django-rest-framework/docs/api-guide/serializers.md

<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.
>
> &mdash; 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.CahrField(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.

---


## 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

**[TODO: Explain model field to serializer field mapping in more detail]**

## 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 = CharField(source='get_absolute_url', read_only=True)
        group = NaturalKeyField()

        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 is to use the primary keys of the related instances.

Alternative representations include serializing using natural keys, serializing complete nested representations, or serializing using a custom representation, such as a URL that uniquely identifies the model instances.

The `PrimaryKeyRelatedField` and `HyperlinkedRelatedField` fields provide alternative flat representations.

The `ModelSerializer` class can itself be used as a field, in order to serialize relationships using nested representations.

The `RelatedField` class may be subclassed to create a custom representation of a relationship.  The subclass should override `.to_native()`, and optionally `.from_native()` if deserialization is supported.

All the relational fields may be used for any relationship or reverse relationship on a model.

## 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