Introduction

Rails 5.1 brings many improvements and plenty of changes. Among them is the change of function that is used when generating UUIDs.

For PostgreSQL >= 9.4, ActiveRecord will now use pgcrypto’s gen_random_uuid function whereas previously uuid-ossp’s uuid_generate_v4 function was used.

Follow these steps to add UUID primary keys to your Rails 5.1+ application.

Add a migration

First we need to enable the PostgreSQL pgcrypto extension in a migration. Lets start by creating a blank migration:

$ rails generate migration enable_pgcrypto_extension

Then change the file to enable the extension. It should look something like this:

class EnablePgcryptoExtension < ActiveRecord::Migration[5.1]
  def change
    enable_extension 'pgcrypto'
  end
end

Change the primary default type

When you generate models ActiveRecord will use Integer as the type for id by default. In Rails 5 you can change this behaviour by adding the following to your config/application.rb file.

config.generators do |g|
  g.orm :active_record, primary_key_type: :uuid
end

Try it out

Now when you run rails generate model post title:string your migration file should look something like this:

class CreatePosts < ActiveRecord::Migration[5.1]
  def change
    create_table :posts, id: :uuid do |t|
      t.string :title

      t.timestamps
    end
  end
end

Further help

It’s most likely you’ve ended up here from a Google search, here are some errors that you might experience that relate to this guide:

I get an error about gen_random_uuid when I run rake db:migrate ?

If you get the following error when you run rake db:migrate then it’s likely you’ve hit the issue I wrote this post above.

function gen_random_uuid() does not exist

I get an error about uuid_generate_v4 when I run rake db:migrate ?

If you get the following error when you run rake db:migrate:

function uuid_generate_v4() does not exist

It’s likely you have old references to uuid_generate_v4 in your code. Most likely a migration like this:

add_column :posts, :id, default: "uuid_generate_v4()", null: false

To ensure all your old as well as your new migrations continue to run simply enable both extensions in a migration:

class EnablePgcryptoExtension < ActiveRecord::Migration[5.1]
  def change
    enable_extension 'uuid-ossp'
    enable_extension 'pgcrypto'
  end
end