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262 lines
10 KiB
Markdown
262 lines
10 KiB
Markdown
# Tutorial 1: Serialization
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## Introduction
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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.
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## Setting up a new environment
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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.
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:::bash
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mkdir ~/env
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virtualenv --no-site-packages ~/env/tutorial
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source ~/env/tutorial/bin/activate
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Now that we're inside a virtualenv environment, we can install our package requirements.
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pip install django
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pip install djangorestframework
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**Note:** To exit the virtualenv environment at any time, just type `deactivate`. For more information see the [virtualenv documentation][virtualenv].
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## Getting started
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Okay, we're ready to get coding.
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To get started, let's create a new project to work with.
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cd ~
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django-admin.py startproject tutorial
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cd tutorial
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Once that's done we can create an app that we'll use to create a simple Web API.
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python manage.py startapp blog
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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"`.
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DATABASES = {
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'default': {
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'ENGINE': 'django.db.backends.sqlite3',
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'NAME': 'tmp.db',
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'USER': '',
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'PASSWORD': '',
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'HOST': '',
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'PORT': '',
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}
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}
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We'll also need to add our new `blog` app and the `rest_framework` app to `INSTALLED_APPS`.
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INSTALLED_APPS = (
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...
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'rest_framework',
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'blog'
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)
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We also need to wire up the root urlconf, in the `tutorial/urls.py` file, to include our blog views.
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urlpatterns = patterns('',
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url(r'^', include('blog.urls')),
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)
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Okay, we're ready to roll.
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## Creating a model to work with
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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.
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from django.db import models
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class Comment(models.Model):
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email = models.EmailField()
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content = models.CharField(max_length=200)
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created = models.DateTimeField(auto_now_add=True)
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Don't forget to sync the database for the first time.
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python manage.py syncdb
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## Creating a Serializer class
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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.
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from blog import models
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from rest_framework import serializers
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class CommentSerializer(serializers.Serializer):
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id = serializers.IntegerField(readonly=True)
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email = serializers.EmailField()
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content = serializers.CharField(max_length=200)
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created = serializers.DateTimeField()
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def restore_object(self, attrs, instance=None):
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"""
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Create or update a new comment instance.
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"""
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if instance:
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instance.email = attrs['email']
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instance.content = attrs['content']
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instance.created = attrs['created']
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return instance
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return models.Comment(**attrs)
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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.
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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.
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## Working with Serializers
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Before we go any further we'll familiarise ourselves with using our new Serializer class. Let's drop into the Django shell.
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python manage.py shell
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Okay, once we've got a few imports out of the way, we'd better create a few comments to work with.
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from blog.models import Comment
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from blog.serializers import CommentSerializer
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from rest_framework.renderers import JSONRenderer
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from rest_framework.parsers import JSONParser
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c1 = Comment(email='leila@example.com', content='nothing to say')
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c2 = Comment(email='tom@example.com', content='foo bar')
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c3 = Comment(email='anna@example.com', content='LOLZ!')
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c1.save()
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c2.save()
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c3.save()
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We've now got a few comment instances to play with. Let's take a look at serializing one of those instances.
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serializer = CommentSerializer(instance=c1)
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serializer.data
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# {'id': 1, 'email': u'leila@example.com', 'content': u'nothing to say', 'created': datetime.datetime(2012, 8, 22, 16, 20, 9, 822774, tzinfo=<UTC>)}
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At this point we've translated the model instance into python native datatypes. To finalise the serialization process we render the data into `json`.
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stream = JSONRenderer().render(serializer.data)
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stream
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# '{"id": 1, "email": "leila@example.com", "content": "nothing to say", "created": "2012-08-22T16:20:09.822"}'
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Deserialization is similar. First we parse a stream into python native datatypes...
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data = JSONParser().parse(stream)
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...then we restore those native datatypes into to a fully populated object instance.
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serializer = CommentSerializer(data)
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serializer.is_valid()
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# True
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serializer.object
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# <Comment: Comment object>
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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.
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## Writing regular Django views using our Serializers
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Let's see how we can write some API views using our new Serializer class.
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We'll start off by creating a subclass of HttpResponse that we can use to render any data we return into `json`.
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Edit the `blog/views.py` file, and add the following.
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from blog.models import Comment
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from blog.serializers import CommentSerializer
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from django.http import HttpResponse
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from django.views.decorators.csrf import csrf_exempt
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from rest_framework.renderers import JSONRenderer
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from rest_framework.parsers import JSONParser
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class JSONResponse(HttpResponse):
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"""
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An HttpResponse that renders it's content into JSON.
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"""
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def __init__(self, data, **kwargs):
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content = JSONRenderer().render(data)
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kwargs['content_type'] = 'application/json'
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super(JSONResponse, self).__init__(content, **kwargs)
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The root of our API is going to be a view that supports listing all the existing comments, or creating a new comment.
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@csrf_exempt
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def comment_root(request):
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"""
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List all comments, or create a new comment.
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"""
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if request.method == 'GET':
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comments = Comment.objects.all()
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serializer = CommentSerializer(instance=comments)
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return JSONResponse(serializer.data)
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elif request.method == 'POST':
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data = JSONParser().parse(request)
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serializer = CommentSerializer(data)
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if serializer.is_valid():
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comment = serializer.object
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comment.save()
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return JSONResponse(serializer.data, status=201)
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else:
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return JSONResponse(serializer.errors, status=400)
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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.
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We'll also need a view which corresponds to an individual comment, and can be used to retrieve, update or delete the comment.
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@csrf_exempt
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def comment_instance(request, pk):
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"""
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Retrieve, update or delete a comment instance.
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"""
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try:
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comment = Comment.objects.get(pk=pk)
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except Comment.DoesNotExist:
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return HttpResponse(status=404)
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if request.method == 'GET':
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serializer = CommentSerializer(instance=comment)
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return JSONResponse(serializer.data)
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elif request.method == 'PUT':
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data = JSONParser().parse(request)
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serializer = CommentSerializer(data, instance=comment)
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if serializer.is_valid():
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comment = serializer.object
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comment.save()
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return JSONResponse(serializer.data)
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else:
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return JSONResponse(serializer.errors, status=400)
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elif request.method == 'DELETE':
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comment.delete()
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return HttpResponse(status=204)
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Finally we need to wire these views up. Create the `blog/urls.py` file:
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from django.conf.urls import patterns, url
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urlpatterns = patterns('blog.views',
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url(r'^$', 'comment_root'),
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url(r'^(?P<pk>[0-9]+)$', 'comment_instance')
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)
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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.
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## Testing our first attempt at a Web API
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**TODO: Describe using runserver and making example requests from console**
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**TODO: Describe opening in a web browser and viewing json output**
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## Where are we now
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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.
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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.
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We'll see how we can start to improve things in [part 2 of the tutorial][tut-2].
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[virtualenv]: http://www.virtualenv.org/en/latest/index.html
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[tut-2]: 2-requests-and-responses.md
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