Nested attributes in Ruby on Rails

Introduction

Nested attributes let you save data for related models in one form. It helps keep things simple when models are connected.

You want to create or update a parent record and its child records together.

You have a form where users can add details for multiple related items at once.

You want to manage a list of related objects inside a single form.

You want to avoid writing separate forms and actions for each related model.

Syntax

Ruby on Rails

class ParentModel < ApplicationRecord
  has_many :child_models
  accepts_nested_attributes_for :child_models
end

Use accepts_nested_attributes_for in the parent model to allow nested data.

Rails will handle saving or updating the child models automatically.

Examples

This lets you create or update a book and its chapters in one go.

Ruby on Rails

class Book < ApplicationRecord
  has_many :chapters
  accepts_nested_attributes_for :chapters
end

Use nested attributes for a one-to-one relation like author and profile.

Ruby on Rails

class Author < ApplicationRecord
  has_one :profile
  accepts_nested_attributes_for :profile
end

Adding allow_destroy: true lets you delete child records through the form.

Ruby on Rails

class Project < ApplicationRecord
  has_many :tasks
  accepts_nested_attributes_for :tasks, allow_destroy: true
end

Sample Program

This example shows a Library model with many Books. The form sends nested book data inside the library data. The controller permits nested attributes and saves all together.

Ruby on Rails

class Library < ApplicationRecord
  has_many :books
  accepts_nested_attributes_for :books
end

# In controller
class LibrariesController < ApplicationController
  def new
    @library = Library.new
    @library.books.build
  end

  def create
    @library = Library.new(library_params)
    if @library.save
      redirect_to @library
    else
      render :new
    end
  end

  private

  def library_params
    params.require(:library).permit(:name, books_attributes: [:title, :author])
  end
end

# Example params from form:
# {
#   library: {
#     name: "City Library",
#     books_attributes: {
#       "0" => { title: "Ruby Basics", author: "Jane" },
#       "1" => { title: "Rails Guide", author: "John" }
#     }
#   }
# }

Important Notes

Remember to permit nested attributes in the controller using permit.

Use build to prepare nested objects for the form.

Nested attributes work best for simple related data, not very complex associations.

Summary

Nested attributes let you save parent and child models together.

Use accepts_nested_attributes_for in the parent model.

Permit nested params in the controller to allow saving related data.