Active Storage

When to use this skill

• Adding file or image uploads to a Rails model
• Configuring cloud storage (S3, GCS, Azure) or local disk
• Generating image variants (thumbnails, resizing, format conversion)
• Setting up direct uploads from the browser to cloud storage
• Validating uploaded file types, sizes, and content
• Serving or downloading attached files
• Preventing N+1 queries on attachments

Principles

1. Triple-validate uploads — check content-type, file extension, AND magic bytes. Never trust the client.
2. Variants are lazy — define named variants on the model, they're generated on first access. Pre-process in background if needed.
3. Eager load attachments — always use with_attached_* scopes to prevent N+1 queries.
4. Direct upload for large files — skip the server roundtrip; upload directly from browser to cloud storage.
5. Proxy mode for private files — use instead of redirect mode when files must stay behind authentication.