Dependent destroy and nullify in Ruby on Rails - Step-by-Step Execution

Concept Flow - Dependent destroy and nullify

Parent object exists

↓

Delete parent object

↓

Dependent destroy

↓

Delete child objects

↓

Child objects removed

When a parent record is deleted, dependent destroy deletes child records, while dependent nullify keeps children but removes their link to the parent.

Execution Sample

Ruby on Rails

class Author < ApplicationRecord
  has_many :books, dependent: :destroy
end

Author.find(1).destroy

Deletes an author and all their books because of dependent: :destroy.

Execution Table

Step	Action	Parent Exists?	Child Exists?	Child Foreign Key	Result
1	Author with id=1 exists	Yes	Yes (3 books)	author_id = 1	Parent and children present
2	Call Author.find(1).destroy	Yes	Yes (3 books)	author_id = 1	Start deletion
3	dependent: :destroy triggers	Yes	Yes (3 books)	author_id = 1	Prepare to delete children
4	Delete each book	Yes	No	null	All 3 books deleted
5	Delete author	No	No	null	Author deleted, children gone
6	End	No	No	null	Deletion complete

💡 Author and all dependent books are deleted, no children remain.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5
Author.exists?(1)	true	true	true	false
Books.count with author_id=1	3	3	0	0

Key Moments - 2 Insights

Why do the child books get deleted when the author is destroyed?

Because dependent: :destroy tells Rails to delete all associated child records before deleting the parent, as shown in execution_table step 3 and 4.

What happens to child records with dependent: :nullify?

Instead of deleting children, Rails sets their foreign key to null, keeping the child records but removing the link to the parent.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what happens to the child books?

A Their foreign keys are set to null

B They remain unchanged

C They are deleted

D They are duplicated

💡 Hint

Check the 'Result' column at step 4 showing 'All 3 books deleted'

At which step does the parent author get deleted?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Action' and 'Result' columns in step 5

If dependent: :nullify was used instead of :destroy, what would happen to the books after author deletion?

A Books would remain with author_id set to null

B Books would remain unchanged with author_id still set

C Books would be deleted

D Books would be duplicated

💡 Hint

Refer to key_moments explanation about dependent: :nullify behavior

Concept Snapshot

dependent: :destroy deletes all child records when parent is deleted.
dependent: :nullify keeps child records but sets their foreign keys to null.
Use dependent options to control child record cleanup.
Deleting parent triggers dependent action automatically.
Choose destroy to remove children, nullify to keep them without parent link.

Full Transcript

In Rails, when you delete a parent record, you can control what happens to its child records using dependent options. With dependent: :destroy, deleting the parent deletes all its children. With dependent: :nullify, deleting the parent keeps the children but removes their link by setting foreign keys to null. The execution table shows step-by-step how deleting an author with dependent: :destroy deletes all their books first, then deletes the author. Variable tracking confirms the author and books no longer exist after deletion. This helps keep your database clean and consistent automatically.