# Generators

`generators` replaces a built-in client generator by name. The names are `uuidv4`, `uuidv7`, and `okid`. Each replacement returns a string. A [generator](/docs/define/table/defaults#generator) runs when insert omits the field.

| Key | Insert |
| --- | --- |
| omitted | Calls the function stored on the column. |
| `uuidv4` | Calls the function you passed, for a `uuidv4` generator. |
| `uuidv7` | Calls the function you passed, for a `uuidv7` generator. |
| `okid` | Calls the function you passed, for an `okid` generator. |

The function takes no arguments. One function covers every column of that name. A bare `t.id()` that took that generator from the schema [id default](/docs/define/schema/defaults) uses it too. Each missing field calls the function once.

### uuidv7

Insert stores this string when `id` is omitted.

```ts title="db.ts"
import { connect } from "okmodel/pg/postgresjs";
import { schema, table, t } from "okmodel/pg";
import { uuidv7 } from "okmodel/ids";

const notes = table("notes", {
  id: t.uuid().default(uuidv7),
  title: t.text(),
});
const app = schema({ tables: [notes] });

export async function add(url: string, title: string) {
  await using db = connect(url, {
    schema: app,
    generators: {
      uuidv7: () => "00000000-0000-7000-8000-000000000000", // [!code focus]
    },
  });
  return db.notes.insert({ title });
}
```

<Callout type="info" title="Left alone">
A function you wrote and passed to `.default()` keeps its own call. A bare `t.id()` stores `gen_random_uuid()`. `t.id({ default: "uuidv7" })` stores `uuidv7()`, which needs Postgres 18. Pick that for a large write-heavy table: the ids are time-ordered and the index stays local. Random ids are fine elsewhere.
</Callout>