Rails Active Storage 2026年完全ガイド：ファイルアップロード、S3連携、面接対策

bin/rails active_storage:install
bin/rails db:migrate

local:
  service: Disk
  root: <%= Rails.root.join("storage") %>

amazon:
  service: S3
  access_key_id: <%= ENV['AWS_ACCESS_KEY_ID'] %>
  secret_access_key: <%= ENV['AWS_SECRET_ACCESS_KEY'] %>
  region: <%= ENV['AWS_REGION'] %>
  bucket: <%= ENV['AWS_BUCKET'] %>

# config/environments/development.rb
config.active_storage.service = :local

# config/environments/production.rb
config.active_storage.service = :amazon

class User < ApplicationRecord
  has_one_attached :avatar
  has_many_attached :documents
end

class UsersController < ApplicationController
  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user, notice: 'User was successfully created.'
    else
      render :new
    end
  end

  private

  def user_params
    params.require(:user).permit(:name, :email, :avatar, documents: [])
  end
end

<%= form_with model: @user do |form| %>
  <%= form.file_field :avatar %>
  <%= form.file_field :documents, multiple: true %>
  <%= form.submit %>
<% end %>

gem 'aws-sdk-s3', require: false

import * as ActiveStorage from "@rails/activestorage"
ActiveStorage.start()

<%= form.file_field :avatar, direct_upload: true %>

addEventListener("direct-upload:progress", event => {
  const { id, progress } = event.detail
  const progressElement = document.getElementById(`direct-upload-${id}`)
  progressElement.style.width = `${progress}%`
})

gem 'active_storage_validations'

class User < ApplicationRecord
  has_one_attached :avatar
  
  validates :avatar, content_type: ['image/png', 'image/jpeg', 'image/gif'],
                     size: { less_than: 5.megabytes }
end

class User < ApplicationRecord
  has_one_attached :avatar
  
  validate :acceptable_avatar
  
  private
  
  def acceptable_avatar
    return unless avatar.attached?
    
    unless avatar.blob.byte_size <= 5.megabytes
      errors.add(:avatar, 'is too large (max 5MB)')
    end
    
    acceptable_types = ['image/jpeg', 'image/png', 'image/gif']
    unless acceptable_types.include?(avatar.content_type)
      errors.add(:avatar, 'must be a JPEG, PNG, or GIF')
    end
  end
end

gem 'image_processing', '~> 1.2'

<%= image_tag @user.avatar.variant(resize_to_limit: [200, 200]) %>

class User < ApplicationRecord
  has_one_attached :avatar do |attachable|
    attachable.variant :thumb, resize_to_limit: [100, 100]
    attachable.variant :medium, resize_to_limit: [300, 300]
    attachable.variant :large, resize_to_limit: [800, 800]
  end
end

<%= image_tag @user.avatar.variant(:thumb) %>

# N+1クエリが発生する
users = User.all
users.each { |user| user.avatar.attached? }

# 最適化されたクエリ
users = User.with_attached_avatar
users.each { |user| user.avatar.attached? }

class ProcessAvatarJob < ApplicationJob
  def perform(user)
    user.avatar.variant(:thumb).processed
    user.avatar.variant(:medium).processed
  end
end

# config/environments/production.rb
config.active_storage.resolve_model_to_route = :cdn
config.action_controller.asset_host = 'https://cdn.example.com'

# プライベートファイルへの署名付きURLの生成
rails_blob_url(@user.document, disposition: 'attachment', expires_in: 5.minutes)