19 KiB
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
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:
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.title = attrs.get('title', instance.title)
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.
stream = JSONRenderer().render(data)
stream
# '{"email": "leila@example.com", "content": "foo bar", "created": "2012-08-22T16:20:09.822"}'
Deserializing objects
Deserialization is similar. First we parse a stream into python native datatypes...
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
# <Comment object at 0x10633b2d0>
>>> serializer.deserialize('json', stream)
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_<fieldname> methods to your Serializer subclass. These are analagous to .clean_<fieldname> 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_<fieldname> 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 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()
Note: Nested serializers are only suitable for read-only representations, as there are cases where they would have ambiguous or non-obvious behavior if used when updating instances. For read-write representations you should always use a flat representation, by using one of the RelatedField subclasses.
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 wind-up bird chronicle', 'author': 'Haruki Murakami'}
# ]
Deserializing multiple objects for creation
To deserialize a list of object data, and create multiple object instances in a single pass, you should also set the many=True flag, and pass a list of data to be deserialized.
This allows you to write views that create multiple items when a POST request is made.
For example:
data = [
{'title': 'The bell jar', 'author': 'Sylvia Plath'},
{'title': 'For whom the bell tolls', 'author': 'Ernest Hemingway'}
]
serializer = BookSerializer(data=data, many=True)
serializer.is_valid()
# True
serializer.save() # `.save()` will be called on each deserialized instance
Deserializing multiple objects for update
You can also deserialize a list of objects as part of a bulk update of multiple existing items. In this case you need to supply both an existing list or queryset of items, as well as a list of data to update those items with.
This allows you to write views that update or create multiple items when a PUT request is made.
# Capitalizing the titles of the books
queryset = Book.objects.all()
data = [
{'id': 3, 'title': 'The Bell Jar', 'author': 'Sylvia Plath'},
{'id': 4, 'title': 'For Whom the Bell Tolls', 'author': 'Ernest Hemingway'}
]
serializer = BookSerializer(queryset, data=data, many=True)
serializer.is_valid()
# True
serialize.save() # `.save()` will be called on each updated or newly created instance.
Bulk updates will update any instances that already exist, and create new instances for data items that do not have a corresponding instance.
When performing a bulk update you may want any items that are not present in the incoming data to be deleted. To do so, pass allow_delete=True to the serializer.
serializer = BookSerializer(queryset, data=data, many=True, allow_delete=True)
serializer.is_valid()
# True
serializer.save() # `.save()` will be called on each updated or newly created instance.
# `.delete()` will be called on any other items in the `queryset`.
Passing allow_delete=True ensures that any update operations will completely overwrite the existing queryset, rather than simply updating any objects found in the incoming data.
How identity is determined when performing bulk updates
Performing a bulk update is slightly more complicated than performing a bulk creation, because the serializer needs a way of determining how the items in the incoming data should be matched against the existing object instances.
By default the serializer class will use the id key on the incoming data to determine the canonical identity of an object. If you need to change this behavior you should override the get_identity method on the Serializer class. For example:
class AccountSerializer(serializers.Serializer):
slug = serializers.CharField(max_length=100)
created = serializers.DateTimeField()
... # Various other fields
def get_identity(self, data):
"""
This hook is required for bulk update.
We need to override the default, to use the slug as the identity.
Note that the data has not yet been validated at this point,
so we need to deal gracefully with incorrect datatypes.
"""
try:
return data.get('slug', None)
except AttributeError:
return None
To map the incoming data items to their corresponding object instances, the .get_identity() method will be called both against the incoming data, and against the serialized representation of the existing objects.
Including extra context
There are some cases where you need to provide extra context to the serializer in addition to the object being serialized. One common case is if you're using a serializer that includes hyperlinked relations, which requires the serializer to have access to the current request so that it can properly generate fully qualified URLs.
You can provide arbitrary additional context by passing a context argument when instantiating the serializer. For example:
serializer = AccountSerializer(account, context={'request': request})
serializer.data
# {'id': 6, 'owner': u'denvercoder9', 'created': datetime.datetime(2013, 2, 12, 09, 44, 56, 678870), 'details': 'http://example.com/accounts/6/details'}
The context dictionary can be used within any serializer field logic, such as a custom .to_native() method, by accessing the self.context attribute.
ModelSerializers
Often you'll want serializer classes that map closely to model definitions.
The ModelSerializer class lets you automatically create a Serializer class with fields that correspond to the Model fields.
class AccountSerializer(serializers.ModelSerializer):
class Meta:
model = Account
By default, all the model fields on the class will be mapped to corresponding serializer fields.
Any foreign keys on the model will be mapped to PrimaryKeyRelatedField if you're using a ModelSerializer, or HyperlinkedRelatedField if you're using a HyperlinkedModelSerializer.
Specifying fields explicitly
You can add extra fields to a ModelSerializer or override the default fields by declaring fields on the class, just as you would for a Serializer class.
class AccountSerializer(serializers.ModelSerializer):
url = serializers.CharField(source='get_absolute_url', read_only=True)
groups = serializers.PrimaryKeyRelatedField(many=True)
class Meta:
model = Account
Extra fields can correspond to any property or callable on the model.
Relational fields
When serializing model instances, there are a number of different ways you might choose to represent relationships. The default representation for ModelSerializer is to use the primary keys of the related instances.
Alternative representations include serializing using hyperlinks, serializing complete nested representations, or serializing with a custom representation.
For full details see the serializer relations documentation.
Specifying which fields should be included
If you only want a subset of the default fields to be used in a model serializer, you can do so using fields or exclude options, just as you would with a ModelForm.
For example:
class AccountSerializer(serializers.ModelSerializer):
class Meta:
model = Account
exclude = ('id',)
Specifiying nested serialization
The default ModelSerializer uses primary keys for relationships, but you can also easily generate nested representations using the depth option:
class AccountSerializer(serializers.ModelSerializer):
class Meta:
model = Account
exclude = ('id',)
depth = 1
The depth option should be set to an integer value that indicates the depth of relationships that should be traversed before reverting to a flat representation.
Specifying which fields should be read-only
You may wish to specify multiple fields as read-only. Instead of adding each field explicitely with the read_only=True attribute, you may use the read_only_fields Meta option, like so:
class AccountSerializer(serializers.ModelSerializer):
class Meta:
model = Account
read_only_fields = ('created', 'modified')
Customising the default fields
You can create customized subclasses of ModelSerializer that use a different set of default fields for the representation, by overriding various get_<field_type>_field methods.
Each of these methods may either return a field or serializer instance, or None.
get_pk_field
Signature: .get_pk_field(self, model_field)
Returns the field instance that should be used to represent the pk field.
get_nested_field
Signature: .get_nested_field(self, model_field)
Returns the field instance that should be used to represent a related field when depth is specified as being non-zero.
get_related_field
Signature: .get_related_field(self, model_field, to_many=False)
Returns the field instance that should be used to represent a related field when depth is not specified, or when nested representations are being used and the depth reaches zero.
get_field
Signature: .get_field(self, model_field)
Returns the field instance that should be used for non-relational, non-pk fields.
Example:
The following custom model serializer could be used as a base class for model serializers that should always exclude the pk by default.
class NoPKModelSerializer(serializers.ModelSerializer):
def get_pk_field(self, model_field):
return None