I am working on a Ruby on Rails app. We are using a PostgreSQL database.

There is a table named `scores` with the following columns: 

```
Column        | Type
--------------+-----------------------
id            | integer
value         | double precision
ran_at        | timestamp
active        | boolean
build_id      | bigint
metric_id     | integer
platform_id   | integer
mode_id       | integer
machine_id    | integer
higher_better | boolean
job_id        | integer
variation_id  | integer
step          | character varying(255)

```

I need to add a *sequence* to `job_id` (note: there is no model for `job`).

How do I create this sequence?