File uploads with Active Storage in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Active Storage in Rails?

Active Storage is a built-in Rails library that makes it easy to upload files to cloud services or local disk and attach them to Active Record models.

beginner

How do you attach a file to a model using Active Storage?

You use the `has_one_attached` or `has_many_attached` macro in the model, then assign a file to the attachment attribute, for example: `user.avatar.attach(params[:avatar])`.

beginner

Which command sets up Active Storage in a Rails app?

Run `rails active_storage:install` to generate the necessary database tables, then run `rails db:migrate` to apply them.

beginner

How can you display an uploaded image in a Rails view using Active Storage?

Use the `image_tag` helper with the attached file, for example: `<%= image_tag user.avatar %>` if `avatar` is attached.

beginner

What is the difference between `has_one_attached` and `has_many_attached`?

`has_one_attached` allows attaching a single file to a model, while `has_many_attached` allows attaching multiple files.

Which command do you run to create Active Storage tables in your database?

A rails active_storage:migrate

B rails generate model ActiveStorage

C rails db:create

D rails active_storage:install

The command `rails active_storage:install` generates the migration files needed for Active Storage tables.

How do you declare a single file attachment in a Rails model?

A has_one_attached :file

B has_many_attached :file

C attach_one :file

D attach_many :file

`has_one_attached` declares a single file attachment for the model.

What method do you use to attach a file from a form to a model instance?

A model.file.attach(params[:file])

B model.attach_file(params[:file])

C model.file = params[:file]

D model.attach(params[:file])

You call `.attach` on the attachment attribute, passing the uploaded file from params.

How can you show an attached image in a Rails view?

A <%= show_image model.file %>

B <%= image_tag model.file %>

C <%= display_image model.file %>

D <%= render_image model.file %>

Use the `image_tag` helper with the attached file to display the image.

Which Active Storage macro allows multiple files to be attached?

A attach_all

B has_one_attached

C has_many_attached

D attach_many

`has_many_attached` is used for multiple file attachments.

Explain the steps to set up file uploads with Active Storage in a new Rails app.

Think about setup, model, controller, and view parts.

Describe how you would attach and display multiple images for a product using Active Storage.

Focus on multiple attachments and rendering them.

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