> For the complete documentation index, see [llms.txt](https://samypesse.gitbook.io/blini/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://samypesse.gitbook.io/blini/guides/create-schema.md).

# Create a schema

A Schema defines the shape of the documents within a collection. It is basically a map of field name to [`Type`](https://github.com/samypesse/blini/tree/4acfa8dba5ac2fc91fc7fc7128682ae3287cbff8/docs/types.md).

It contains specs for transforming and validating each fields.

## Create

Setup your schema for your collection by creating a new instance of `Blini.Schema`.

`Schema` takes only one argument a map of `field: String` to `type: Type`.

```javascript
const { Schema, Type } = require('blini');

const userSchema = new Schema({
    username: Type.String({
        validation: [
            Validation.required(),
            Validation.minLength(3),
            Validation.maxLength(30)
        ],
        index: {
            unique: true
        }
    }),
    books: Type.Set(Type.Ref('Book'), {
        validations: [
            Validation.default([]),
        ]
    })
});
```

## Native types

Native types like `String`, `Number` and `ObjectID`.

## Validation

All types (natives and iterables) are taking as last argument a spec of options.

The `validation` option lets you specify an array of validations functions to use.

See [Validations](/blini/references/validations.md) for more details about the helper validation methods (note that a validation can be any function taking a `value` and retuning the transformed value or throwing an error).

## Iterable

Blini doesn't have a type for mixed/object or array, instead it's using immutable data structure: `Type.List`, `Type.Set` and `Type.Map`.

Each of these types take a `Type` as first argument, it will be used to normalize and validate inner data.

```javascript
const postSchema = new Schema({
    title: Type.String(),
    body:  Type.String(),
    tags:  Type.Set(Type.String())
});
```

## Sub-documents

Since `Schema` inherits from `Type`, you can add schema for shaping sub-documents.

```javascript
const commentSchema = new Schema({
    author: Type.Ref('Author'),
    body:   Type.String()
});

const postSchema = new Schema({
    title:    Type.String(),
    body:     Type.String(),
    author:   Type.Ref('Author'),
    comments: Type.List(commentSchema)
});
```

## References