How to Use Active Model Serializer in Ruby on Rails

Use ActiveModel::Serializer in Ruby on Rails by creating a serializer class for your model that defines attributes to include in JSON output. Then, render your model or collection with the serializer in your controller to get clean, structured JSON responses.

Syntax

Create a serializer class inheriting from ActiveModel::Serializer. Inside, use attributes to list model fields to include in JSON. Optionally, define custom methods for computed fields.

In your controller, render the model or collection with render json: @object, and Rails uses the serializer automatically.

ruby

class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :email

  def name
    "User: " + object.name
  end
end

Example

This example shows a User model with a serializer that customizes the name attribute. The controller renders JSON using the serializer.

ruby

# app/models/user.rb
class User < ApplicationRecord
  # attributes: id, name, email
end

# app/serializers/user_serializer.rb
class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :email

  def name
    "User: " + object.name
  end
end

# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def show
    user = User.find(params[:id])
    render json: user
  end
end

Output

{
  "user": {
    "id": 1,
    "name": "User: Alice",
    "email": "[e-mail]"
  }
}

Common Pitfalls

• Not creating a serializer class for your model, so Rails falls back to default JSON rendering.
• Forgetting to list attributes in the serializer, resulting in missing fields in output.
• Using instance variables instead of object inside serializer methods.
• Rendering JSON without specifying the serializer when multiple serializers exist.

ruby

## Wrong: Missing serializer
render json: @user

## Right: Serializer defined and used automatically
render json: @user

## Wrong: Using instance variable in serializer
class UserSerializer < ActiveModel::Serializer
  def name
    "User: " + @user.name  # wrong
  end
end

## Right: Use object
class UserSerializer < ActiveModel::Serializer
  def name
    "User: " + object.name
  end
end

Quick Reference

Concept	Description
Serializer Class	Inherit from ActiveModel::Serializer to define JSON structure
attributes	List model fields to include in JSON output
Custom Methods	Define methods for computed or formatted fields
Rendering	Use render json: @model in controller to apply serializer
object	Reference the model instance inside serializer methods

Key Takeaways

Create a serializer class inheriting from ActiveModel::Serializer for each model you want to format.

List attributes inside the serializer to control what appears in JSON output.

Use the object keyword inside serializer methods to access model data.

Render your model or collection with render json: @object to apply the serializer automatically.

Avoid common mistakes like missing serializers or incorrect instance variable usage.