This guide describes how to use Active Model serializers to build non-trivial JSON services in Rails. By reading this guide, you will learn:
* When to use the built-in Active Model serialization
* When to use a custom serializer for your models
* How to use serializers to encapsulate authorization concerns
* How to create serializer templates to describe the application-wide structure of your serialized JSON
* How to build resources not backed by a single database table for use with JSON services
This guide covers an intermediate topic and assumes familiarity with Rails conventions. It is suitable for applications that expose a
JSON API that may return different results based on the authorization status of the user.
endprologue.
h3. Serialization
By default, Active Record objects can serialize themselves into JSON by using the `to_json` method. This method takes a series of additional
parameter to control which properties and associations Rails should include in the serialized output.
When building a web application that uses JavaScript to retrieve JSON data from the server, this mechanism has historically been the primary
way that Rails developers prepared their responses. This works great for simple cases, as the logic for serializing an Active Record object
is neatly encapsulated in Active Record itself.
However, this solution quickly falls apart in the face of serialization requirements based on authorization. For instance, a web service
may choose to expose additional information about a resource only if the user is entitled to access it. In addition, a JavaScript front-end
may want information that is not neatly described in terms of serializing a single Active Record object, or in a different format than.
In addition, neither the controller nor the model seems like the correct place for logic that describes how to serialize an model object
*for the current user*.
Serializers solve these problems by encapsulating serialization in an object designed for this purpose. If the default +to_json+ semantics,
with at most a few configuration options serve your needs, by all means continue to use the built-in +to_json+. If you find yourself doing
hash-driven-development in your controllers, juggling authorization logic and other concerns, serializers are for you!
h3. The Most Basic Serializer
A basic serializer is a simple Ruby object named after the model class it is serializing.
<ruby>
class PostSerializer
def initialize(post, scope)
@post, @scope = post, scope
end
def as_json
{ post: { title: @post.name, body: @post.body } }
end
end
</ruby>
A serializer is initialized with two parameters: the model object it should serialize and an authorization scope. By default, the
authorization scope is the current user (+current_user+) but you can use a different object if you want. The serializer also
implements an +as_json+ method, which returns a Hash that will be sent to the JSON encoder.
Rails will transparently use your serializer when you use +render :json+ in your controller.
<ruby>
class PostsController < ApplicationController
def show
@post = Post.find(params[:id])
render json: @post
end
end
</ruby>
Because +respond_with+ uses +render :json+ under the hood for JSON requests, Rails will automatically use your serializer when
you use +respond_with+ as well.
h4. +serializable_hash+
In general, you will want to implement +serializable_hash+ and +as_json+ to allow serializers to embed associated content
directly. The easiest way to implement these two methods is to have +as_json+ call +serializable_hash+ and insert the root.
<ruby>
class PostSerializer
def initialize(post, scope)
@post, @scope = post, scope
end
def serializable_hash
{ title: @post.name, body: @post.body }
end
def as_json
{ post: serializable_hash }
end
end
</ruby>
h4. Authorization
Let's update our serializer to include the email address of the author of the post, but only if the current user has superuser
access.
<ruby>
class PostSerializer
def initialize(post, scope)
@post, @scope = post, scope
end
def as_json
{ post: serializable_hash }
end
def serializable_hash
hash = post
hash.merge!(super_data) if super?
hash
end
private
def post
{ title: @post.name, body: @post.body }
end
def super_data
{ email: @post.email }
end
def super?
@scope.superuser?
end
end
</ruby>
h4. Testing
One benefit of encapsulating our objects this way is that it becomes extremely straight-forward to test the serialization
logic in isolation.
<ruby>
require "ostruct"
class PostSerializerTest < ActiveSupport::TestCase
# For now, we use a very simple authorization structure. These tests will need
# refactoring if we change that.
plebe = OpenStruct.new(super?: false)
god = OpenStruct.new(super?: true)
post = OpenStruct.new(title: "Welcome to my blog!", body: "Blah blah blah", email: "tenderlove@gmail.com")
test "a regular user sees just the title and body" do
json = PostSerializer.new(post, plebe).to_json
hash = JSON.parse(json)
assert_equal post.title, hash.delete("title")
assert_equal post.body, hash.delete("body")
assert_empty hash
end
test "a superuser sees the title, body and email" do
json = PostSerializer.new(post, god).to_json
hash = JSON.parse(json)
assert_equal post.title, hash["title"]
assert_equal post.body, hash["body"]
assert_equal post.email, hash["email"]
assert_empty hash
end
end
</ruby>
It's important to note that serializer objects define a clear interface specifically for serializing an existing object.
In this case, the serializer expects to receive a post object with +name+, +body+ and +email+ attributes and an authorization
scope with a +super?+ method.
By defining a clear interface, it's must easier to ensure that your authorization logic is behaving correctly. In this case,
the serializer doesn't need to concern itself with how the authorization scope decides whether to set the +super?+ flag, just
whether it is set. In general, you should document these requirements in your serializer files and programatically via tests.
The documentation library +YARD+ provides excellent tools for describing this kind of requirement:
<ruby>
class PostSerializer
# @param [~body, ~title, ~email] post the post to serialize
# @param [~super] scope the authorization scope for this serializer
def initialize(post, scope)
@post, @scope = post, scope
end
# ...
end
</ruby>
h3. Attribute Sugar
To simplify this process for a number of common cases, Rails provides a default superclass named +ActiveModel::Serializer+
that you can use to implement your serializers.
For example, you will sometimes want to simply include a number of existing attributes from the source model into the outputted
JSON. In the above example, the +title+ and +body+ attributes were always included in the JSON. Let's see how to use
+ActiveModel::Serializer+ to simplify our post serializer.
<ruby>
class PostSerializer < ActiveModel::Serializer
attributes :title, :body
def initialize(post, scope)
@post, @scope = post, scope
end
def serializable_hash
hash = attributes
hash.merge!(super_data) if super?
hash
end
private
def super_data
{ email: @post.email }
end
def super?
@scope.superuser?
end
end
</ruby>
First, we specified the list of included attributes at the top of the class. This will create an instance method called
+attributes+ that extracts those attributes from the post model.
NOTE: Internally, +ActiveModel::Serializer+ uses +read_attribute_for_serialization+, which defaults to +read_attribute+, which defaults to +send+. So if you're rolling your own models for use with the serializer, you can use simple Ruby accessors for your attributes if you like.
Next, we use the attributes methood in our +serializable_hash+ method, which allowed us to eliminate the +post+ method we hand-rolled
earlier. We could also eliminate the +as_json+ method, as +ActiveModel::Serializer+ provides a default +as_json+ method for
us that calls our +serializable_hash+ method and inserts a root. But we can go a step further!
<ruby>
class PostSerializer < ActiveModel::Serializer
attributes :title, :body
private
def attributes
hash = super
hash.merge!(email: post.email) if super?
hash
end
def super?
@scope.superuser?
end
end
</ruby>
The superclass provides a default +initialize+ method as well as a default +serializable_hash+ method, which uses
+attributes+. We can call +super+ to get the hash based on the attributes we declared, and then add in any additional
attributes we want to use.
NOTE: +ActiveModel::Serializer+ will create an accessor matching the name of the current class for the resource you pass in. In this case, because we have defined a PostSerializer, we can access the resource with the +post+ accessor.
h3. Associations
In most JSON APIs, you will want to include associated objects with your serialized object. In this case, let's include
the comments with the current post.
<ruby>
class PostSerializer < ActiveModel::Serializer
attributes :title. :body
has_many :comments
private
def attributes
hash = super
hash.merge!(email: post.email) if super?
hash
end
def super?
@scope.superuser?
end
end
</ruby>
The default +serializable_hash+ method will include the comments as embedded objects inside the post.
<javascript>
{
post: {
title: "Hello Blog!",
body: "This is my first post. Isn't it fabulous!",
comments: [
{
title: "Awesome",
body: "Your first post is great"
}
]
}
}
</javascript>
Rails uses the same logic to generate embedded serializations as it does when you use +render :json+. In this case,
because you didn't define a +CommentSerializer+, Rails used the default +as_json+ on your comment object.
If you define a serializer, Rails will automatically instantiate it with the existing authorization scope.
<ruby>
class CommentSerializer
def initialize(comment, scope)
@comment, @scope = comment, scope
end
def serializable_hash
{ title: @comment.title }
end
def as_json
{ comment: serializable_hash }
end
end
</ruby>
If we define the above comment serializer, the outputted JSON will change to:
<javascript>
{
post: {
title: "Hello Blog!",
body: "This is my first post. Isn't it fabulous!",
comments: [{ title: "Awesome" }]
}
}
</javascript>
Let's imagine that our comment system allows an administrator to kill a comment, and we only want to allow
users to see the comments they're entitled to see. By default, +has_many :comments+ will simply use the
+comments+ accessor on the post object. We can override the +comments+ accessor to limit the comments used
to just the comments we want to allow for the current user.
<ruby>
class PostSerializer < ActiveModel::Serializer
attributes :title. :body
has_many :comments
private
def attributes
hash = super
hash.merge!(email: post.email) if super?
hash
end
def comments
post.comments_for(scope)
end
def super?
@scope.superuser?
end
end
</ruby>
+ActiveModel::Serializer+ will still embed the comments, but this time it will use just the comments
for the current user.
NOTE: The logic for deciding which comments a user should see still belongs in the model layer. In general, you should encapsulate concerns that require making direct Active Record queries in scopes or public methods on your models.
h3. Customizing Associations
Not all front-ends expect embedded documents in the same form. In these cases, you can override the
default +serializable_hash+, and use conveniences provided by +ActiveModel::Serializer+ to avoid having to
build up the hash manually.
For example, let's say our front-end expects the posts and comments in the following format:
<javascript>
{
post: {
id: 1
title: "Hello Blog!",
body: "This is my first post. Isn't it fabulous!",
comments: [1,2]
},
comments: [
{
id: 1
title: "Awesome",
body: "Your first post is great"
},
{
id: 2
title: "Not so awesome",
body: "Why is it so short!"
}
]
}
</javascript>
We could achieve this with a custom +as_json+ method. We will also need to define a serializer for comments.
<ruby>
class CommentSerializer < ActiveModel::Serializer
attributes :id, :title, :body
# define any logic for dealing with authorization-based attributes here
end
class PostSerializer < ActiveModel::Serializer
attributes :title. :body
has_many :comments
def as_json
{ post: serializable_hash }.merge!(associations)
end
def serializable_hash
post_hash = attributes
post_hash.merge!(association_ids)
post_hash
end
private
def attributes
hash = super
hash.merge!(email: post.email) if super?
hash
end
def comments
post.comments_for(scope)
end
def super?
@scope.superuser?
end
end
</ruby>
Here, we used two convenience methods: +associations+ and +association_ids+. The first,
+associations+, creates a hash of all of the define associations, using their defined
serializers. The second, +association_ids+, generates a hash whose key is the association
name and whose value is an Array of the association's keys.
The +association_ids+ helper will use the overridden version of the association, so in
this case, +association_ids+ will only include the ids of the comments provided by the
+comments+ method.
h3. Special Association Serializers
So far, associations defined in serializers use either the +as_json+ method on the model
or the defined serializer for the association type. Sometimes, you may want to serialize
associated models differently when they are requested as part of another resource than
when they are requested on their own.
For instance, we might want to provide the full comment when it is requested directly,
but only its title when requested as part of the post. To achieve this, you can define
a serializer for associated objects nested inside the main serializer.
<ruby>
class PostSerializer < ActiveModel::Serializer
class CommentSerializer < ActiveModel::Serializer
attributes :id, :title
end
# same as before
# ...
end
</ruby>
In other words, if a +PostSerializer+ is trying to serialize comments, it will first
look for +PostSerializer::CommentSerializer+ before falling back to +CommentSerializer+
and finally +comment.as_json+.
h3. Optional Associations
In some cases, you will want to allow a front-end to decide whether to include associated
content or not. You can achieve this easily by making an association *optional*.
<ruby>
class PostSerializer < ActiveModel::Serializer
attributes :title. :body
has_many :comments, :optional => true
# ...
end
</ruby>
If an association is optional, it will not be included unless the request asks for it
with an +including+ parameter. The +including+ parameter is a comma-separated list of
optional associations to include. If the +including+ parameter includes an association
you did not specify in your serializer, it will receive a +401 Forbidden+ response.
h3. Overriding the Defaults
h4. Authorization Scope
By default, the authorization scope for serializers is +:current_user+. This means
that when you call +render json: @post+, the controller will automatically call
its +current_user+ method and pass that along to the serializer's initializer.
If you want to change that behavior, simply use the +serialization_scope+ class
method.
<ruby>
class PostsController < ApplicationController
serialization_scope :current_app
end
</ruby>
You can also implement an instance method called (no surprise) +serialization_scope+,
which allows you to define a dynamic authorization scope based on the current request.
WARNING: If you use different objects as authorization scopes, make sure that they all implement whatever interface you use in your serializers to control what the outputted JSON looks like.
h4. Parameter to Specify Included Optional Associations
In most cases, you should be able to use the default +including+ parameter to specify
which optional associations to include. If you are already using that parameter name or
want to reserve it for some reason, you can specify a different name by using the
