> For the complete documentation index, see [llms.txt](https://docs.lingoql.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.lingoql.com/sub0/creating-database-models/table-relationships.md).

# Table Relationships

Use `foreign_key` to define relationships between tables or collections in a Sub0 database model.

### Defining a relationship

To create a relationship, add a `foreign_key` object to a field.

```json
{
  "user_id": {
    "type": "Number",
    "foreign_key": {
      "of": "_user",
      "key": "id"
    }
  }
}
```

This declares that:

* `user_id` references the `id` field on the `_user` table
* The relationship is enforced at the database level where supported

***

### `foreign_key` properties

| Property | Type     | Description                          |
| -------- | -------- | ------------------------------------ |
| `of`     | `String` | Target table/collection name         |
| `key`    | `String` | Referenced field on the target table |

***

### One-to-many relationships

This is the most common relationship pattern in Sub0.

**Example:** A user can have many transactions.

```json
{
  "user_id": {
    "type": "Number",
    "foreign_key": {
      "of": "_user",
      "key": "id"
    }
  }
}
```

* One `_user` record → many `_transaction` records
* Defined by placing the foreign key on the “many” side

***

### Many-to-one relationships

Implicitly handled by the same structure.

```json
{
  "order_id": {
    "type": "Number",
    "foreign_key": {
      "of": "_order",
      "key": "id"
    }
  }
}
```

***

### Required vs optional relationships

Relationships are **required by default**.

To allow a nullable relationship, mark the field as optional:

```json
{
  "user_id": {
    "type": "Number",
    "optional": true,
    "foreign_key": {
      "of": "_user",
      "key": "id"
    }
  }
}
```

***

### Relationships across databases

Sub0 applies relationship definitions according to each database engine:

* **PostgreSQL / MySQL / MariaDB**\
  Foreign key constraints are enforced at the database level.
* **MongoDB**\
  Relationship fields can still be defined in the model, but MongoDB does not enforce foreign key constraints natively.

***

### Nested objects vs relationships

Use relationships when:

* Data lives in separate tables/collections
* Records must be independently queryable
* Referential integrity matters

Use `JSON_OBJECT` when:

* Data is tightly coupled
* No independent lifecycle is required
* Joins are unnecessary

***

### Summary

* Relationships are declared using `foreign_key`
* Defined at the field level
* Database-agnostic and enforced appropriately
* Optional relationships are explicitly marked

Next, see [Indexing](/sub0/creating-database-models/indexing.md) for `indexable` and `fts` field options.


---

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