Seed data in Ruby on Rails - Mini Project: Build & Apply

Seed Data in Rails

📖 Scenario: You are building a simple Rails app to manage a bookstore. You want to add some initial books to your database so you can see them when you run the app.

🎯 Goal: Create seed data for the Book model with exact titles and authors. Then configure a count variable to track how many books you add. Finally, write the code to insert these books into the database using Rails seed conventions.

📋 What You'll Learn

Create a list of books with exact titles and authors

Add a variable to count the number of books

Use a loop to create each book record in the database

Ensure the seed file ends with a message showing how many books were added

💡 Why This Matters

🌍 Real World

Seed data helps you quickly fill your app's database with useful starting information, so you can test and develop features without manual data entry.

💼 Career

Knowing how to write seed files is important for backend and full-stack developers working with Rails to set up databases efficiently.

1

Create the books list

Create a variable called books that holds an array of hashes. Each hash should have the keys :title and :author with these exact entries: { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' }, { title: '1984', author: 'George Orwell' }, and { title: 'To Kill a Mockingbird', author: 'Harper Lee' }.

Ruby on Rails

# Create the books array with hashes for title and author
# Your code here

Hint

Use an array [] with hashes {} inside. Each hash needs :title and :author keys.

2

Add a book count variable

Create a variable called book_count and set it to 0. This will keep track of how many books you add to the database.

Ruby on Rails

books = [
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
]
# Create book_count variable and set to 0
# Your code here

Hint

Just write book_count = 0 below the books array.

3

Create books in the database

Use a books.each do |book| loop to create each book in the database with Book.create!(title: book[:title], author: book[:author]). Inside the loop, increase book_count by 1 using book_count += 1.

Ruby on Rails

books = [
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
]

book_count = 0

# Loop over books and create each one, increment book_count
# Your code here

Hint

Use books.each do |book| ... end and inside call Book.create! with the right keys. Don't forget to add book_count += 1.

4

Add a completion message

Add a final line that outputs a message with puts showing exactly: "Added #{book_count} books to the database."

Ruby on Rails

books = [
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
]

book_count = 0

books.each do |book|
  Book.create!(title: book[:title], author: book[:author])
  book_count += 1
end

# Add a puts line to show how many books were added
# Your code here

Hint

Use puts with double quotes and #{book_count} inside to show the number.

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