bin/rails active_storage:install
bin/rails db:migrate
bundle add image_processing

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

test:
  service: Disk
  root: <%= Rails.root.join("tmp/storage") %>

amazon:
  service: S3
  access_key_id: <%= Rails.application.credentials.dig(:aws, :access_key_id) %>
  secret_access_key: <%= Rails.application.credentials.dig(:aws, :secret_access_key) %>
  region: eu-west-1
  bucket: <%= Rails.application.credentials.dig(: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 do |attachable|
    attachable.variant :thumb, resize_to_limit: [100, 100]
    attachable.variant :medium, resize_to_limit: [300, 300]
  end
end

class Event < ApplicationRecord
  has_many_attached :photos
  has_many_attached :documents
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/webp"]
    unless acceptable_types.include?(avatar.content_type)
      errors.add(:avatar, "must be a JPEG, PNG, or WebP")
    end
  end
end

gem "active_storage_validations"

class User < ApplicationRecord
  has_one_attached :avatar

  validates :avatar,
    content_type: ["image/png", "image/jpeg", "image/webp"],
    size: { less_than: 5.megabytes }
end

# Resize to fit (maintains aspect ratio)
resize_to_limit: [300, 300]

# Resize and crop to exact dimensions
resize_to_fill: [300, 300]

# With format conversion
resize_to_limit: [300, 300], format: :webp, saver: { quality: 80 }

<% if user.avatar.attached? %>
  <%= image_tag user.avatar.variant(:thumb), alt: user.name %>
<% else %>
  <%= image_tag "default-avatar.png", alt: "Default" %>
<% end %>

# test/models/user_test.rb
require "test_helper"

class UserAttachmentTest < ActiveSupport::TestCase
  setup do
    @user = users(:one)
  end

  test "attaches an avatar" do
    @user.avatar.attach(
      io: File.open(Rails.root.join("test/fixtures/files/avatar.jpg")),
      filename: "avatar.jpg",
      content_type: "image/jpeg"
    )

    assert @user.avatar.attached?
  end

  test "rejects oversized avatar" do
    @user.avatar.attach(
      io: StringIO.new("x" * 6.megabytes),
      filename: "large.jpg",
      content_type: "image/jpeg"
    )

    assert_not @user.valid?
    assert_includes @user.errors[:avatar], "is too large (max 5MB)"
  end

  test "rejects invalid content type" do
    @user.avatar.attach(
      io: File.open(Rails.root.join("test/fixtures/files/document.pdf")),
      filename: "doc.pdf",
      content_type: "application/pdf"
    )

    assert_not @user.valid?
    assert @user.errors[:avatar].any?
  end
end

# test/controllers/users_controller_test.rb
require "test_helper"

class UsersUploadTest < ActionDispatch::IntegrationTest
  setup do
    @user = users(:one)
    sign_in @user
  end

  test "uploads avatar" do
    avatar = fixture_file_upload("avatar.jpg", "image/jpeg")

    patch user_path(@user), params: { user: { avatar: avatar } }

    assert @user.reload.avatar.attached?
  end

  test "removes avatar" do
    @user.avatar.attach(
      io: File.open(Rails.root.join("test/fixtures/files/avatar.jpg")),
      filename: "avatar.jpg",
      content_type: "image/jpeg"
    )

    delete remove_avatar_user_path(@user)

    assert_not @user.reload.avatar.attached?
  end
end

test/fixtures/files/
├── avatar.jpg
├── document.pdf
└── photo.png

class UsersController < ApplicationController
  def update
    if @user.update(user_params)
      redirect_to @user, notice: "Profile updated"
    else
      render :edit, status: :unprocessable_entity
    end
  end

  def remove_avatar
    @user.avatar.purge
    redirect_to edit_user_path(@user), notice: "Avatar removed"
  end

  private

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

def event_params
  params.require(:event).permit(:name, photos: [], documents: [])
end

<%= form_with model: @user do |f| %>
  <div>
    <%= f.label :avatar %>
    <%= f.file_field :avatar, accept: "image/png,image/jpeg,image/webp" %>

    <% if @user.avatar.attached? %>
      <%= image_tag @user.avatar.variant(:thumb), class: "rounded mt-2" %>
    <% end %>
  </div>
  <%= f.submit %>
<% end %>

// app/javascript/application.js
import * as ActiveStorage from "@rails/activestorage"
ActiveStorage.start()

<%= f.file_field :photos, multiple: true, direct_upload: true %>

# Prevent N+1 on attachments
User.with_attached_avatar.limit(10)

# Multiple attachments
Event.with_attached_photos.with_attached_documents