How to Seed Database in Rails: Simple Guide with Examples

In Rails, you seed the database by adding data to the db/seeds.rb file and then running rails db:seed. This command loads the seed file and inserts the data into your database.

Syntax

Use the db/seeds.rb file to write Ruby code that creates records in your database. Then run rails db:seed in the terminal to execute this file and add the data.

Key parts:

• db/seeds.rb: The file where you define seed data.
• rails db:seed: Command to run the seed file.

ruby

User.create(name: 'Alice', email: '[e-mail]')
Product.create(name: 'Book', price: 9.99)

Example

This example shows how to seed a simple database with users and products. Running rails db:seed will add these records.

ruby

# db/seeds.rb
User.create(name: 'Alice', email: '[e-mail]')
User.create(name: 'Bob', email: '[e-mail]')

Product.create(name: 'Book', price: 9.99)
Product.create(name: 'Pen', price: 1.49)

Output

Created 2 users and 2 products in the database.

Common Pitfalls

Common mistakes include:

• Not running rails db:seed after editing seeds.rb.
• Running seeds multiple times without clearing data, causing duplicates.
• Using create! without handling validation errors.

To avoid duplicates, use find_or_create_by instead of create.

ruby

# Wrong way (duplicates on multiple runs)
User.create(name: 'Alice', email: '[e-mail]')

# Right way (avoids duplicates)
User.find_or_create_by(email: '[e-mail]') do |user|
  user.name = 'Alice'
end

Quick Reference

Command	Description
rails db:seed	Runs the seed file to populate the database
rails db:reset	Drops, creates, migrates, and seeds the database
find_or_create_by	Creates a record only if it doesn't exist to avoid duplicates
db/seeds.rb	File where seed data is defined

Key Takeaways

Write seed data in db/seeds.rb using Ruby code to create records.

Run rails db:seed to load seed data into your database.

Use find_or_create_by to prevent duplicate records when seeding multiple times.

Edit seeds.rb and rerun rails db:seed whenever you want to update seed data.

Use rails db:reset to clear and reseed your database in one command.