Dynamic Nested Attributes

Overview

Implement Rails nested attributes with dynamic add/remove functionality using Turbo Streams and Simple Form. This pattern allows users to add and remove associated records inline within a parent form.

When to Use

• Building forms where users need to manage multiple child records (has_many associations)
• Adding/removing nested items without page refresh
• Bulk creation or editing of associated records
• Forms requiring progressive disclosure of additional fields

Key Components

1. Form Object or Model

• Accepts nested attributes for the association
• Use accepts_nested_attributes_for :association_name in the model or form object

2. Main Form View

Create a form that includes:

• simple_fields_for for rendering existing nested items
• A container element with an ID for appending new items (e.g., #accessories)
• A link to add new items that triggers a Turbo Stream request

Example:

= simple_form_for resource do |f|
  = f.simple_fields_for :items do |ff|
    = render 'item_fields', f: ff, resource:

= render 'add_button', index: resource.items.size

Add Button Partial (_add_button.html.slim):

-# locals: (index:)
= link_to icon('add'), new_parent_item_path(index: index),
  id: 'add_button', class: 'btn', data: { turbo_stream: true }

3. Nested Fields Partial

Create a partial (e.g., _item_fields.html.slim) that:

• Wraps fields in a unique container with an ID based on index
• Includes a data controller for remove functionality
• Shows a delete button for all records
• Includes all form inputs for the nested item

Example:

fieldset id="item_#{f.index}" controller='destroy-nested-attributes'
  = f.hidden_field :_destroy, data: { destroy_nested_attributes_target: 'input' }
  = f.input :name
  = f.input :quantity
  .form-row__actions
    = button_tag icon('delete'), type: 'button', class: 'btn btn-delete',
      data: { action: 'destroy-nested-attributes#perform' }

4. Controller Actions

Implement a new action that:

• Builds a new nested item
• Accepts index parameter for tracking position

Example:

def new
  @item = Item.new
end

5. Turbo Stream Response

Create a new.turbo_stream.slim view that:

• Updates the “add” button with incremented index
• Appends the new nested fields to the container
• Uses index parameter to ensure unique field names
• Works with non-persisted parents by using a symbol and empty URL

Example:

= turbo_stream.replace 'add_button', partial: 'add_button', locals: { index: params[:index].to_i + 1 }

= simple_form_for :parent, url: '' do |f|
  = f.simple_fields_for :items_attributes, @item, index: params[:index] do |ff|
    = turbo_stream.append 'items', partial: 'item_fields', locals: { f: ff }

6. Remove Stimulus Controller

Create a Stimulus controller to handle client-side removal:

Example JavaScript:

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ['input']
  static classes = ['destroyed']

  connect() {
    if (!this.hasDestroyedClass) {
      this.element.setAttribute(`data-${this.identifier}-destroyed-class`, 'is-hidden')
    }
  }

  perform() {
    this.inputTarget.value = '1'
    this.element.classList.add(this.destroyedClass)
  }
}

Implementation Checklist

• Add accepts_nested_attributes_for to model/form object
• Create main form with simple_fields_for and container element
• Create nested fields partial with remove functionality
• Implement controller new action with index support
• Create turbo_stream response view
• Add Stimulus controller for client-side removal
• Update routes to support nested resource creation
• Update strong parameters to permit nested attributes
• Add policy authorization if using Pundit

Common Patterns

Dynamic Collections Based on Parent Selection

Pass filtered collections to nested partials:

= render 'item_fields', f: ff, resource:, collection: resource.items

Routes Example

If there is not an existing new route in use, use the following pattern

resources :items, only: [:new]

If one does exist, create a new namespaced controller

namespace :parent do
  resources :items, only: [:new]
end

Strong Parameters Example

def item_params
  params.require(:parent).permit(
    :category,
    :subcategory,
    items_attributes: %i[
      name
      quantity
      part_id
      optional
      hidden
    ]
  )
end

Related Patterns

• Turbo Frame inline editing
• Stimulus data controller integration
• Form object pattern for bulk operations
• Policy-scoped collections for associations