Collection definition (Mongoose Model).
Apart from what you define, we will add:

* Timestamps: created_at, updated_at
* *TODO* last user: created_by: updated_by
* Version control: \_\_v

#### Model: backend.options (Object)

Mongoose options. Mostly to specify collection name.

#### Model: backend.permissions: Object(read, list, create, update, delete)

Label definition for permissions, all properties are required.

```json
{
"permissions": {
"read": "View users",
"list": "List users",
"create": "Create users",
"update": "Update users",
"delete": "Delete users"
}
}
```

#### Model: backend.formatter: Function

Function to transform models.

It will be called just before output.

*NOTE* Do not use it to hide properties, use `restricted` instead.

#### Model: backend.schema: Object

Example

```json
{
"schema": {
"username": {
"type": "String"
}
}
}
```

* label: String

Label that will be displayed at forms and list.

Optional for: Array, Object types.

* type: String
* Date
* String
* Number
* ObjectId
* Mixed
* Array
Require a `items` property.
* Object
Require a `properties` property.

*NOTE* while mongoose support custom type, we actually don't because
the schema, edit manually atm.

* restricted (boolean | object)

With an object, you can specify what to do in each operation.

```js
{
"restricted": {
"create": false, // no restricted
"update": true, // always restricted
"read": "permission-xxx" // check permission
}
}
```

`restricted = true` is a shortcut of: `{create: false, update: false, read: true}`
`restricted = false` is a shortcut of: `{create: false, update: false, read: false}`

Fields examples:

* Type Array of ObjectIds (OneToMany relation)
```json
{
"field": {
"type": "Array",
"items": {
"label": "list_of_x",
"type": "ObjectId",
"ref": "x",
"restricted": false
}
}
}
```

* Type Object
```yml
{
"data": {
"type": "Object",
"properties": {
"first_name": {
"label": "First name",
"type": "String",
"restricted": false
},
"last_name": {
"label": "Last name",
"type": "String",
"restricted": false
}
}
}
}
```