> 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/anatomy-of-models.md).

# Anatomy of Models

Use this reference to understand Sub0 database model properties, field types, relationships, and indexing.

### Core field properties

| Property         | Type      | Description                                                  |
| ---------------- | --------- | ------------------------------------------------------------ |
| `type`           | `String`  | Defines the data type of the field. Required for all fields. |
| `optional`       | `Boolean` | If `true`, the field may be `null`. Defaults to `false`.     |
| `primary_key`    | `Boolean` | Marks the field as the primary key of the model.             |
| `auto_increment` | `Boolean` | Enables automatic value generation for numeric primary keys. |
| `max_length`     | `Number`  | Sets the maximum allowed length for string values.           |
| `indexable`      | `Boolean` | Creates a database index for the field.                      |
| `fts`            | `Boolean` | Enables full-text search indexing for the field.             |
| `foreign_key`    | `Object`  | Declares a relationship to another model.                    |

### Foreign key properties

Use `foreign_key` to reference another table or collection.

| Sub-property | Type     | Description                              |
| ------------ | -------- | ---------------------------------------- |
| `of`         | `String` | The target model to reference            |
| `key`        | `String` | The referenced field on the target model |

### Supported field types

| `type` Value  | Description                               |
| ------------- | ----------------------------------------- |
| `Number`      | Integer numeric value                     |
| `Float`       | Floating-point numeric value              |
| `String`      | Text value                                |
| `Boolean`     | Boolean (`true` / `false`)                |
| `DateTime`    | Timestamp value                           |
| `JSON_OBJECT` | Structured object with defined properties |
| `JSON_ARRAY`  | Array of values of a single type          |

### JSON object fields

Use `type: "JSON_OBJECT"` for nested structured data. Each object must define `properties`.

#### Object properties

| Property     | Type      | Description                             |
| ------------ | --------- | --------------------------------------- |
| `properties` | `Object`  | Defines the schema of the nested object |
| `optional`   | `Boolean` | Allows the entire object to be `null`   |

Nested fields inside `properties` follow the **same anatomy rules** as top-level fields.

### JSON array fields

Use `type: "JSON_ARRAY"` for lists of values with one field type.

#### Array properties

| Property        | Type      | Description                                     |
| --------------- | --------- | ----------------------------------------------- |
| `property_type` | `String`  | Defines the type of values allowed in the array |
| `optional`      | `Boolean` | Allows the array to be `null`                   |

> `property_type` is **only valid for JSON arrays** and is used to qualify the type of each array element.

### Indexing and search properties

| Property    | Description                                         |
| ----------- | --------------------------------------------------- |
| `indexable` | Enables standard database indexing                  |
| `fts`       | Enables full-text search indexing (where supported) |

### Notes

* All fields must declare `type`
* Nested fields behave exactly like top-level fields
* Constraints and indexes compile to the underlying database engine
* Model definitions are database-agnostic and work across **MongoDB, MySQL/MariaDB, and PostgreSQL**

Next, see [Creating Models](/sub0/creating-database-models/creating-models.md) for real-world database model examples.


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