Rails Active Storage Expert Skill

Rails Active Storage Expert

Implement file uploads, attachments, image processing, and cloud storage in Rails applications using Active Storage.

Philosophy

Core Principles:

Always run the install generator — Active Storage needs its migrations and config
Use libvips over ImageMagick — 10x faster, 1/10 memory usage
Prevent N+1 queries — Always use with_attached_<name> scopes
Handle missing attachments — Never assume an attachment exists
Proxy in production, redirect in development — Proxy mode works with CDNs
Use named variants — Define variants on the model, not in views

When To Use This Skill

Adding file uploads to a Rails model
Configuring cloud storage (S3, GCS, Azure)
Implementing image resizing/transformation (variants)
Setting up direct uploads from the browser
Debugging attachment issues (missing files, broken variants)
Migrating between storage services
Testing file uploads

Instructions

Step 1: Verify Setup

ALWAYS check if Active Storage is installed first:

# Check for Active Storage tables
bin/rails runner "puts ActiveStorage::Blob.table_exists?"

# Check for storage.yml
ls config/storage.yml

# Check for Active Storage migrations
ls db/migrate/*active_storage*

If not installed:

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

This creates three tables:

active_storage_blobs — file metadata (filename, content_type, byte_size, checksum)
active_storage_attachments — polymorphic join table connecting models to blobs
active_storage_variant_records — tracks generated variants

Verify image processing is available:

# Check for libvips (preferred)
vips --version

# Or ImageMagick (fallback)
convert --version

Add to Gemfile if needed:

gem "image_processing", "~> 1.2"  # Required for variants

Step 2: Configure Storage Service

config/storage.yml — Define available services:

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

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

Set the active service per environment:

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

# config/environments/test.rb
config.active_storage.service = :test

# config/environments/production.rb
config.active_storage.service = :amazon  # or :google, :azure

For cloud services, see reference.md for S3, GCS, and Azure configuration.

Step 3: Declare Attachments on Models

Single file attachment:

class User < ApplicationRecord
  has_one_attached :avatar
end

Multiple file attachments:

class Message < ApplicationRecord
  has_many_attached :images
end

With named variants (ALWAYS do this):

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

With a specific storage service:

class User < ApplicationRecord
  has_one_attached :avatar, service: :s3
end

Step 4: Handle Attachments in Controllers

Strong parameters — permit attachment params:

# has_one_attached
def user_params
  params.expect(user: [:name, :email, :avatar])
end

# has_many_attached
def message_params
  params.expect(message: [:title, :content, images: []])
end

Attaching files programmatically:

# From an upload
user.avatar.attach(params[:avatar])

# From an IO object
user.avatar.attach(
  io: File.open("/path/to/file.jpg"),
  filename: "avatar.jpg",
  content_type: "image/jpeg"
)

# From a URL (download first)
user.avatar.attach(
  io: URI.open("https://example.com/photo.jpg"),
  filename: "photo.jpg"
)

Step 5: Display Attachments in Views

ALWAYS check if attached before rendering:

<%# WRONG — will raise if no avatar %>
<%= image_tag user.avatar %>

<%# CORRECT — guard against missing attachment %>
<%= image_tag user.avatar if user.avatar.attached? %>

<%# BEST — use variant with guard %>
<% if user.avatar.attached? %>
  <%= image_tag user.avatar.variant(:thumb) %>
<% end %>

Use named variants (not inline transforms):

<%# WRONG — variant defined in view %>
<%= image_tag user.avatar.variant(resize_to_limit: [100, 100]) %>

<%# CORRECT — named variant from model %>
<%= image_tag user.avatar.variant(:thumb) %>

Step 6: Prevent N+1 Queries

This is the #1 performance mistake with Active Storage.

# WRONG — N+1 query for each user's avatar
@users = User.all
# In view: user.avatar triggers a query per user

# CORRECT — eager load attachments
@users = User.with_attached_avatar

# For has_many_attached
@messages = Message.with_attached_images

When rendering variants, also load variant records:

# In view, after eager loading attachments:
message.images.with_all_variant_records.each do |image|
  image_tag image.variant(:thumb).processed.url
end

Step 7: Form Setup

Simple file upload:

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

Multiple files:

<%= form_with model: @message do |form| %>
  <%= form.file_field :images, multiple: true %>
  <%= form.submit %>
<% end %>

Preserve existing has_many_attached files on update:

<%# Without this, updating replaces ALL existing attachments %>
<% @message.images.each do |image| %>
  <%= form.hidden_field :images, multiple: true, value: image.signed_id %>
<% end %>
<%= form.file_field :images, multiple: true %>

Direct uploads (upload to cloud before form submit):

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

Requires Active Storage JavaScript. See reference.md for setup.

Step 8: Removing Attachments

# Synchronous — deletes blob + file from storage immediately
user.avatar.purge

# Async — deletes via background job (preferred in controllers)
user.avatar.purge_later

Allow users to remove attachments via a checkbox:

<% if @user.avatar.attached? %>
  <%= image_tag @user.avatar.variant(:thumb) %>
  <%= form.check_box :remove_avatar, label: "Remove avatar" %>
<% end %>

# In controller or model
after_save :purge_avatar, if: -> { saved_change_to_attribute?(:remove_avatar) }

# Or handle in controller:
def update
  @user.update(user_params)
  @user.avatar.purge_later if params[:user][:remove_avatar] == "1"
end

Step 9: Testing

Use file_fixture_upload in integration tests:

class UsersControllerTest < ActionDispatch::IntegrationTest
  test "creates user with avatar" do
    post users_path, params: {
      user: {
        name: "Test",
        avatar: file_fixture_upload("avatar.png", "image/png")
      }
    }

    user = User.order(:created_at).last
    assert user.avatar.attached?
  end
end

Place test files in test/fixtures/files/.

Clean up uploaded files after tests:

# test/test_helper.rb or application_system_test_case.rb
class ActionDispatch::IntegrationTest
  def after_teardown
    super
    FileUtils.rm_rf(ActiveStorage::Blob.service.root)
  end
end

For parallel tests, isolate storage per process:

class ActiveSupport::TestCase
  parallelize_setup do |i|
    ActiveStorage::Blob.service.root = "#{ActiveStorage::Blob.service.root}-#{i}"
  end
end

See reference.md for fixture-based attachment testing.

Common Agent Mistakes

1. Forgetting the install generator

Active Storage won't work without its migrations. Always run bin/rails active_storage:install and bin/rails db:migrate.

2. Missing image processing dependency

Variants require gem "image_processing" AND either libvips or ImageMagick installed on the system. Without these, variant() calls will raise.

3. Wrong variant processor syntax

# WRONG — MiniMagick syntax with Vips processor
variant(resize: "100x100")

# CORRECT — Vips syntax (Rails 7+ default)
variant(resize_to_limit: [100, 100])

4. Not handling missing attachments

# WILL RAISE if no avatar attached
user.avatar.variant(:thumb)

# SAFE
user.avatar.variant(:thumb) if user.avatar.attached?

5. N+1 queries with attachments

Always use with_attached_<name> scope when loading collections. This is the most common performance issue.

6. Defining variants in views instead of models

Variants defined inline in views are hard to maintain and can't be preprocessed. Define them on the model using the block syntax.

7. Replacing instead of appending with has_many_attached

By default, attaching new files to has_many_attached replaces all existing attachments. Use hidden fields with signed_id to preserve existing ones.

Quick Reference

Variant Transforms (Vips — Rails 7+ default)

| Transform | Syntax | Description | |-----------|--------|-------------| | Resize to fit | resize_to_limit: [w, h] | Shrinks to fit within bounds, preserves aspect ratio | | Resize to fill | resize_to_fill: [w, h] | Fills bounds exactly, crops excess | | Resize and pad | resize_to_fit: [w, h] | Fits within bounds, may be smaller | | Resize exact | resize_and_pad: [w, h] | Fits within bounds, pads to exact size | | Convert format | format: :webp | Convert to another format | | Quality | saver: { quality: 80 } | Set output quality | | Rotate | rotate: 90 | Rotate by degrees |

Key Methods

# Attachment status
record.avatar.attached?        # => true/false
record.avatar.blank?           # => true/false (opposite of attached?)

# File metadata
record.avatar.filename         # => "avatar.png"
record.avatar.content_type     # => "image/png"
record.avatar.byte_size        # => 12345
record.avatar.blob.metadata    # => {"identified"=>true, "width"=>100, "height"=>100, "analyzed"=>true}

# URLs
url_for(record.avatar)                    # Redirect URL
rails_blob_path(record.avatar)            # Path helper
rails_storage_proxy_path(record.avatar)   # Proxy URL

# Downloading
record.avatar.download         # => binary string
record.avatar.open { |f| ... } # => yields Tempfile

# Purging
record.avatar.purge            # Sync delete
record.avatar.purge_later      # Async delete

Configuration Options

# config/application.rb or environment files
config.active_storage.variant_processor = :vips          # :vips (default) or :mini_magick
config.active_storage.resolve_model_to_route = :rails_storage_proxy  # Enable proxy mode
config.active_storage.draw_routes = false                # Disable default routes (for custom auth)
config.active_storage.track_variants = true              # Track variant records (default)
config.active_storage.service = :local                   # Set storage service

Agent Skills: Rails Active Storage Expert

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