# Column Modifiers Reference

Column modifiers are chainable methods called on the `column` builder to define column settings and constraints.

## Modifiers List

### `pk()`

Marks a column as the primary key.

```typescript
column.int().pk()
```

:::important
Every table in your schema must contain exactly one primary key.
:::

### `auto()`

Enables auto-increment for numeric columns (`int`, `float`, `number`).

```typescript
column.int().pk().auto()
```

### `unique()`

Marks a column as unique, automatically creating a unique database index.

```typescript
column.text().unique()
```

### `index()`

Creates a standard index on the column for quick query filtering and cursor sorting.

```typescript
column.int().index()
```

### `optional()`

Marks a column as optional. This allows the value to be `undefined`.

```typescript
column.text().optional() // Inferred type: string | undefined
```

### `nullable()`

Allows the column value to be explicitly `null`.

```typescript
column.text().nullable() // Inferred type: string | null
```

### `default(value)`

Sets a default value for the column when a row is inserted without one.

```typescript
column.bool().default(true)
column.text().default('N/A')
```

### `validate(validator)`

Defines a custom validation function that runs before inserts and updates.

```typescript
validate(validator: (value: T) => string | null | undefined): Column
```

If validation fails, the validator should return a custom error message `string`. If the value is valid, return `null` or `undefined`.

```typescript
column.text().validate((val) => {
  return val.length >= 3 ? null : 'Username is too short';
})
```

:::note
Custom validators override built-in type checks for that column.
To access the custom validator function programmatically, import the `ValidateFn` symbol:

```typescript
import { ValidateFn } from 'locality-idb';
const validator = emailColumn[ValidateFn];
```
:::

### `onUpdate(updater)`

Sets a function to dynamically update the column value during update operations.

```typescript
onUpdate(updater: (currentValue: T) => T): Column
```

Typically used for updating timestamps like `updatedAt`:

```typescript
column.timestamp().onUpdate(() => getTimestamp())
```

:::tip[Info]
* The updater function overrides any value provided in the update payload.
* To access this function programmatically, import the `OnUpdate` symbol:

```typescript
import { OnUpdate } from 'locality-idb';
const updater = updatableColumn[OnUpdate];
```
:::