Dependent destroy and nullify in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does dependent: :destroy do in a Rails association?

It deletes all associated child records when the parent record is deleted, ensuring no orphaned records remain.

beginner

What is the effect of dependent: :nullify in a Rails model association?

It sets the foreign key of associated child records to NULL instead of deleting them when the parent is deleted.

intermediate

When should you use dependent: :destroy instead of dependent: :nullify?

Use destroy when child records should be removed with the parent to keep data clean. Use nullify when child records should remain but lose their link to the deleted parent.

beginner

What happens if you don't specify any dependent option in a Rails association?

The associated child records remain unchanged when the parent is deleted, which can lead to orphaned records with invalid foreign keys.

intermediate

Explain the difference between dependent: :destroy and dependent: :delete_all.

destroy runs callbacks on each child before deleting, while delete_all deletes all children directly in the database without callbacks.

What does dependent: :nullify do when the parent record is deleted?

A Sets child records' foreign keys to NULL

B Deletes all child records

C Raises an error

D Does nothing

dependent: :nullify sets the foreign key in child records to NULL, keeping the child records but removing their link to the parent.

Which dependent option runs callbacks on child records before deletion?

A nullify

B destroy

C delete_all

D restrict_with_exception

dependent: :destroy runs callbacks on each child record before deleting it.

If you want to keep child records but remove their link to the parent, which option do you use?

A dependent: :restrict_with_error

B dependent: :destroy

C dependent: :delete_all

D dependent: :nullify

dependent: :nullify keeps child records but sets their foreign key to NULL.

What risk exists if you omit the dependent option in a has_many association?

A Orphaned child records may remain

B Child records are automatically deleted

C Parent record cannot be deleted

D Foreign keys are set to NULL

Without dependent, deleting a parent leaves child records with foreign keys pointing to a non-existent parent, causing orphaned records.

Which dependent option deletes child records directly in the database without callbacks?

A destroy

B nullify

C delete_all

D restrict_with_exception

dependent: :delete_all deletes child records directly in the database, skipping callbacks.

Describe how dependent: :destroy and dependent: :nullify affect child records when a parent record is deleted in Rails.

Think about whether child records are deleted or kept.

Explain why it is important to use the dependent option in Rails associations.

Consider what happens to child records when the parent is removed.

Practice

1. What does dependent: :destroy do in a Rails model association?

easy

A. Deletes associated records when the parent record is deleted.

B. Sets associated records' foreign keys to null when the parent is deleted.

C. Prevents deletion of the parent record if associated records exist.

D. Copies associated records to a backup table before deletion.

Solution

1. Step 1: Understand the role of dependent option

   The dependent option in Rails associations controls what happens to associated records when the parent is deleted.

2. Step 2: Identify behavior of :destroy

   The :destroy option deletes all associated records automatically when the parent record is deleted.

3. Final Answer:

   Deletes associated records when the parent record is deleted. -> Option A

4. Quick Check:

   dependent: :destroy = deletes associated records [OK]

Hint: Destroy deletes children; nullify just clears foreign keys [OK]

Common Mistakes:

• Confusing :destroy with :nullify behavior
• Thinking :destroy prevents parent deletion
• Assuming it backs up records before deletion

2. Which is the correct syntax to set an association to nullify dependent records in Rails?

easy

A. has_many :comments, dependent: :delete_all

B. has_many :comments, dependent: :remove

C. has_many :comments, dependent: :nullify

D. has_many :comments, dependent: :destroy_all

Solution

1. Step 1: Recall valid dependent options in Rails

   Rails supports :destroy, :delete_all, and :nullify as valid dependent options.

2. Step 2: Identify the option that sets foreign keys to null

   dependent: :nullify sets the foreign keys of associated records to null instead of deleting them.

3. Final Answer:

   has_many :comments, dependent: :nullify -> Option C

4. Quick Check:

   dependent: :nullify = sets foreign keys to null [OK]

Hint: Use :nullify to keep records but clear foreign keys [OK]

Common Mistakes:

• Using :destroy_all which is invalid
• Confusing :delete_all with :nullify
• Using non-existent :remove option

3. Given the models:

class Post < ApplicationRecord
  has_many :comments, dependent: :nullify
end

class Comment < ApplicationRecord
  belongs_to :post, optional: true
end

What happens to comments when a post is deleted?

medium

A. Comments remain but their post_id is set to null.

B. All comments are deleted from the database.

C. Comments remain unchanged with the same post_id.

D. An error occurs because comments must belong to a post.

Solution

1. Step 1: Analyze the dependent option in Post model

   The dependent: :nullify option means when a post is deleted, associated comments' foreign keys (post_id) are set to null.

2. Step 2: Check if comments can have null post_id

   The belongs_to :post, optional: true allows comments to exist without a post, so setting post_id to null is valid.

3. Final Answer:

   Comments remain but their post_id is set to null. -> Option A

4. Quick Check:

   dependent: :nullify + optional: true = nullify foreign keys [OK]

Hint: Nullify keeps records but clears foreign keys if optional true [OK]

Common Mistakes:

• Assuming comments get deleted with post
• Ignoring optional: true and expecting errors
• Thinking comments keep old post_id after deletion

4. Identify the error in this Rails model code:

class User < ApplicationRecord
  has_many :posts, dependent: :nullify
end

class Post < ApplicationRecord
  belongs_to :user
end

medium

A. belongs_to should be has_many in Post model.

B. No error; code works as expected.

C. dependent: :nullify is invalid syntax for has_many.

D. Posts cannot have null user_id because belongs_to is not optional.

Solution

1. Step 1: Understand dependent: :nullify effect

   It sets foreign keys of associated posts to null when a user is deleted.

2. Step 2: Check belongs_to association in Post

   belongs_to :user by default requires user_id to be present (not null). Since it's not marked optional, nullifying user_id causes validation or DB errors.

3. Final Answer:

   Posts cannot have null user_id because belongs_to is not optional. -> Option D

4. Quick Check:

   dependent: :nullify requires optional belongs_to [OK]

Hint: Use optional: true with nullify to avoid errors [OK]

Common Mistakes:

• Assuming nullify works without optional: true
• Thinking dependent: :nullify is invalid syntax
• Confusing belongs_to with has_many in Post

5. You have a Library model with many Books. You want to delete a library and automatically delete all its books. But if a Book has Reviews, you want to keep the reviews but remove the book reference (set book_id to null). Which setup correctly achieves this?

hard

A. Library has_many :books, dependent: :destroy; Book has_many :reviews, dependent: :destroy; Review belongs_to :book, optional: true

B. Library has_many :books, dependent: :destroy; Book has_many :reviews, dependent: :nullify; Review belongs_to :book, optional: true

C. Library has_many :books, dependent: :nullify; Book has_many :reviews, dependent: :destroy; Review belongs_to :book

D. Library has_many :books, dependent: :nullify; Book has_many :reviews, dependent: :nullify; Review belongs_to :book

Solution

1. Step 1: Set Library to delete all books on deletion

   Using dependent: :destroy on has_many :books deletes all books when a library is deleted.

2. Step 2: Set Book to nullify reviews instead of deleting

   Using dependent: :nullify on has_many :reviews keeps reviews but sets their book_id to null.

3. Step 3: Allow reviews to have null book_id

   Marking belongs_to :book, optional: true in Review allows book_id to be null without errors.

4. Final Answer:

   Library has_many :books, dependent: :destroy; Book has_many :reviews, dependent: :nullify; Review belongs_to :book, optional: true -> Option B

5. Quick Check:

   destroy deletes children; nullify clears foreign keys with optional true [OK]

Hint: Destroy parents, nullify grandchildren with optional belongs_to [OK]

Common Mistakes:

• Using nullify on library-books deletes nothing
• Destroying reviews deletes data you want to keep
• Not setting optional: true on Review's belongs_to