In the context of Rails API, we begin to talk about a serializer. It is a data-formatting layer that acts like a view in the traditional MVC framework.

Let’s say a client or a server makes an HTTP GET request to our Rails API with the following endpoint:

get '/api/v1/items'

For organization’s purposes, endpoints are namespaced so that APIs are taken care of separately from controllers. Additionally, versioning is also namespaced. Each namespace gets a subfolder under the controllers directory in a Rails app (see below).


In our routes file, a specific verb + endpoint combo is mapped to a specific controller action; in our simple example, an index action. A resources syntax comes handy in generating a list of RESTful routes and route_helpers.

# routes.rb
Rails.application.routes.draw do
  namespace :api do
    namespace :v1 do
      resources :items, only: [:index]

Within the index action, we call render json:, which is a built-in rails method that takes 2 steps under the hood: 1) data prep and 2) json encoding. In the first step, render json: calls .as_json method that formats a json structure. Then, in the second step, it calls .to_json method that turns a formatted object into a string.

class Api::V1::ItemsController < ApplicationController

  # without serializer
  def index
    render json: Item.all

  # with serializer
  def index
    render json:


Now, I’ve mentioned earlier that a serializer is a data-formatting layer. By data-formatting, it means that data can be displayed partially (or selectively). In other words, the serializer allows us to modify our object during our data prep phase, so that we can customize what data we want to send back in a response. The serializer takes in the entire class (i.e., Item.all) and returns an array of altered hashes containing certain attributes.

There is a cool gem called fast_jsonapi, made by Netflix. Basically it puts a json in a specific format so that it’s more readable. With this gem installed in our app, we can generate a serializer from the command line. Specify a model name and whichever attributes you want to select and display.

$ rails g serializer <model_name> <attribute_1> <attribute_2>

Running the command will result in the creation of a brand new serializer folder with an item_serializer.rb file. Furthermore, a custom attribute (i.e., greeting) can be added as shown below.

class ItemSerializer
  include FastJsonapi::ObjectSerializer
  attributes :id, :name, :description, :unit_price, :merchant_id

  attribute :greeting |object| do
    "Item #{} is great!"

Back to the controller. Before rendering a response, we can pass in Item.all to a serializer and control what we choose to show (and not to show). Isn’t this cool?!