source: schemas.py

# Schemas

> A machine-readable [schema] describes what resources are available via the API, what their URLs are, how they are represented and what operations they support.
>
> &mdash; Heroku, [JSON Schema for the Heroku Platform API][cite]

API schemas are a useful tool that allow for a range of use cases, including
generating reference documentation, or driving dynamic client libraries that
can interact with your API.

## Representing schemas internally

REST framework uses [Core API][coreapi] in order to model schema information in
a format-independent representation. This information can then be rendered
into various different schema formats, or used to generate API documentation.

When using Core API, a schema is represented as a `Document` which is the
top-level container object for information about the API. Available API
interactions are represented using `Link` objects. Each link includes a URL,
HTTP method, and may include a list of `Field` instances, which describe any
parameters that may be accepted by the API endpoint. The `Link` and `Field`
instances may also include descriptions, that allow an API schema to be
rendered into user documentation.

Here's an example of an API description that includes a single `search`
endpoint:

    coreapi.Document(
        title='Flight Search API',
        url='https://api.example.org/',
        content={
            'search': coreapi.Link(
                url='/search/',
                action='get',
                fields=[
                    coreapi.Field(
                        name='from',
                        required=True,
                        location='query',
                        description='City name or airport code.'
                    ),
                    coreapi.Field(
                        name='to',
                        required=True,
                        location='query',
                        description='City name or airport code.'
                    ),
                    coreapi.Field(
                        name='date',
                        required=True,
                        location='query',
                        description='Flight date in "YYYY-MM-DD" format.'
                    )
                ],
                description='Return flight availability and prices.'
            )
        }
    )

## Schema output formats

In order to be presented in an HTTP response, the internal representation
has to be rendered into the actual bytes that are used in the response.

[Core JSON][corejson] is designed as a canonical format for use with Core API.
REST framework includes a renderer class for handling this media type, which
is available as `renderers.CoreJSONRenderer`.

Other schema formats such as [Open API][open-api] (Formerly "Swagger"),
[JSON HyperSchema][json-hyperschema], or [API Blueprint][api-blueprint] can
also be supported by implementing a custom renderer class.

## Schemas vs Hypermedia

It's worth pointing out here that Core API can also be used to model hypermedia
responses, which present an alternative interaction style to API schemas.

With an API schema, the entire available interface is presented up-front
as a single endpoint. Responses to individual API endpoints are then typically
presented as plain data, without any further interactions contained in each
response.

With Hypermedia, the client is instead presented with a document containing
both data and available interactions. Each interaction results in a new
document, detailing both the current state and the available interactions.

Further information and support on building Hypermedia APIs with REST framework
is planned for a future version.

---

# Adding a schema

You'll need to install the `coreapi` package in order to add schema support
for REST framework.

    pip install coreapi

REST framework includes functionality for auto-generating a schema,
or allows you to specify one explicitly. There are a few different ways to
add a schema to your API, depending on exactly what you need.

## Using DefaultRouter

If you're using `DefaultRouter` then you can include an auto-generated schema,
simply by adding a `schema_title` argument to the router.

    router = DefaultRouter(schema_title='Server Monitoring API')

The schema will be included at the root URL, `/`, and presented to clients
that include the Core JSON media type in their `Accept` header.

    $ http http://127.0.0.1:8000/ Accept:application/vnd.coreapi+json
    HTTP/1.0 200 OK
    Allow: GET, HEAD, OPTIONS
    Content-Type: application/vnd.coreapi+json

    {
        "_meta": {
            "title": "Server Monitoring API"
        },
        "_type": "document",
        ...
    }

This is a great zero-configuration option for when you want to get up and
running really quickly. If you want a little more flexibility over the
schema output then you'll need to consider using `SchemaGenerator` instead.

## Using SchemaGenerator

The most common way to add a schema to your API is to use the `SchemaGenerator`
class to auto-generate the `Document` instance, and to return that from a view.

This option gives you the flexibility of setting up the schema endpoint
with whatever behaviour you want. For example, you can apply different
permission, throttling or authentication policies to the schema endpoint.

Here's an example of using `SchemaGenerator` together with a view to
return the schema.

**views.py:**

    from rest_framework.decorators import api_view, renderer_classes
    from rest_framework import renderers, schemas

    generator = schemas.SchemaGenerator(title='Bookings API')

    @api_view()
    @renderer_classes([renderers.CoreJSONRenderer])
    def schema_view(request):
        return generator.get_schema()

**urls.py:**

    urlpatterns = [
        url('/', schema_view),
        ...
    ]

You can also serve different schemas to different users, depending on the
permissions they have available. This approach can be used to ensure that
unauthenticated requests are presented with a different schema to
authenticated requests, or to ensure that different parts of the API are
made visible to different users depending on their role.

In order to present a schema with endpoints filtered by user permissions,
you need to pass the `request` argument to the `get_schema()` method, like so:

    @api_view()
    @renderer_classes([renderers.CoreJSONRenderer])
    def schema_view(request):
        return generator.get_schema(request=request)

## Explicit schema definition

An alternative to the auto-generated approach is to specify the API schema
explicitly, by declaring a `Document` object in your codebase. Doing so is a
little more work, but ensures that you have full control over the schema
representation.

    import coreapi
    from rest_framework.decorators import api_view, renderer_classes
    from rest_framework import renderers

    schema = coreapi.Document(
        title='Bookings API',
        content={
            ...
        }
    )

    @api_view()
    @renderer_classes([renderers.CoreJSONRenderer])
    def schema_view(request):
        return schema

## Static schema file

A final option is to write your API schema as a static file, using one
of the available formats, such as Core JSON or Open API.

You could then either:

* Write a schema definition as a static file, and [serve the static file directly][static-files].
* Write a schema definition that is loaded using `Core API`, and then
  rendered to one of many available formats, depending on the client request.

---

# API Reference

## SchemaGenerator

A class that deals with introspecting your API views, which can be used to
generate a schema.

Typically you'll instantiate `SchemaGenerator` with a single argument, like so:

    generator = SchemaGenerator(title='Stock Prices API')

Arguments:

* `title` - The name of the API. **required**
* `patterns` - A list of URLs to inspect when generating the schema. Defaults to the project's URL conf.
* `urlconf` - A URL conf module name to use when generating the schema. Defaults to `settings.ROOT_URLCONF`.

### get_schema()

Returns a `coreapi.Document` instance that represents the API schema.

    @api_view
    @renderer_classes([renderers.CoreJSONRenderer])
    def schema_view(request):
        return generator.get_schema()

Arguments:

* `request` - The incoming request. Optionally used if you want to apply per-user permissions to the schema-generation.

---

## Core API

This documentation gives a brief overview of the components within the `coreapi`
package that are used to represent an API schema.

Note that these classes are imported from the `coreapi` package, rather than
from the `rest_framework` package.

### Document

Represents a container for the API schema.

#### `title`

A name for the API.

#### `url`

A canonical URL for the API.

#### `content`

A dictionary, containing the `Link` objects that the schema contains.

In order to provide more structure to the schema, the `content` dictionary
may be nested, typically to a second level. For example:

    content={
        "bookings": {
            "list": Link(...),
            "create": Link(...),
            ...
        },
        "venues": {
            "list": Link(...),
            ...
        },
        ...
    }

### Link

Represents an individual API endpoint.

#### `url`

The URL of the endpoint. May be a URI template, such as `/users/{username}/`.

#### `action`

The HTTP method associated with the endpoint. Note that URLs that support
more than one HTTP method, should correspond to a single `Link` for each.

#### `fields`

A list of `Field` instances, describing the available parameters on the input.

#### `description`

A short description of the meaning and intended usage of the endpoint.

### Field

Represents a single input parameter on a given API endpoint.

#### `name`

A descriptive name for the input.

#### `required`

A boolean, indicated if the client is required to included a value, or if
the parameter can be omitted.

#### `location`

Determines how the information is encoded into the request. Should be one of
the following strings:

**"path"**

Included in a templated URI. For example a `url` value of `/products/{product_code}/` could be used together with a `"path"` field, to handle API inputs in a URL path such as `/products/slim-fit-jeans/`.

These fields will normally correspond with [named arguments in the project URL conf][named-arguments].

**"query"**

Included as a URL query parameter. For example `?search=sale`. Typically for `GET` requests.

These fields will normally correspond with pagination and filtering controls on a view.

**"form"**

Included in the request body, as a single item of a JSON object or HTML form. For example `{"colour": "blue", ...}`. Typically for `POST`, `PUT` and `PATCH` requests. Multiple `"form"` fields may be included on a single link.

These fields will normally correspond with serializer fields on a view.

**"body"**

Included as the complete request body. Typically for `POST`, `PUT` and `PATCH` requests. No more than one `"body"` field may exist on a link. May not be used together with `"form"` fields.

These fields will normally correspond with views that use `ListSerializer` to validate the request input, or with file upload views.

#### `encoding`

**"application/json"**

JSON encoded request content. Corresponds to views using `JSONParser`.
Valid only if either one or more `location="form"` fields, or a single
`location="body"` field is included on the `Link`.

**"multipart/form-data"**

Multipart encoded request content. Corresponds to views using `MultiPartParser`.
Valid only if one or more `location="form"` fields is included on the `Link`.

**"application/x-www-form-urlencoded"**

URL encoded request content. Corresponds to views using `FormParser`. Valid
only if one or more `location="form"` fields is included on the `Link`.

**"application/octet-stream"**

Binary upload request content. Corresponds to views using `FileUploadParser`.
Valid only if a `location="body"` field is included on the `Link`.

#### `description`

A short description of the meaning and intended usage of the input field.


[cite]: https://blog.heroku.com/archives/2014/1/8/json_schema_for_heroku_platform_api
[coreapi]: http://www.coreapi.org/
[corejson]: http://www.coreapi.org/specification/encoding/#core-json-encoding
[open-api]: https://openapis.org/
[json-hyperschema]: http://json-schema.org/latest/json-schema-hypermedia.html
[api-blueprint]: https://apiblueprint.org/
[static-files]: https://docs.djangoproject.com/en/dev/howto/static-files/
[named-arguments]: https://docs.djangoproject.com/en/dev/topics/http/urls/#named-groups
