Association callbacks in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are association callbacks in Rails?

Association callbacks are special methods that run automatically when you add or remove objects from an association, like has_many or has_one. They help you run code during these changes.

beginner

Name two common association callbacks in Rails.

Two common association callbacks are before_add and after_remove. They run before adding an object to an association and after removing an object, respectively.

intermediate

How do you define a before_add callback on a has_many association?

You add it inside the association like this:
has_many :comments, before_add: :check_comment
Then define the method check_comment in your model to run before adding a comment.

intermediate

What happens if a before_add callback throws an exception?

If a before_add callback raises an error, the object will NOT be added to the association. This lets you stop unwanted changes safely.

intermediate

Can association callbacks access the object being added or removed? How?

Yes, association callbacks receive the object being added or removed as an argument. For example:
before_add :method_name where method_name(added_object) can use that object inside.

Which callback runs before adding an object to a Rails association?

A before_add

B after_add

C before_save

D after_remove

The before_add callback runs right before an object is added to an association.

What argument does an association callback method receive?

A No arguments

B The parent model

C The object being added or removed

D The database connection

Association callbacks receive the object being added or removed as an argument.

If a before_add callback raises an error, what happens?

A The object is not added

B The object is still added

C The callback is ignored

D The entire transaction rolls back

Raising an error in before_add stops the object from being added.

Which association type supports before_add callbacks?

A All of the above

B has_many

C has_one

D belongs_to

before_add is commonly used with has_many associations.

Where do you specify association callbacks in Rails?

A In the routes file

B In the controller

C In the database schema

D Inside the association declaration

You specify callbacks like before_add inside the association declaration in the model.

Explain what association callbacks are and give an example of when you might use one.

Think about how Rails lets you run code when you add or remove related objects.

Describe how to add a before_add callback to a has_many association and what happens if the callback raises an error.

Focus on the association declaration and callback behavior.

Practice

1. What is the purpose of after_add and after_remove callbacks in Rails associations?

easy

A. To define database indexes on associations

B. To validate the presence of associated records

C. To preload associated records for performance

D. To run code automatically when related records are added or removed

Solution

1. Step 1: Understand association callbacks

   Rails provides after_add and after_remove callbacks to trigger code when associated records change.

2. Step 2: Identify their purpose

   These callbacks run automatically when you add or remove related records, helping keep data in sync.

3. Final Answer:

   To run code automatically when related records are added or removed -> Option D

4. Quick Check:

   Association callbacks = automatic code on add/remove [OK]

Hint: Callbacks run code on add/remove related records [OK]

Common Mistakes:

• Confusing callbacks with validations
• Thinking callbacks preload data
• Assuming callbacks define indexes

2. Which is the correct syntax to add an after_add callback to a has_many association named comments in a Rails model?

easy

A. has_many :comments, after_add: :notify_comment_added

B. has_many :comments, after_add => :notify_comment_added

C. has_many :comments do after_add :notify_comment_added end

D. has_many :comments, callback_after_add: :notify_comment_added

Solution

1. Step 1: Recall correct hash syntax for options

   Rails uses symbol keys with colon and value pairs like after_add: :method_name.

2. Step 2: Match syntax to association

   The correct syntax is has_many :comments, after_add: :notify_comment_added.

3. Final Answer:

   has_many :comments, after_add: :notify_comment_added -> Option A

4. Quick Check:

   Use colon syntax for options in associations [OK]

Hint: Use colon syntax for options in associations [OK]

Common Mistakes:

• Using hash rocket => instead of colon :
• Trying to define callback inside block
• Using wrong option name like callback_after_add

3. Given the following code, what will be printed when a new comment is added to a Post?

class Post < ApplicationRecord
  has_many :comments, after_add: :log_comment_added

  def log_comment_added(comment)
    puts "Comment added: #{comment.body}"
  end
end

post = Post.new
post.comments << Comment.new(body: 'Nice post!')

medium

A. No output

B. Comment added: Nice post!

C. Error: undefined method log_comment_added

D. Comment added:

Solution

1. Step 1: Understand after_add callback behavior

   The after_add callback calls log_comment_added with the new comment when added.

2. Step 2: Check the method output

   The method prints "Comment added: " plus the comment's body, which is "Nice post!".

3. Final Answer:

   Comment added: Nice post! -> Option B

4. Quick Check:

   after_add calls method with new record [OK]

Hint: after_add calls method with added record [OK]

Common Mistakes:

• Expecting no output
• Thinking callback is not triggered on << operator
• Assuming method is missing error

4. Identify the error in this Rails model code using after_remove callback:

class User < ApplicationRecord
  has_many :posts, after_remove: :notify_post_removed

  def notify_post_removed
    puts 'Post removed'
  end
end

medium

A. The callback method must accept the removed record as an argument

B. The callback name should be after_remove_post

C. The after_remove option is not valid for has_many

D. The method should be private

Solution

1. Step 1: Check callback method signature

   Callbacks like after_remove receive the removed record as an argument.

2. Step 2: Compare method definition

   The method notify_post_removed has no parameters, so it will cause an error.

3. Final Answer:

   The callback method must accept the removed record as an argument -> Option A

4. Quick Check:

   Callback methods need removed record argument [OK]

Hint: Callback methods must accept the changed record argument [OK]

Common Mistakes:

• Defining callback method without parameters
• Using invalid callback option name
• Thinking method visibility causes error

5. You want to track when tags are added or removed from an Article model using has_many :tags. You want to update a counter on Article each time tags change. Which is the best way to implement this using association callbacks?

hard

A. Use before_save callback on Article to count tags

B. Override the tags= setter method to update the counter

C. Use after_add and after_remove callbacks on has_many :tags to update the counter method

D. Manually call a method to update the counter after changing tags

Solution

1. Step 1: Understand the goal

   You want to update a counter automatically when tags are added or removed.

2. Step 2: Choose appropriate callback

   after_add and after_remove on has_many :tags run code exactly when tags change.

3. Step 3: Compare other options

   Overriding setter or using before_save is less direct and can miss changes; manual calls are error-prone.

4. Final Answer:

   Use after_add and after_remove callbacks on has_many :tags to update the counter method -> Option C

5. Quick Check:

   Use association callbacks for automatic related record updates [OK]

Hint: Use after_add and after_remove for automatic updates [OK]

Common Mistakes:

• Overriding setter unnecessarily
• Using before_save which may miss association changes
• Forgetting to automate updates