django-rest-framework/docs/tutorial/1-serialization.md
2012-09-03 15:57:43 +01:00

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# 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.
mkdir ~/env
virtualenv --no-site-packages ~/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.
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 `djangorestframework` app to `INSTALLED_APPS`.
INSTALLED_APPS = (
...
'djangorestframework',
'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 project named `serializers.py` and add the following.
from blog import models
from djangorestframework import serializers
class CommentSerializer(serializers.Serializer):
email = serializers.EmailField()
content = serializers.CharField(max_length=200)
created = serializers.DateTimeField()
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 djangorestframework.renderers import JSONRenderer
from djangorestframework.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
# {'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`.
stream = JSONRenderer().render(serializer.data)
stream
# '{"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...
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 djangorestframework.renderers import JSONRenderer
from djangorestframework.parsers import JSONParser
from django.http import HttpResponse
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.
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.error_data, status=400)
We'll also need a view which corrosponds to an individual comment, and can be used to retrieve, update or delete the comment.
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.error_data, status=400)
elif request.method == 'DELETE':
comment.delete()
return HttpResponse(status=204)
Finally we need to wire these views up, in the `tutorial/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