# Serializers
> Expanding the usefulness of the serializers is something that we would
like to address. However, it's not a trivial problem, and it
will take some serious design work.
>
> — Russell Keith-Magee, [Django users group][cite]
Serializers allow complex data such as querysets and model instances to be converted to native Python datatypes that can then be easily rendered into `JSON`, `XML` or other content types. Serializers also provide deserialization, allowing parsed data to be converted back into complex types, after first validating the incoming data.
REST framework's serializers work very similarly to Django's `Form` and `ModelForm` classes. It provides a `Serializer` class which gives you a powerful, generic way to control the output of your responses, as well as a `ModelSerializer` class which provides a useful shortcut for creating serializers that deal with model instances and querysets.
## Declaring Serializers
Let's start by creating a simple object we can use for example purposes:
class Comment(object):
def __init__(self, email, content, created=None):
self.email = email
self.content = content
self.created = created or datetime.datetime.now()
comment = Comment(email='leila@example.com', content='foo bar')
We'll declare a serializer that we can use to serialize and deserialize `Comment` objects.
Declaring a serializer looks very similar to declaring a form:
from rest_framework 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):
"""
Given a dictionary of deserialized field values, either update
an existing model instance, or create a new model instance.
"""
if instance is not None:
instance.email = attrs.get('email', instance.email)
instance.content = attrs.get('content', instance.content)
instance.created = attrs.get('created', instance.created)
return instance
return 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.
The `restore_object` method is optional, and is only required if we want our serializer to support deserialization into fully fledged object instances. If we don't define this method, then deserializing data will simply return a dictionary of items.
## Serializing objects
We can now use `CommentSerializer` to serialize a comment, or list of comments. Again, using the `Serializer` class looks a lot like using a `Form` class.
serializer = CommentSerializer(comment)
serializer.data
# {'email': u'leila@example.com', 'content': u'foo bar', 'created': datetime.datetime(2012, 8, 22, 16, 20, 9, 822774)}
At this point we've translated the model instance into Python native datatypes. To finalise the serialization process we render the data into `json`.
from rest_framework.renderers import JSONRenderer
json = JSONRenderer().render(serializer.data)
json
# '{"email": "leila@example.com", "content": "foo bar", "created": "2012-08-22T16:20:09.822"}'
### Customizing field representation
Sometimes when serializing objects, you may not want to represent everything exactly the way it is in your model.
If you need to customize the serialized value of a particular field, you can do this by creating a `transform_` method. For example if you needed to render some markdown from a text field:
description = serializers.TextField()
description_html = serializers.TextField(source='description', read_only=True)
def transform_description_html(self, obj, value):
from django.contrib.markup.templatetags.markup import markdown
return markdown(value)
These methods are essentially the reverse of `validate_` (see *Validation* below.)
## Deserializing objects
Deserialization is similar. First we parse a stream into Python native datatypes...
from StringIO import StringIO
from rest_framework.parsers import JSONParser
stream = StringIO(json)
data = JSONParser().parse(stream)
...then we restore those native datatypes into a fully populated object instance.
serializer = CommentSerializer(data=data)
serializer.is_valid()
# True
serializer.object
#
When deserializing data, we can either create a new instance, or update an existing instance.
serializer = CommentSerializer(data=data) # Create new instance
serializer = CommentSerializer(comment, data=data) # Update `instance`
By default, serializers must be passed values for all required fields or they will throw validation errors. You can use the `partial` argument in order to allow partial updates.
serializer = CommentSerializer(comment, data={'content': u'foo bar'}, partial=True) # Update `instance` with partial data
## Validation
When deserializing data, you always need to call `is_valid()` before attempting to access the deserialized object. If any validation errors occur, the `.errors` property will contain a dictionary representing the resulting error messages. For example:
serializer = CommentSerializer(data={'email': 'foobar', 'content': 'baz'})
serializer.is_valid()
# False
serializer.errors
# {'email': [u'Enter a valid e-mail address.'], 'created': [u'This field is required.']}
Each key in the dictionary will be the field name, and the values will be lists of strings of any error messages corresponding to that field. The `non_field_errors` key may also be present, and will list any general validation errors.
When deserializing a list of items, errors will be returned as a list of dictionaries representing each of the deserialized items.
#### Field-level validation
You can specify custom field-level validation by adding `.validate_` methods to your `Serializer` subclass. These are analogous to `.clean_` methods on Django forms, but accept slightly different arguments.
They take a dictionary of deserialized attributes as a first argument, and the field name in that dictionary as a second argument (which will be either the name of the field or the value of the `source` argument to the field, if one was provided).
Your `validate_` methods should either just return the `attrs` dictionary or raise a `ValidationError`. For example:
from rest_framework import serializers
class BlogPostSerializer(serializers.Serializer):
title = serializers.CharField(max_length=100)
content = serializers.CharField()
def validate_title(self, attrs, source):
"""
Check that the blog post is about Django.
"""
value = attrs[source]
if "django" not in value.lower():
raise serializers.ValidationError("Blog post is not about Django")
return attrs
#### Object-level validation
To do any other validation that requires access to multiple fields, add a method called `.validate()` to your `Serializer` subclass. This method takes a single argument, which is the `attrs` dictionary. It should raise a `ValidationError` if necessary, or just return `attrs`. For example:
from rest_framework import serializers
class EventSerializer(serializers.Serializer):
description = serializers.CharField(max_length=100)
start = serializers.DateTimeField()
finish = serializers.DateTimeField()
def validate(self, attrs):
"""
Check that the start is before the stop.
"""
if attrs['start'] < attrs['finish']:
raise serializers.ValidationError("finish must occur after start")
return attrs
## Saving object state
To save the deserialized objects created by a serializer, call the `.save()` method:
if serializer.is_valid():
serializer.save()
The default behavior of the method is to simply call `.save()` on the deserialized object instance. You can override the default save behaviour by overriding the `.save_object(obj)` method on the serializer class.
The generic views provided by REST framework call the `.save()` method when updating or creating entities.
## Dealing with nested objects
The previous examples are fine for dealing with objects that only have simple datatypes, but sometimes we also need to be able to represent more complex objects, where some of the attributes of an object might not be simple datatypes such as strings, dates or integers.
The `Serializer` class is itself a type of `Field`, and can be used to represent relationships where one object type is nested inside another.
class UserSerializer(serializers.Serializer):
email = serializers.EmailField()
username = serializers.CharField(max_length=100)
class CommentSerializer(serializers.Serializer):
user = UserSerializer()
content = serializers.CharField(max_length=200)
created = serializers.DateTimeField()
If a nested representation may optionally accept the `None` value you should pass the `required=False` flag to the nested serializer.
class CommentSerializer(serializers.Serializer):
user = UserSerializer(required=False) # May be an anonymous user.
content = serializers.CharField(max_length=200)
created = serializers.DateTimeField()
Similarly if a nested representation should be a list of items, you should pass the `many=True` flag to the nested serialized.
class CommentSerializer(serializers.Serializer):
user = UserSerializer(required=False)
edits = EditItemSerializer(many=True) # A nested list of 'edit' items.
content = serializers.CharField(max_length=200)
created = serializers.DateTimeField()
Validation of nested objects will work the same as before. Errors with nested objects will be nested under the field name of the nested object.
serializer = CommentSerializer(comment, data={'user': {'email': 'foobar', 'username': 'doe'}, 'content': 'baz'})
serializer.is_valid()
# False
serializer.errors
# {'user': {'email': [u'Enter a valid e-mail address.']}, 'created': [u'This field is required.']}
## Dealing with multiple objects
The `Serializer` class can also handle serializing or deserializing lists of objects.
#### Serializing multiple objects
To serialize a queryset or list of objects instead of a single object instance, you should pass the `many=True` flag when instantiating the serializer. You can then pass a queryset or list of objects to be serialized.
queryset = Book.objects.all()
serializer = BookSerializer(queryset, many=True)
serializer.data
# [
# {'id': 0, 'title': 'The electric kool-aid acid test', 'author': 'Tom Wolfe'},
# {'id': 1, 'title': 'If this is a man', 'author': 'Primo Levi'},
# {'id': 2, 'title': 'The wi