path: root/docs/tutorial/1-serialization.md

# Tutorial 1: Serialization

## Introduction

This tutorial will walk you through the building blocks that make up REST framework.   It'll take a little while to get through, but it'll give you a comprehensive understanding of how everything fits together.

## Setting up a new environment

Before we do anything else we'll create a new virtual environment, using [virtualenv].  This will make sure our package configuration is keep nicely isolated from any other projects we're working on.

    :::bash
    mkdir ~/env
    virtualenv ~/env/tutorial
    source ~/env/tutorial/bin/activate

Now that we're inside a virtualenv environment, we can install our package requirements.

    pip install django
    pip install djangorestframework

**Note:** To exit the virtualenv environment at any time, just type `deactivate`.  For more information see the [virtualenv documentation][virtualenv].

## Getting started

Okay, we're ready to get coding.
To get started, let's create a new project to work with.

    cd ~
    django-admin.py startproject tutorial
    cd tutorial

Once that's done we can create an app that we'll use to create a simple Web API.

    python manage.py startapp blog

The simplest way to get up and running will probably be to use an `sqlite3` database for the tutorial.  Edit the `tutorial/settings.py` file, and set the default database `"ENGINE"` to `"sqlite3"`, and `"NAME"` to `"tmp.db"`.

    DATABASES = {
        'default': {
            'ENGINE': 'django.db.backends.sqlite3',
            'NAME': 'tmp.db',
            'USER': '',
            'PASSWORD': '',
            'HOST': '',
            'PORT': '',
        }
    }

We'll also need to add our new `blog` app and the `rest_framework` app to `INSTALLED_APPS`.

    INSTALLED_APPS = (
        ...
        'rest_framework',
        'blog'
    )

We also need to wire up the root urlconf, in the `tutorial/urls.py` file, to include our blog views.

    urlpatterns = patterns('',
        url(r'^', include('blog.urls')),
    )

Okay, we're ready to roll.

## Creating a model to work with

For the purposes of this tutorial we're going to start by creating a simple `Comment` model that is used to store comments against a blog post.  Go ahead and edit the  `blog` app's `models.py` file.

    from django.db import models

    class Comment(models.Model):        
        email = models.EmailField()
        content = models.CharField(max_length=200)
        created = models.DateTimeField(auto_now_add=True)

Don't forget to sync the database for the first time.

    python manage.py syncdb

## Creating a Serializer class

We're going to create a simple Web API that we can use to edit these comment objects with.  The first thing we need is a way of serializing and deserializing the objects into representations such as `json`.  We do this by declaring serializers that work very similarly to Django's forms.  Create a file in the `blog` directory named `serializers.py` and add the following.

    from blog import models
    from rest_framework import serializers


    class CommentSerializer(serializers.Serializer):
        id = serializers.IntegerField(readonly=True)
        email = serializers.EmailField()
        content = serializers.CharField(max_length=200)
        created = serializers.DateTimeField(readonly=True)
        
        def restore_object(self, attrs, instance=None):
            """
            Create or update a new comment instance.
            """
            if instance:
                instance.email = attrs['email']
                instance.content = attrs['content']
                instance.created = attrs['created']
                return instance
            return models.Comment(**attrs)

The first part of serializer class defines the fields that get serialized/deserialized.  The `restore_object` method defines how fully fledged instances get created when deserializing data.

We can actually also save ourselves some time by using the `ModelSerializer` class, as we'll see later, but for now we'll keep our serializer definition explicit.  

## Working with Serializers

Before we go any further we'll familiarise ourselves with using our new Serializer class.  Let's drop into the Django shell.

    python manage.py shell

Okay, once we've got a few imports out of the way, we'd better create a few comments to work with.

    from blog.models import Comment
    from blog.serializers import CommentSerializer
    from rest_framework.renderers import JSONRenderer
    from rest_framework.parsers import JSONParser

    c1 = Comment(email='leila@example.com', content='nothing to say')
    c2 = Comment(email='tom@example.com', content='foo bar')
    c3 = Comment(email='anna@example.com', content='LOLZ!')
    c1.save()
    c2.save()
    c3.save()

We've now got a few comment instances to play with.  Let's take a look at serializing one of those instances.

    serializer = CommentSerializer(instance=c1)
    serializer.data
    # {'id': 1, 'email': u'leila@example.com', 'content': u'nothing to say', 'created': datetime.datetime(2012, 8, 22, 16, 20, 9, 822774, tzinfo=<UTC>)}

At this point we've translated the model instance into python native datatypes.  To finalise the serialization process we render the data into `json`.

    content = JSONRenderer().render(serializer.data)
    content
    # '{"id": 1, "email": "leila@example.com", "content": "nothing to say", "created": "2012-08-22T16:20:09.822"}'

Deserialization is similar.  First we parse a stream into python native datatypes... 

    import StringIO

    stream = StringIO.StringIO(content)
    data = JSONParser().parse(stream)

...then we restore those native datatypes into to a fully populated object instance.

    serializer = CommentSerializer(data)
    serializer.is_valid()
    # True
    serializer.object
    # <Comment: Comment object>
    
Notice how similar the API is to working with forms.  The similarity should become even more apparent when we start writing views that use our serializer.

## Writing regular Django views using our Serializers

Let's see how we can write some API views using our new Serializer class.
We'll start off by creating a subclass of HttpResponse that we can use to render any data we return into `json`.

Edit the `blog/views.py` file, and add the following.

    from blog.models import Comment
    from blog.serializers import CommentSerializer
    from django.http import HttpResponse
    from django.views.decorators.csrf import csrf_exempt
    from rest_framework.renderers import JSONRenderer
    from rest_framework.parsers import JSONParser


    class JSONResponse(HttpResponse):
        """
        An HttpResponse that renders it's content into JSON.
        """

        def __init__(self, data, **kwargs):
            content = JSONRenderer().render(data)
            kwargs['content_type'] = 'application/json'
            super(JSONResponse, self).__init__(content, **kwargs)


The root of our API is going to be a view that supports listing all the existing comments, or creating a new comment.

    @csrf_exempt
    def comment_root(request):
        """
        List all comments, or create a new comment.
        """
        if request.method == 'GET':
            comments = Comment.objects.all()
            serializer = CommentSerializer(instance=comments)
            return JSONResponse(serializer.data)

        elif request.method == 'POST':
            data = JSONParser().parse(request)
            serializer = CommentSerializer(data)
            if serializer.is_valid():
                comment = serializer.object
                comment.save()
                return JSONResponse(serializer.data, status=201)
            else:
                return JSONResponse(serializer.errors, status=400)

Note that because we want to be able to POST to this view from clients that won't have a CSRF token we need to mark the view as `csrf_exempt`.  This isn't something that you'd normally want to do, and REST framework views actually use more sensible behavior than this, but it'll do for our purposes right now. 

We'll also need a view which corresponds to an individual comment, and can be used to retrieve, update or delete the comment.

    @csrf_exempt
    def comment_instance(request, pk):
        """
        Retrieve, update or delete a comment instance.
        """
        try:
            comment = Comment.objects.get(pk=pk)
        except Comment.DoesNotExist:
            return HttpResponse(status=404)
 
        if request.method == 'GET':
            serializer = CommentSerializer(instance=comment)
            return JSONResponse(serializer.data)
    
        elif request.method == 'PUT':
            data = JSONParser().parse(request)
            serializer = CommentSerializer(data, instance=comment)
            if serializer.is_valid():
                comment = serializer.object
                comment.save()
                return JSONResponse(serializer.data)
            else:
                return JSONResponse(serializer.errors, status=400)

        elif request.method == 'DELETE':
            comment.delete()
            return HttpResponse(status=204)

Finally we need to wire these views up. Create the `blog/urls.py` file:

    from django.conf.urls import patterns, url

    urlpatterns = patterns('blog.views',
        url(r'^$', 'comment_root'),
        url(r'^(?P<pk>[0-9]+)$', 'comment_instance')
    )

It's worth noting that there's a couple of edge cases we're not dealing with properly at the moment.  If we send malformed `json`, or if a request is made with a method that the view doesn't handle, then we'll end up with a 500 "server error" response.  Still, this'll do for now.

## Testing our first attempt at a Web API

**TODO: Describe using runserver and making example requests from console**

**TODO: Describe opening in a web browser and viewing json output**

## Where are we now

We're doing okay so far, we've got a serialization API that feels pretty similar to Django's Forms API, and some regular Django views.

Our API views don't do anything particularly special at the moment, beyond serve `json` responses, and there's some error handling edge cases we'd still like to clean up, but it's a functioning Web API.

We'll see how we can start to improve things in [part 2 of the tutorial][tut-2].

[virtualenv]: http://www.virtualenv.org/en/latest/index.html
[tut-2]: 2-requests-and-responses.md