File uploads with Active Storage in Ruby on Rails

Introduction

Active Storage helps you easily add file uploads to your Rails app. It manages storing files and linking them to your data.

You want users to upload profile pictures.

You need to attach documents to records like invoices or reports.

You want to store images for blog posts or products.

You want to handle file uploads without writing complex code.

You want to serve uploaded files securely and efficiently.

Syntax

Ruby on Rails

class User < ApplicationRecord
  has_one_attached :avatar
end

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

has_one_attached is for one file per record.

Use has_many_attached if you want multiple files.

Examples

This allows an article to have many images uploaded.

Ruby on Rails

class Article < ApplicationRecord
  has_many_attached :images
end

Enables direct uploads to cloud storage for faster uploads.

Ruby on Rails

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

Attaches the uploaded file to the user record in the controller.

Ruby on Rails

user.avatar.attach(params[:user][:avatar])

Sample Program

This example shows a User model with one attached avatar file. The form lets users pick a file to upload. The controller saves the user and attaches the file.

Ruby on Rails

class User < ApplicationRecord
  has_one_attached :avatar
end

# Controller example
class UsersController < ApplicationController
  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user, notice: "User created with avatar!"
    else
      render :new
    end
  end

  private

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

# View (new.html.erb)
<%= form_with model: @user, local: true do |form| %>
  <%= form.label :name %>
  <%= form.text_field :name %>

  <%= form.label :avatar %>
  <%= form.file_field :avatar %>

  <%= form.submit "Create User" %>
<% end %>

Important Notes

Remember to run rails active_storage:install and migrate your database before using Active Storage.

Uploaded files are stored in a service like local disk, Amazon S3, or Google Cloud Storage configured in config/storage.yml.

Use rails server and check browser DevTools Network tab to see file upload requests.

Summary

Active Storage makes file uploads easy and secure in Rails.

Use has_one_attached or has_many_attached to link files to models.

Forms use file_field to let users pick files to upload.

Practice

1. What is the purpose of has_one_attached in a Rails model when using Active Storage?

easy

A. To link a single uploaded file to a model instance

B. To create a database table for storing files

C. To validate the file type before upload

D. To automatically resize uploaded images

Solution

1. Step 1: Understand Active Storage attachment macros

   The has_one_attached macro in a Rails model declares a one-to-one association between the model and a single uploaded file.

2. Step 2: Differentiate from other options

   It does not create tables, validate file types, or resize images by itself; those are handled elsewhere.

3. Final Answer:

   To link a single uploaded file to a model instance -> Option A

4. Quick Check:

   has_one_attached = single file link [OK]

Hint: Remember: has_one_attached means one file per model [OK]

Common Mistakes:

• Confusing has_one_attached with database table creation
• Thinking it validates or processes files automatically
• Mixing it up with has_many_attached for multiple files

2. Which of the following is the correct way to add a file input field in a Rails form for uploading a file with Active Storage?

easy

A. <%= form.file_field :avatar %>

B. <%= form.text_field :avatar %>

C. <%= form.submit :avatar %>

D. <%= form.select :avatar %>

Solution

1. Step 1: Identify the correct form helper for file uploads

   The file_field helper creates a file input allowing users to select files for upload.

2. Step 2: Exclude other input types

   Text fields, submit buttons, and select dropdowns do not allow file selection.

3. Final Answer:

   <%= form.file_field :avatar %> -> Option A

4. Quick Check:

   file_field = file input [OK]

Hint: Use file_field for file inputs in forms [OK]

Common Mistakes:

• Using text_field instead of file_field for uploads
• Confusing submit button with file input
• Using select dropdown for file selection

3. Given the following model and form code, what will be the result after submitting a file?

class User < ApplicationRecord
  has_one_attached :profile_pic
end

# In the form view:
<%= form_with model: @user do |form| %>
  <%= form.file_field :profile_pic %>
  <%= form.submit %>
<% end %>

medium

A. The file will be ignored because has_one_attached is missing

B. The uploaded file will be attached to the user's profile_pic

C. The form will raise a syntax error due to missing multipart option

D. The file will be saved as a string in the profile_pic column

Solution

1. Step 1: Check model attachment declaration

   The model uses has_one_attached :profile_pic, so it can attach one file to the user.

2. Step 2: Understand form_with default behavior

   Rails 6+ automatically sets multipart for file uploads in form_with, so the file will be sent correctly.

3. Final Answer:

   The uploaded file will be attached to the user's profile_pic -> Option B

4. Quick Check:

   has_one_attached + file_field = file attached [OK]

Hint: form_with sets multipart automatically for file_field [OK]

Common Mistakes:

• Assuming file is saved as string in DB column
• Thinking multipart option must be manually added
• Believing file is ignored without extra code

4. Identify the error in this code snippet for attaching multiple files using Active Storage:

class Product < ApplicationRecord
  has_one_attached :images
end

# In the form:
<%= form.file_field :images, multiple: true %>

medium

A. The model should not declare any attachment for multiple files

B. file_field does not support the multiple option

C. The form must use text_field instead of file_field

D. has_one_attached should be has_many_attached for multiple files

Solution

1. Step 1: Check model attachment type

   The model uses has_one_attached, which supports only one file, but the form allows multiple files.

2. Step 2: Correct attachment macro for multiple files

   To attach multiple files, the model must use has_many_attached.

3. Final Answer:

   has_one_attached should be has_many_attached for multiple files -> Option D

4. Quick Check:

   Multiple files need has_many_attached [OK]

Hint: Use has_many_attached for multiple file uploads [OK]

Common Mistakes:

• Using has_one_attached with multiple: true
• Thinking file_field can't handle multiple files
• Not declaring attachment in model for uploads

5. You want to allow users to upload multiple images for a gallery and display thumbnails after upload. Which combination of model and view code correctly implements this with Active Storage?

hard

A. Model: has_many_attached :photosView: form.text_field :photos

B. Model: has_one_attached :photosView: form.file_field :photos

C. Model: has_many_attached :photosView: form.file_field :photos, multiple: true

D. Model: has_one_attached :photosView: form.file_field :photos, multiple: true

Solution

1. Step 1: Choose correct model attachment for multiple files

   To attach multiple images, use has_many_attached :photos in the model.

2. Step 2: Use file_field with multiple option in the form

   The form must use file_field :photos, multiple: true to allow selecting multiple files.

3. Step 3: Display thumbnails after upload

   With this setup, you can iterate over @model.photos to show thumbnails.

4. Final Answer:

   Model: has_many_attached :photos and View: form.file_field :photos, multiple: true -> Option C

5. Quick Check:

   Multiple files need has_many_attached + multiple: true [OK]

Hint: Multiple files = has_many_attached + file_field multiple: true [OK]

Common Mistakes:

• Using has_one_attached for multiple files
• Omitting multiple: true in file_field
• Using text_field instead of file_field for uploads