Seeding Data In Ruby On Rails

A good db/seeds.rb, helps every app grow


Slides & Code Samples (MIT)

What We'll Cover

  • What Are Seeds?
  • Horror Stories On Why Seeds Are Useful
  • We'll Check The Vibe
  • Approaches To Seeding
  • Testing Seeds
  • What do good seeds look like?

What Are Seeds?

They're bits of data we use to pre-populate our database.

They could be anything from a User ever developer will use for local development, to chunks of data used for in all environments.

What Are Seeds?

In Ruby On Rails, we often use the rails db:seed command and you should be able to run it multiple times without fear of it going wrong.

Personally I just run bin/setup & expect it to do everything I need.

What Are Seeds?

Ruby on Rails stores them in the db/seeds.rb file.

โ–ธ app/
โ–ธ bin/
โ–ธ config/
โ–พ db/

What Are Seeds?

The out of the box the seeds file looks like:

# This file should contain all the record creation needed to seed the database with its default values.
# The data can then be loaded with the rails db:seed command (or created alongside the database with db:setup).
# Examples:
#   movies = Movie.create([{ name: 'Star Wars' }, { name: 'Lord of the Rings' }])
#   Character.create(name: 'Luke', movie: movies.first)

What Are Seeds?

If I'm lucky, I'd pick up a project where I can safely rerun the seeds:

# db/seeds.rb

User.find_or_create_by!(email: '') do |user|
  user.password = "12345678"
  user.password_confirmation = "12345678"
end if Rails.env.development?

What Are Seeds?

Sometimes they'd look a bit more like:

# db/seeds.rb

#ย Last updated 10 years ago - DON'T USE RUN IN PRODUCTION
TaxRate.find_or_create_by!(origin: 'NL', destination: 'GB') do |tax_rate|
  tax_rate.amount = "1.10"


Horror Stories On Why Seeds Are Useful

Horror Stories On Why Seeds Are Useful

The Stolen Laptop

Horror Stories On Why Seeds are Useful

"Staging" Environment Emailing Real Users

Horror Stories On Why Seeds are Useful

The poor developer experience

Check the Vibe

Who has worked on a project which had no seeds at all?


Who has worked on a project where they were sent a database dump to develop locally?


Who has worked on a project they felt the seeds were well maintained?


Approaches To Seeding

Approaches To Seeding

  • Explicitly Defined Seeds
  • Faker Generated Seeds
  • Fixtures & Factories Generated Seeds
  • Anonymised Production Database
  • Plain Old Ruby Objects

Explicitly Defined Seeds

# db/seeds.rb
# Load all the files in db/seeds folder
Dir[File.join(Rails.root, 'db', 'seeds', '*.rb')].sort.each do |seed|
  load seed
# db/seeds/companies.seeds.rb
Company.find_or_create_by!(name: 'Hatch') do |company|
  company.other_details = "Awesome Stuff"
  company.projects = [ 'Acme')]

Faker Generated Seeds

require 'faker'      #=> "Christophe Bartell" #=> ""

Faker Generated Seeds

# db/seeds.rb
require 'faker'

User.find_or_create_by!(email: '') do |user| =
  user.password = "12345678"
  user.password_confirmation = "12345678"

  100.times do
    user.posts << Faker::Job.title)
end if ENV['SEED_USER']

Fixtures Generated Seeds

$ rails db:fixtures:load FIXTURES=users,posts

There is a command, it will load fixtures into your current development environment.

I've not seen this approach used in the wild, when I tried it I had trouble with foreign keys (Plus I've never worked on a project with decent fixtures).

Factories Generated Seeds

FactoryBot.define do
  factory :user do
    name { }
    password {, "12345678") }
    sequence(:email) { |n| "email#{n}" }

    factory :user_with_channel do
      accounts { [build(:account_with_channel, owner: instance)] }

Factories Generated Seeds

# db/seeds.rb
require 'factory_bot'

if ENV['SEED_USER'] && User.where(email: '').zero?
  FactoryBot.create(:user_with_channel, email: ''))

I've been really liking this approach, it makes me want to have really good factories & run rails db:reset often.

Anonymised Production Database

require 'evil_seed'
EvilSeed.configure do |config|
  config.root('User', 'created_at > ?', Time.current.beginning_of_day)

    name  { }
    email { }


Plain Old Ruby Objects

Have you ever seen this?

class User < ApplicationRecord
  belongs_to :plan
end Plan.premium).plan.display_adverts?
# Outputs: false

Plain Old Ruby Objects

# models/plan.rb
Plan =, :display_adverts, keyword_init: true) do
  def self.all
    @all ||= [ 'free', display_adverts: true), 'premium', display_adverts: false)

  alias_method :display_adverts?, :display_adverts

Plain Old Ruby Objects

class User < ApplicationRecord
  def plan
    @plan ||= Plan.all.find { |plan| == plan_id }
end 'premium').plan.display_adverts?
# Outputs: false

Testing Seeds

Testing Seeds

# spec/db/seeds_spec.rb
RSpec.describe 'Rails.application' do
  describe '#load_seed' do
    subject { Rails.application.load_seed }
    before { ENV['SEED_USER'] = 'enabled' }
    after { ENV['SEED_USER'] = nil }

    it do
      expect { subject }.to change(User, :count).by(1)
        .and change(Channel, :count).by(1)

Testing Seeds

If you're using FactoryBot you can Lint factories to find out quickly why something is failing.

# spec/factory_bot_spec.rb
require "rails_helper"

describe FactoryBot do
  it { FactoryBot.lint(traits: true) }

What do good seeds look like?

How do you know if you've made good seeds?

What do good seeds look like?

  • How often are your developers reaching for production data?
  • Running the app in development, do you have paginated collections?
  • Do your developers often reset their database?
  • Are people reviewing code looking at preview environments?

What do good seeds look like?

Confidence to run bin/setup every time a developer changes branch, be able to use them in preview environments.

You can achieve this by a mix of PORO, Factories & partial anonymised dumps.
Make sure you have some cheap tests in there.


I want you to take a project with incomplete seeds, and make them 10% better.

Then let me know how it goes!

Thank you!
@MikeRogers0 on Twitter

Hello Everyone! I'm going to be talking seeding in Rails, it's something which we all probably know about but most the projects we're using probably are have empty files.

I'm going to throw some code at you during this talk! I'll drop links afterwards, but you can find them here. Also this talk is up on GitHub & written in markdown, if you want to take it & change it you can :D

This is what I'm going to be covering in this. The idea is by the end of it, you'll want to take any existing/new project and just add a few seeds. Plus I have some cool tricks to get a lot of value from your seed data.

Could be a user, could be Tax Rates or chucks of fixed data. The idea is that is development, or a preview environment we have a good representation of production so we can build a better product. And in production, environments we use them to make sure we have data we expect to be there (e.g. Tax Rates) & it's the same across all environments.

Rails has some built in commands to make this easier for us, and also their is the awesome bin/setup file. Recently I've been getting into the mindset that every time a developer changes branches, they should feel confident in the `bin/setup` file to rebuild a fresh environment. I think that's something awesome to aim for.

They also live here:

The default file sucks so hard. I hate it. I don't think you'd ever want your seeds to look like this at all.

If I'm lucky they look like this. Where there is actually something a bit more useful going on.

Maybe you'll also get this, or just a blank file. You might also get something a bit suspect. This is taken from a real project I worked on. I wish I had written that comment, but I didn't. In the production DB that value is different now. They we're made years ago & were to much of a hassle to keep up to date.

First story: This was a few years ago now, but a developer was sent a copy of the production database to use locally. Then someone broke into their office on a weekend (Smashed the door in) and stole their laptop. They had to tell the customer about it (Code + DB lost) & it was very uncomfortable. As we all work from home, this one is becoming more important to think about. Aside: I heard that Facebook the developers dev environment is in a cloud machine the SSH into. I'm super excited about GitHub Codespaces for not having the codebase/DB on local machines, but instead of cloud machine which is destroyed at the end of each session.

Second story: - Preview environment didn't have basic auth on it & passwords were the same. - The preview environments ran the cronjobs same as production - The dataset was an anonymised snapshot of production, often a few months behind (It would take a while to run & was very manual). - The preview environment used MailTrap to stop emails going out. But one day our client wanted to see the emails in their inbox. So some emails weren't anonymised in the snapshot. Then one day the client said "I want an exact copy of production in staging". Lots of whoopsies there, but if we had better process the preview environments sample data, it would have been avoidable.

You always kind of know a project is going to be rubbish when you need to open console to create a user to login with. It's kind of nice to go through a local application with a developer & show them lots of pages with real feeling content on it. Plus it's even better if there are 100s of records to make N+1s obvious. This is something I'm quite passionate about, like if we want people to really enjoy working with Rails we need to care about this kind of low hanging fruit.

Before we continue! Let's check the vibes of the room! Throw up some emojis!

Who has picked up a project where it was just blank? I feel it's pretty common.

I used to work in agencies quite a lot & it was pretty common to just take a copy of the production DB.

I don't think I've ever worked on a project which was amazing. I'm going to show you a very cool trick I've started doing which I think is the way to go.

So know we know the pain we're trying to avoid, how do we do it?

These are the ones you'd find in you `db/seeds.rb` file. The files can become very big pretty fast. I've seen them broken up into smaller files before Sometimes they might if statements to handles different environments and whatnot.

We all know faker? I find fuzz testing in development is kind of nice to find weird use cases. If I don't use Faker, I normally end up rolling my face over the keyboard. It make me think about the HTML components I'm designing.

I like this, especially in development environments

You can also take it to the next level, and use the data you'll use in your tests to the mess with your app. When I was experimenting with this, I found this command.

So I wondered what would happen if I just used these instead! I generally have pretty flushed out factories, which are good representations of my expected data.

I'm really liking this approach! ThoughtBot doesn't encourage this, but I've been using it a little & I really like it. The happy side effect I'm noticing it is: - Easier to write tests, as I'm looking at what the test will see. - I'm more incentivised to make working factories. I think if your app is simple, and you're starting fresh this could be a valid approach.

But what if you're picking up a project where there is nothing? Or the database is intense! Evil Martians have this pretty cool library for taking snapshots of user data & the relationships. So potentially you could automate a subset of data to be available for your local development & review apps. Potentially you could also run this in a way where you can log who is requesting what data, weather via version control or something else. I started using this in a project with a 8GB database recently which had a real mess of a database. We ended up using it to create a dump of our DB, which took about ~20 minutes to run & then we used our local machines.

Sometimes I find data is the same in all environments, but is in the database. So lets say you have a user with a Plan relationship. The plans are all the same for each environment.

So what if we pulled all the data from that table & put it into a Struct Along with some helper methods

Then rewrote our model to look like this. We avoid having to touch the database for this information, plus it's now the same across all environments. Plus all the data changes are in version control.

DO IT!! I started just adding a file where I'd call the load_seed function with the various ENVs which do stuff & I'd just check stuff happens in my database. It doesn't have to be anything fancy, but it will pick up on any whoopsies. I seriously love this test in my code, it picks up some really random mistakes sometimes.

Plus as I use FactoryBot for my seeds right now, I often call the linter as part of the test suite. It's a really cool! From Factories I'm going to write anyway, I'm able to use them more effectively.

How do you know when we've made good seeds? Most developers won't tell you something is wrong, we're all frogs will sit in boiling water sometimes.

Instead, look at measurable behaviour! - Developers touching prod is bad - Making problems obvious quickly is a win. - Are developers committing schema changes which aren't real! - This is more for Product Owners, but are they going to preview environments & looking at stuff? Or are they testing in prod?

So what should we be doing? So you should be using seeds, they're a good foundation for any app.

Make a project better