Uniqueness validation in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a duplicate record is saved with uniqueness validation?

Given a Rails model with validates :email, uniqueness: true, what will happen if you try to save two records with the same email?

Ruby on Rails

class User < ApplicationRecord
  validates :email, uniqueness: true
end

user1 = User.create(email: '[e-mail]')
user2 = User.new(email: '[e-mail]')
success = user2.save

A The second save will succeed but overwrite the first record silently.

B The second save will succeed and create a duplicate record because uniqueness validation only works on update.

C The second save will raise a runtime error due to a database constraint violation.

D The second save will fail and <code>success</code> will be false because the email is not unique.

💡 Hint

Think about what the uniqueness validation does before saving a record.

The validates :email, uniqueness: true ensures that no two records have the same email. When you try to save a second record with an existing email, the validation fails and save returns false.

📝 Syntax

intermediate

Which option correctly adds case-insensitive uniqueness validation on username?

You want to ensure usernames are unique regardless of letter case. Which validation code is correct?

Ruby on Rails

class User < ApplicationRecord
  # Add validation here
end

A validates_uniqueness_of :username, case_sensitive: false

B validates :username, uniqueness: { case_sensitive: false }

C validates :username, uniqueness: true, case_sensitive: false

D validates :username, unique: { case_sensitive: false }

💡 Hint

Look for the correct syntax for options inside the uniqueness validator.

The correct syntax is validates :username, uniqueness: { case_sensitive: false }. The option case_sensitive must be inside the uniqueness hash.

🔧 Debug

advanced

Why does uniqueness validation fail with concurrent record creation?

Consider this code:

class User < ApplicationRecord
  validates :email, uniqueness: true
end

# Two threads run:
User.create(email: '[e-mail]')
User.create(email: '[e-mail]')

Why might both records be created despite the validation?

A Because uniqueness validation is not atomic and race conditions can allow duplicates before database constraints catch them.

B Because Rails validations are skipped on create by default.

C Because the validation only checks for uniqueness on update, not on create.

D Because the database automatically allows duplicates unless a unique index is added.

💡 Hint

Think about what happens when two processes check uniqueness at the same time.

Uniqueness validation happens in Ruby code before saving. If two processes check at the same time, both may pass validation before either saves, causing duplicates. This is a race condition. To fully prevent duplicates, a unique database index is needed.

❓ state_output

advanced

What is the value of user.errors[:email] after a failed uniqueness validation?

Given this code:

class User < ApplicationRecord
  validates :email, uniqueness: true
end

user1 = User.create(email: '[e-mail]')
user2 = User.new(email: '[e-mail]')
user2.valid?
errors = user2.errors[:email]

What is the value of errors?

A ["can't be blank"]

B [] (empty array)

C ["has already been taken"]

D ["is invalid"]

💡 Hint

What message does Rails add for uniqueness validation failures?

When uniqueness validation fails, Rails adds the error message "has already been taken" to the attribute's errors.

🧠 Conceptual

expert

Why should you add a unique index in the database when using uniqueness validation?

Which is the best explanation for adding a unique index on a column that has Rails uniqueness validation?

A To ensure data integrity at the database level and prevent duplicates even with concurrent writes.

B To speed up the Rails validation process by caching results in the database.

C Because Rails validations do not run on production environments by default.

D To allow Rails to automatically rollback transactions on validation failure.

💡 Hint

Think about what happens if two users save the same data at the same time.

Rails validations run in application code and can miss duplicates during concurrent writes. A unique index in the database guarantees no duplicates can be saved, ensuring data integrity.