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Seed your database with the Faker gem. Then seed 9.4x faster with upsert_all and activerecord-import.


I cover 3 things in this article —

  1. Using the faker gem in a Rails app to seed 10,000 fake records.

  2. Fixing our slow seeds by making them 9.4x faster, using upsert_all or the activerecord-import gem (since our naive method is very inefficient).

  3. Finally, I show you how to split your database seeds into 3 separate files — one file for each Rails environment. This way, you won't seed your production database with lots of faker generated records.

Table of Contents

Every new Ruby on Rails app includes a seeds.rb file — it's a simple Ruby script that we use to add initial data to an empty database.

In this article, I'm going to show you how to combine seeds.rb with the faker gem to generate heaps of fake testing data (we're going to generate 10,000 fake Post records with faker).

But we're not going to stop there. By default, Rails will seed our database incredibly inefficiently, so we're going to speed up our seeding by 9x (minimum), by using the new upsert_all method introduced in Rails 6 (and the activerecord-import gem for older apps).

Finally, I'm going to show you how to split the seeds.rb file into 3 separate files, to seed our development, test and production environments separately.

Let's go!

Scaffolding a sample app

To start, I'm going to scaffold a simple app to use in this article (and I recommend you follow along with me).

Our simple Rails app will use the default sqlite3 database, and include a basic Post model. We can generate this by running —

rails new test_faker_gem_seeding && cd test_faker_gem_seeding
rails g scaffold Post title:string body:string
rails db:migrate
rails server

These commands will generate the sample app for us, and start the rails server. Let's also add a Post from the rails console, to give our app some initial data —

# run in your terminal
rails c

# run in the rails console
Post.create(title: 'test', body: 'we will use the faker gem to generate more posts')
 id: 1,
 title: "test",
 body: "we will use the faker gem to generate more posts",
 created_at: Wed, 12 Jul 2023 02:17:58.063703000 UTC +00:00,
 updated_at: Wed, 12 Jul 2023 02:17:58.063703000 UTC +00:00>

If you visit in your browser, the sample app will be running, and your single Post will be there —

The sample Ruby on Rails app with a single Post. The next step is to automatically generate fake Post records with the faker gem.
The sample Ruby on Rails app with a single Post. The next step is to automatically generate fake Post records with the faker gem.

Looks good! Our sample Rails app is basic, but it's enough for this article.

In the next section, we're going to add the faker gem to our app and use it to generate Posts for us in our seeds.rb file.

Seeding our database with the faker gem

Now that we've got our sample Rails app running, we're going to use the faker gem (and some simple Ruby code) to generate fake Posts.

We're not just going to seed a paltry few Posts either — we're going to seed 10,000 Posts into our Rails database.

This will be slow! Initially, Rails / ActiveRecord will seed our 10,000 Posts incredibly inefficiently. In a later section, we're going to use the activerecord-import gem to fix this.

The faker gem is pretty simple — it generates fake data. This makes it perfect for generating seed data in our Rails apps (faker can also be handy for testing, but we won't cover that today).

Here are a couple of fun examples I grabbed from the official faker gem docs

faker_gem_examples #=> "eliza@mann.test" #=> "seize collaborative mindshare"
Faker::Movies::HarryPotter.quote #=> "I solemnly swear that I am up to no good."
Faker::Quotes::Shakespeare.romeo_and_juliet_quote # => "O Romeo, Romeo! wherefore art thou Romeo?."

To install the faker gem, I recommend adding it to your Gemfile under the :development group (and maybe also :test) like —

group :development, :test do
  gem "faker", "~> 3.2"

and then install it by running bundle install.

You could also just run bundle add faker, but I highly recommend keeping it out of your :production environment (unless you need it there too, but you probably don't).

Next, test that the faker gem is installed correctly by running rails console and then -

=> "Chris Stanton II"

Looks good! Time to write some database seeds.

A basic seeds.rb file for our Rails app might look like this —

puts "starting to seed the database"

Post.create(title:, body:

puts "finished seeding the database"

Here, we're using the Faker::Company generator to generate a title: and body: for our Post, rather than hard-coding values.

The faker gem includes heaps of other generators for generating fake data (over 200!) — poke around a bit and find some you like!

Let's seed our database and see what data faker generates for us.

We can run rails db:reset to delete our database, recreate it, and re-seed it —

rails db:reset

Dropped database 'db/development.sqlite3'
Dropped database 'db/test.sqlite3'
Created database 'db/development.sqlite3'
Created database 'db/test.sqlite3'
starting to seed the database
finished seeding the database

I wrote more about the rails db: command and database migrations in another post — Rails Generate Migration Reference Guide.

We can open the rails console and view our Post

  id: 1,
  title: "Mills-Cole",
  body: "disintermediate collaborative users",
  created_at: Wed, 12 Jul 2023 03:27:53.318704000 UTC +00:00,
  updated_at: Wed, 12 Jul 2023 03:27:53.318704000 UTC +00:00>]

Looks good! We've seeded our database with a single Post, with a faker-generated title and body. Nice!

We're just getting started though — since our seeds.rb file is just a plain Ruby file, it's pretty straightforward to generate more records (lots more 😉).

Let's try it. Instead of one Post, let's generate 10,000

puts "starting to seed the database"
10000.times do |iter|
  puts iter
  Post.create(title:, body:
puts "finished seeding the database"

The faker gem can generate unique values with .uniqe (ex: By default though, faker doesn't guarantee unique values. Read more in the faker docs.

Run rails db:reset and give it a couple of seconds to run, then open up the Rails console again with rails c. We can sample a random Post by running -

 id: 6948,
 title: "Hudson, Turner and Kuvalis",
 body: "empower one-to-one content",
 created_at: Wed, 12 Jul 2023 03:35:57.240784000 UTC +00:00,
 updated_at: Wed, 12 Jul 2023 03:35:57.240784000 UTC +00:00>

Looks good! Thanks to the faker gem, we've seeded our database with 10,000 Posts (and saved ourselves a lot of typing along the way 😅).

That's it for this section. Feel free to take a break and play around with the faker gem a bit!

You might have noticed something though — running rails db:reset is slow. You probably had to wait 5-10 seconds. We're going to fix this!

The next section is all about fixing our slow seeding.

Fixing our slow seeds with upsert_all and activerecord-import

Note: Thanks to Redditor /u/EOengineer for showing me the upsert_all method, included in Rails 6+. Turns out it's even faster than the ActiveRecord Import gem!

You might have noticed that seeding all 10,000 Posts takes a while. But how long is a while?

We can Benchmark our seeding time in the rails console

Benchmark.measure { Rails.application.load_seed }

We use the Rails.application.load_seed command to load our database seeds from the Rails console.

We wrap Benchmark.measure around our call to track the execution time of our seeds (we care about the @real=7.5 value).

7.5 seconds to seed our database. Too slow!

If you have a more complex seeds.rb file, your seeds will take even longer.

Why are our seeds so slow?

When we called Benchmark, we saw a lot of SQL INSERT statements in the rails console, like this —

# 10,000 INSERTS like this
TRANSACTION (0.0ms)  begin transaction
Post Create (0.1ms)  INSERT INTO "posts" ...
TRANSACTION (0.2ms)  commit transaction

When we seed our database, Rails is generating a new INSERT statement for each Post that we're creating — this is very slow!

If we could somehow combine all these INSERT statements into a single INSERT, we could speed up our database seeding by a lot.

Well, fortunately, there's an easy fix — Rails 6 introduced the upsert_all method (and similar insert_all method).

According to the upsert_all docs —

[upsert_all will] update or insert (upsert) multiple records into the database in a single SQL INSERT statement. It does not instantiate any models nor does it trigger Active Record callbacks or validations.



Unfortunately, upsert_all is Rails 6+ only. However, I've included a second section which uses the activerecord-import gem instead, which supports Rails 3+.

Note: upsert is a "portmanteau" — it's a combination of the words 'update' and 'insert'. In the case of upsert_all, we use it to insert a collection of records if they don't exist or update them if they do exist.

Bulk inserting seed data with upsert_all (Rails 6+)

From Rails 6 onwards, Rails includes the upsert_all method to help address our problem (too many INSERT calls).

Note: We could have used insert_all here instead since we're creating records. upsert_all sounded cooler though 😎

Let's refactor our seeds.rb file to call upsert_all rather than create

puts "starting to seed the database"

posts = []
10000.times do |iter|
  # construct a hash of our Post values
  posts << { title:, body: }

# create all our Posts with a single INSERT
Post.upsert_all posts
puts "finished seeding the database"

Using Post.upsert_all requires us to make some changes to our seeds.rb file.

First, we construct an array of hashes, with each hash containing the data for a Post. Then, we call Post.upsert_all posts to insert all 10,000 Posts together.

If you re-run the database seeds (rails db:seed or rails db:reset), you will notice that they run a lot quicker!

We can Benchmark our seeds again to check how much quicker upsert_all is —

Benchmark.measure { Rails.application.load_seed }

We've gone from 7.5 seconds down to 0.8 seconds — that's a speedup of 9.4x! 🤯

This speedup is from creating all our Posts together, rather than creating them one by one.


Bulk inserting seed data with the ActiveRecord Import gem (Rails 3+)

If you're not running Rails 6+, you won't be able to use upsert_all or insert_all to bulk-insert your records. That's OK — instead, you can use the ActiveRecord Import gem to do the same thing.

Just like upsert_all, the activerecord-import gem will combine all our INSERT statements, so we can seed our Posts in bulk.

I recommend you read through the activerecord-import gem's README.

Let's add the gem to our Gemfile, in the same group as the faker gem —

group :development, :test do
  gem "faker", "~> 3.2"
  gem "activerecord-import", "~> 1.4"

Don't forget to bundle after adding the activerecord-import gem.

Next, we need to refactor our seeds.rb code.

Adjust your seeds.rb file to look like this —

puts "starting to seed the database"

posts = []
10000.times do |iter|
  # construct a hash of our Post values
  posts << { title:, body: }

# use .import from the activerecord-import gem
Post.import posts
puts "finished seeding the database"

This seeds.rb file, using activerecord-import, is nearly identical to the upsert_all file. All we've changed is Post.upsert_all — we've replaced it with Post.import.

Run these seeds, and you'll notice that they run a lot quicker than our naive seeds.rb file.

Let's Benchmark our activerecord-import seeding —

Benchmark.measure { Rails.application.load_seed }

 @real=1.5487040001899004, # 5x faster than before!

From 7.5 seconds down to 1.5 - 5x faster!

Note that this is about 1.8x slower than using upsert_all (from the previous section), but still a massive improvement over our naive seeds.rb file.

Splitting our seed files by Rails environments

We've used the faker gem to seed 10,000 fake Posts into our database, and now our seeds are running much quicker thanks to either upsert_all or activerecord-import.

However, we probably don't want to seed our production database with all this fake data!

I've got a handy way to split up your database seeds, by creating a new seeds file for each Rails environment.

Disclaimer: copied this method from a great StackOverflow thread.

I also wrote about this method more in a separate article — Split your database seeds.rb by Rails environment.

First, create a seeds/ folder inside the /db folder of your Ruby on Rails app —

mkdir seeds/

And then, inside the seeds/ folder, we're going to create 3 new files, one for each environment —

touch development.rb test.rb production.rb

Note: I've provided the shell commands so you can copy/paste them. You can also create the seeds/ folder and seed files manually if that's easier.

Next, copy the contents of your seeds.rb file into your seeds/development.rb file —

# COPIED from seeds.rb
puts "starting to seed the database"

posts = []
10000.times do |iter|
  posts <<, body:
Post.import posts

puts "finished seeding the database"

And then edit your main seeds.rb file to be the following —

puts Rails.env.downcase

# load the correct seeds file for our Rails environment
load(Rails.root.join( 'db', 'seeds', "#{Rails.env.downcase}.rb"))

That's it! Now, when you run rails db:seed, Rails will use the seed file for the active Rails.env environment.

This also works for other commands which run rails db:seed, like rails db:reset.

Now you can keep your production database free from all the faker-generated Posts, but still use a large dataset in your development environment.

If you want to seed your local database with seeds for a different environment, you can do that too with —

# production seeds
rails db:seed RAILS_ENV=production

# test seeds
rails db:seed RAILS_ENV=test


I hope you found this article useful!

I think developing on a large dataset is important — it helps you experience your app in a more production-like environment.

I've found this a helpful way to spot N+1 queries that creep in — you won't notice them on an empty database, but you will with 10,000 Posts!

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