Seed data in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output after running this seed file?

Given this Rails seed file snippet, what will be the count of users in the database after running rails db:seed?

Ruby on Rails

User.create([{ name: 'Alice' }, { name: 'Bob' }])
User.create(name: 'Charlie')
User.create(name: 'Alice')

A 2

B 4

C 3

D 1

💡 Hint

Each create call adds records unless validations prevent it.

The first line creates 2 users, second line adds 1 more, and the third adds another user named 'Alice'. Total is 4 users.

📝 Syntax

intermediate

Which seed syntax will cause a syntax error?

Identify which option will cause a syntax error when running rails db:seed.

A User.create(name: 'Dave', age: 30)

B User.create([{ name: 'Eve' }, { name: 'Frank' }])

C User.create(name: 'Grace' age: 25)

D User.create([{ name: 'Heidi' }])

💡 Hint

Check for missing commas between hash keys.

Option C is missing a comma between name: 'Grace' and age: 25, causing a syntax error.

❓ state_output

advanced

What will be the value of Product.count after seeding?

Consider this seed code snippet. What is the final count of products in the database?

Ruby on Rails

Product.destroy_all
Product.create(name: 'Pen', price: 1.5)
Product.create(name: 'Notebook', price: 3.0)
Product.create(name: 'Pen', price: 1.5)

A 3

B 2

C 0

D 1

💡 Hint

Destroy_all clears all records before creating new ones.

All three create calls add products. No uniqueness validation is shown, so duplicates are allowed. Total is 3.

🔧 Debug

advanced

Why does this seed file fail with a validation error?

This seed file fails when run. What is the most likely cause?

Ruby on Rails

Category.create(name: 'Books')
Category.create(name: 'Books')

A The database connection is not established.

B The <code>Category</code> model does not allow creation without an ID.

C The seed file syntax is invalid due to missing commas.

D The <code>name</code> field must be unique and duplicates cause validation failure.

💡 Hint

Check model validations for uniqueness constraints.

If name is validated as unique in the model, creating two categories with the same name causes a validation error.

🧠 Conceptual

expert

What is the best way to ensure idempotent seed data in Rails?

You want to run rails db:seed multiple times without creating duplicate records. Which approach achieves this?

A Use <code>Model.find_or_create_by</code> with unique attributes to avoid duplicates.

B Use <code>Model.create</code> without checks; duplicates are acceptable.

C Manually delete all records before seeding with <code>Model.destroy_all</code>.

D Run <code>rails db:seed</code> only once and never again.

💡 Hint

Think about how to check if a record exists before creating it.

Using find_or_create_by checks for existing records with given attributes and creates only if none exist, making seeds safe to run multiple times.

Practice

1. What is the main purpose of seed data in a Rails application?

easy

A. To run the Rails server

B. To create database tables automatically

C. To generate user interface templates

D. To fill the database with initial example data for testing and development

Solution

1. Step 1: Understand what seed data is

   Seed data is used to add example or default data into the database to help developers test and develop features easily.

2. Step 2: Identify the correct purpose

   Among the options, only filling the database with initial example data matches the purpose of seed data.

3. Final Answer:

   To fill the database with initial example data for testing and development -> Option D

4. Quick Check:

   Seed data purpose = Fill database with example data [OK]

Hint: Seed data = example info in database for testing [OK]

Common Mistakes:

• Confusing seed data with database schema creation
• Thinking seed data runs the server
• Assuming seed data generates UI templates

2. Which command correctly runs the seed file to load data into the database in Rails?

easy

A. rails generate seed

B. bin/rails db:seed

C. bin/rails db:create

D. rails server

Solution

1. Step 1: Recall the command to run seed data

   The standard command to load seed data in Rails is bin/rails db:seed.

2. Step 2: Check other options

   rails generate seed is not a valid command, bin/rails db:create creates the database, and rails server starts the server.

3. Final Answer:

   bin/rails db:seed -> Option B

4. Quick Check:

   Run seed file = bin/rails db:seed [OK]

Hint: Run seeds with bin/rails db:seed command [OK]

Common Mistakes:

• Using rails generate seed instead of db:seed
• Confusing db:create with db:seed
• Trying to run seeds with rails server

3. Given this seed code in db/seeds.rb:

3.times do |i|
  User.create(name: "User #{i + 1}")
end

What will be the names of the users created after running bin/rails db:seed?

medium

A. ["User 1", "User 2"]

B. ["User 0", "User 1", "User 2"]

C. ["User 1", "User 2", "User 3"]

D. ["User 3", "User 2", "User 1"]

Solution

1. Step 1: Understand the loop and index

   The loop runs 3 times with i values 0, 1, 2.

2. Step 2: Calculate user names

   Each user name is "User " plus i + 1, so names are "User 1", "User 2", "User 3".

3. Final Answer:

   ["User 1", "User 2", "User 3"] -> Option C

4. Quick Check:

   Loop 3 times with i+1 = User 1, 2, 3 [OK]

Hint: Remember loop index starts at 0, add 1 for names [OK]

Common Mistakes:

• Using i without adding 1, causing off-by-one names
• Assuming loop runs 2 times instead of 3
• Mixing order of names

4. What is wrong with this seed code snippet?

User.create(name: "Alice")
User.create(name: "Bob")
User.create(name: )

medium

A. Missing value for name in the last User.create call

B. User.create cannot be called multiple times

C. Names must be unique, so this will fail

D. The code should be inside a loop

Solution

1. Step 1: Check syntax of each User.create call

   The first two calls have valid name values. The last call has name: with no value, which is a syntax error.

2. Step 2: Verify other options

   Calling User.create multiple times is allowed. Names do not have to be unique unless validated. Loop is optional.

3. Final Answer:

   Missing value for name in the last User.create call -> Option A

4. Quick Check:

   Empty value after name: causes syntax error [OK]

Hint: Check for missing values after keys in hashes [OK]

Common Mistakes:

• Thinking multiple create calls are invalid
• Assuming uniqueness is required by default
• Believing loops are mandatory for seeds

5. You want to seed your database with categories but only if they don't already exist. Which seed code snippet correctly does this?

hard

A. ['Books', 'Movies', 'Music'].each do |name|
Category.find_or_create_by(name: name)
end

B. ['Books', 'Movies', 'Music'].each do |name|
Category.create(name: name)
end

C. Category.delete_all
['Books', 'Movies', 'Music'].each do |name|
Category.create(name: name)
end

D. Category.create(['Books', 'Movies', 'Music'])

Solution

1. Step 1: Understand the requirement to avoid duplicates

   We want to add categories only if they don't exist yet, so we must check before creating.

2. Step 2: Analyze each option

   ['Books', 'Movies', 'Music'].each do |name| Category.create(name: name) end always creates new records, causing duplicates. ['Books', 'Movies', 'Music'].each do |name| Category.find_or_create_by(name: name) end uses find_or_create_by which checks existence before creating. Category.delete_all ['Books', 'Movies', 'Music'].each do |name| Category.create(name: name) end deletes all categories first, which is destructive. Category.create(['Books', 'Movies', 'Music']) tries to create with an array directly, which is invalid syntax.

3. Final Answer:

   ['Books', 'Movies', 'Music'].each do |name| Category.find_or_create_by(name: name) end -> Option A

4. Quick Check:

   Use find_or_create_by to avoid duplicates [OK]

Hint: Use find_or_create_by to prevent duplicate seed entries [OK]

Common Mistakes:

• Creating records without checking duplicates
• Deleting all data before seeding unnecessarily
• Passing array directly to create without hashes