Generate UUID fields in Phoenix with Postgresql

If you need to add a UUID field (not as a primary key) to an Ecto schema and want PostgreSQL to handle its generation automatically, you can set the default value in your migration using the uuid_generate_v4() function.

Creating the UUID Field in a Migration

Define your migration like this:

defmodule App.Repo.Migrations.AddUuidFieldToUser do
  use Ecto.Migration

  def change do
    alter table(:users) do
      add(:some_new_field, :uuid, default: fragment("uuid_generate_v4()"))
    end
  end
end

Handling Missing uuid-ossp Extension

If your database does not have the uuid-ossp module enabled, running the migration may result in this error:

** (Postgrex.Error) ERROR 42883 (undefined_function): function uuid_generate_v4() does not exist

To enable it, log into your PostgreSQL database and execute:

CREATE EXTENSION IF NOT EXISTS "uuid-ossp";

Adding the Extension via a Migration

If you don’t have direct access to the database, you can enable the extension through a migration:

defmodule App.Repo.Migrations.AddUuidGenerateV4ExtensionToDatabase do
  use Ecto.Migration

  def change do
    execute("CREATE EXTENSION IF NOT EXISTS \"uuid-ossp\";")
  end
end

For more details on the uuid-ossp module, check the PostgreSQL documentation.

Updating the Schema

Once the migration is applied, update your Ecto schema to include the new UUID field:

defmodule App.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    ...
    field(:some_new_field, :binary_id)
    timestamps()
  end

  ...
end

Now your UUID field is automatically generated when inserting new records. 🚀