
• Specifying the root option, e.g. root: 'specific_name', during the serializer's initialization:

ActiveModelSerializers::SerializableResource.new(foo, root: 'bar')

#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.

Specifically, 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).

You can also tell what to set as serialization_scope for specific actions.

For example, use admin_user only for Admin::PostSerializer and current_user for rest.

class PostsController < ActionController::Base

  before_action only: :edit do
    self.class.serialization_scope :admin_user
  end

  def show
    render json: @post, serializer: PostSerializer
  end

  def edit
    @post.save
    render json: @post, serializer: Admin::PostSerializer
  end

  private

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

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

#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

Overriding association serializer lookup

If you want to define a specific serializer lookup for your associations, you can override the ActiveModel::Serializer.serializer_for method to return a serializer class based on defined conditions.

class MySerializer < ActiveModel::Serializer
  def self.serializer_for(model, options)
    return SparseAdminSerializer if model.class == 'Admin'
    super
  end

  # the rest of the serializer
end