> For the complete documentation index, see [llms.txt](https://crystal-garage.gitbook.io/lustra-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://crystal-garage.gitbook.io/lustra-docs/model/column-types/primary-keys.md).

# Primary Keys

Lustra expects each persisted model to declare a primary key. Primary keys are used by relations, persistence, and default ordering.

{% hint style="warning" %}
Compound primary keys are not supported.
{% endhint %}

## Declaring a Primary Key Column

You can declare the primary key directly:

```crystal
class Product
  include Lustra::Model

  column id : Int64, primary: true, presence: false
end
```

Use `presence: false` for database-generated values such as `serial` and `bigserial`.

## `primary_key` Helper

Lustra also provides a `primary_key` helper:

```crystal
class Product
  include Lustra::Model

  self.table = "products"

  primary_key "product_id", :uuid
end
```

Arguments:

| Argument | Description                              |
| -------- | ---------------------------------------- |
| `name`   | Database column name. Default: `"id"`.   |
| `type`   | Primary key type. Default: `:bigserial`. |

Built-in primary key types include:

| Type         | Generated column                                              |
| ------------ | ------------------------------------------------------------- |
| `:bigserial` | `Int64`, `presence: false`                                    |
| `:serial`    | `Int32`, `presence: false`                                    |
| `:text`      | `String`, `presence: true`                                    |
| `:int`       | `Int32`, `presence: true`                                     |
| `:bigint`    | `Int64`, `presence: true`                                     |
| `:uuid`      | `UUID`, generated by Lustra before validation for new records |

For the default case, this:

```crystal
primary_key
```

is equivalent to:

```crystal
column id : Int64, primary: true, presence: false
```

For UUID primary keys, Lustra assigns `UUID.random` on new records before validation when the value has not already been set.


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