mirror of
https://github.com/encode/django-rest-framework.git
synced 2024-11-24 10:34:03 +03:00
330 lines
15 KiB
Markdown
330 lines
15 KiB
Markdown
<a class="github" href="serializers.py"></a>
|
|
|
|
# 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:
|
|
|
|
class CommentSerializer(serializers.Serializer):
|
|
email = serializers.EmailField()
|
|
content = serializers.CharField(max_length=200)
|
|
created = serializers.DateTimeField()
|
|
|
|
def restore_object(self, attrs, instance=None):
|
|
if instance is not None:
|
|
instance.title = attrs['title']
|
|
instance.content = attrs['content']
|
|
instance.created = attrs['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.
|
|
|
|
## 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
|
|
|
|
## Serializing querysets
|
|
|
|
To serialize a queryset instead of an object instance, you should pass the `many=True` flag when instantiating the serializer.
|
|
|
|
queryset = Comment.objects.all()
|
|
serializer = CommentSerializer(queryset, many=True)
|
|
serializer.data
|
|
# [{'email': u'leila@example.com', 'content': u'foo bar', 'created': datetime.datetime(2012, 8, 22, 16, 20, 9, 822774)}, {'email': u'jamie@example.com', 'content': u'baz', 'created': datetime.datetime(2013, 1, 12, 16, 12, 45, 104445)}]
|
|
|
|
## 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` and `.non_field_errors` properties will contain the resulting error messages.
|
|
|
|
### 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
|
|
|
|
Serializers also include a `.save()` method that you can override if you want to provide a method of persisting the state of a deserialized object. The default behavior of the method is to simply call `.save()` on the deserialized object instance.
|
|
|
|
The generic views provided by REST framework call the `.save()` method when updating or creating entities.
|
|
|
|
## Dealing with nested objects
|
|
|
|
The previous example is 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.Field()
|
|
username = serializers.Field()
|
|
|
|
class CommentSerializer(serializers.Serializer):
|
|
user = UserSerializer()
|
|
title = serializers.Field()
|
|
content = serializers.Field()
|
|
created = serializers.Field()
|
|
|
|
---
|
|
|
|
**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.
|
|
|
|
---
|
|
|
|
## 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.
|
|
|
|
## Creating custom fields
|
|
|
|
If you want to create a custom field, you'll probably want to override either one or both of the `.to_native()` and `.from_native()` methods. These two methods are used to convert between the intial datatype, and a primative, serializable datatype. Primative datatypes may be any of a number, string, date/time/datetime or None. They may also be any list or dictionary like object that only contains other primative objects.
|
|
|
|
The `.to_native()` method is called to convert the initial datatype into a primative, serializable datatype. The `from_native()` method is called to restore a primative datatype into it's initial representation.
|
|
|
|
Let's look at an example of serializing a class that represents an RGB color value:
|
|
|
|
class Color(object):
|
|
"""
|
|
A color represented in the RGB colorspace.
|
|
"""
|
|
def __init__(self, red, green, blue):
|
|
assert(red >= 0 and green >= 0 and blue >= 0)
|
|
assert(red < 256 and green < 256 and blue < 256)
|
|
self.red, self.green, self.blue = red, green, blue
|
|
|
|
class ColourField(serializers.WritableField):
|
|
"""
|
|
Color objects are serialized into "rgb(#, #, #)" notation.
|
|
"""
|
|
def to_native(self, obj):
|
|
return "rgb(%d, %d, %d)" % (obj.red, obj.green, obj.blue)
|
|
|
|
def from_native(self, data):
|
|
data = data.strip('rgb(').rstrip(')')
|
|
red, green, blue = [int(col) for col in data.split(',')]
|
|
return Color(red, green, blue)
|
|
|
|
|
|
By default field values are treated as mapping to an attribute on the object. If you need to customize how the field value is accessed and set you need to override `.field_to_native()` and/or `.field_from_native()`.
|
|
|
|
As an example, let's create a field that can be used represent the class name of the object being serialized:
|
|
|
|
class ClassNameField(serializers.Field):
|
|
def field_to_native(self, obj, field_name):
|
|
"""
|
|
Serialize the object's class name.
|
|
"""
|
|
return obj.__class__
|
|
|
|
---
|
|
|
|
# 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][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
|
|
|
|
|
|
|
|
[cite]: https://groups.google.com/d/topic/django-users/sVFaOfQi4wY/discussion
|
|
[relations]: relations.md
|