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<li class="main"><a href="#serializers">Serializers</a></li>
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<li><a href="#declaring-serializers">Declaring Serializers</a></li>
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<li><a href="#serializing-objects">Serializing objects</a></li>
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<li><a href="#deserializing-objects">Deserializing objects</a></li>
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<li><a href="#validation">Validation</a></li>
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<li><a href="#dealing-with-nested-objects">Dealing with nested objects</a></li>
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<li><a href="#creating-custom-fields">Creating custom fields</a></li>
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<li class="main"><a href="#modelserializers">ModelSerializers</a></li>
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<li><a href="#specifying-fields-explicitly">Specifying fields explicitly</a></li>
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<li><a href="#relational-fields">Relational fields</a></li>
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<li><a href="#specifying-which-fields-should-be-included">Specifying which fields should be included</a></li>
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<li><a href="#specifiying-nested-serialization">Specifiying nested serialization</a></li>
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<li><a href="#customising-the-default-fields-used-by-a-modelserializer">Customising the default fields used by a ModelSerializer</a></li>
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<p><a class="github" href="https://github.com/tomchristie/django-rest-framework/blob/restframework2/rest_framework/serializers.py"><span class="label label-info">serializers.py</span></a></p>
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<h1 id="serializers">Serializers</h1>
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<blockquote>
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<p>Expanding the usefulness of the serializers is something that we would
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like to address. However, it's not a trivial problem, and it
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will take some serious design work. Any offers to help out in this
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area would be gratefully accepted.</p>
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<p>— Russell Keith-Magee, <a href="https://groups.google.com/d/topic/django-users/sVFaOfQi4wY/discussion">Django users group</a></p>
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</blockquote>
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<p>Serializers allow complex data such as querysets and model instances to be converted to native python datatypes that can then be easily rendered into <code>JSON</code>, <code>XML</code> or other content types. Serializers also provide deserialization, allowing parsed data to be converted back into complex types, after first validating the incoming data.</p>
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<p>REST framework's serializers work very similarly to Django's <code>Form</code> and <code>ModelForm</code> classes. It provides a <code>Serializer</code> class which gives you a powerful, generic way to control the output of your responses, as well as a <code>ModelSerializer</code> class which provides a useful shortcut for creating serializers that deal with model instances and querysets.</p>
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<h2 id="declaring-serializers">Declaring Serializers</h2>
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<p>Let's start by creating a simple object we can use for example purposes:</p>
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<pre class="prettyprint lang-py"><code>class Comment(object):
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def __init__(self, email, content, created=None):
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self.email = email
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self.content = content
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self.created = created or datetime.datetime.now()
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comment = Comment(email='leila@example.com', content='foo bar')
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</code></pre>
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<p>We'll declare a serializer that we can use to serialize and deserialize <code>Comment</code> objects.
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Declaring a serializer looks very similar to declaring a form:</p>
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<pre class="prettyprint lang-py"><code>class CommentSerializer(serializers.Serializer):
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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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if instance:
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instance.title = attrs['title']
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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 Comment(**attrs)
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</code></pre>
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<p>The first part of serializer class defines the fields that get serialized/deserialized. The <code>restore_object</code> method defines how fully fledged instances get created when deserializing data. The <code>restore_object</code> method is optional, and is only required if we want our serializer to support deserialization.</p>
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<h2 id="serializing-objects">Serializing objects</h2>
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<p>We can now use <code>CommentSerializer</code> to serialize a comment, or list of comments. Again, using the <code>Serializer</code> class looks a lot like using a <code>Form</code> class.</p>
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<pre class="prettyprint lang-py"><code>serializer = CommentSerializer(instance=comment)
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serializer.data
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# {'email': u'leila@example.com', 'content': u'foo bar', 'created': datetime.datetime(2012, 8, 22, 16, 20, 9, 822774)}
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</code></pre>
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<p>At this point we've translated the model instance into python native datatypes. To finalise the serialization process we render the data into <code>json</code>.</p>
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<pre class="prettyprint lang-py"><code>stream = JSONRenderer().render(data)
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stream
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# '{"email": "leila@example.com", "content": "foo bar", "created": "2012-08-22T16:20:09.822"}'
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</code></pre>
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<h2 id="deserializing-objects">Deserializing objects</h2>
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<p>Deserialization is similar. First we parse a stream into python native datatypes... </p>
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<pre class="prettyprint lang-py"><code>data = JSONParser().parse(stream)
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</code></pre>
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<p>...then we restore those native datatypes into a fully populated object instance.</p>
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<pre class="prettyprint lang-py"><code>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 object at 0x10633b2d0>
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>>> serializer.deserialize('json', stream)
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</code></pre>
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<h2 id="validation">Validation</h2>
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<p>When deserializing data, you always need to call <code>is_valid()</code> before attempting to access the deserialized object. If any validation errors occur, the <code>.errors</code> and <code>.non_field_errors</code> properties will contain the resulting error messages.</p>
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<p><strong>TODO: Describe validation in more depth</strong></p>
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<h2 id="dealing-with-nested-objects">Dealing with nested objects</h2>
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<p>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,
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where some of the attributes of an object might not be simple datatypes such as strings, dates or integers.</p>
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<p>The <code>Serializer</code> class is itself a type of <code>Field</code>, and can be used to represent relationships where one object type is nested inside another.</p>
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<pre class="prettyprint lang-py"><code>class UserSerializer(serializers.Serializer):
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email = serializers.EmailField()
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username = serializers.CharField()
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def restore_object(self, attrs, instance=None):
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return User(**attrs)
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class CommentSerializer(serializers.Serializer):
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user = UserSerializer()
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title = serializers.CharField()
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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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return Comment(**attrs)
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</code></pre>
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<h2 id="creating-custom-fields">Creating custom fields</h2>
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<p>If you want to create a custom field, you'll probably want to override either one or both of the <code>.to_native()</code> and <code>.from_native()</code> 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.</p>
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<p>The <code>.to_native()</code> method is called to convert the initial datatype into a primative, serializable datatype. The <code>from_native()</code> method is called to restore a primative datatype into it's initial representation.</p>
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<p>Let's look at an example of serializing a class that represents an RGB color value:</p>
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<pre class="prettyprint lang-py"><code>class Color(object):
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"""
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A color represented in the RGB colorspace.
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"""
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def __init__(self, red, green, blue):
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assert(red >= 0 and green >= 0 and blue >= 0)
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assert(red < 256 and green < 256 and blue < 256)
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self.red, self.green, self.blue = red, green, blue
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class ColourField(serializers.WritableField):
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"""
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Color objects are serialized into "rgb(#, #, #)" notation.
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"""
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def to_native(self, obj):
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return "rgb(%d, %d, %d)" % (obj.red, obj.green, obj.blue)
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def from_native(self, data):
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data = data.strip('rgb(').rstrip(')')
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red, green, blue = [int(col) for col in data.split(',')]
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return Color(red, green, blue)
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</code></pre>
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<p>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 <code>.field_to_native()</code> and/or <code>.field_from_native()</code>.</p>
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<p>As an example, let's create a field that can be used represent the class name of the object being serialized:</p>
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<pre class="prettyprint lang-py"><code>class ClassNameField(serializers.WritableField):
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def field_to_native(self, obj, field_name):
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"""
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Serialize the object's class name, not an attribute of the object.
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"""
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return obj.__class__.__name__
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def field_from_native(self, data, field_name, into):
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"""
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We don't want to set anything when we revert this field.
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"""
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pass
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</code></pre>
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<hr />
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<h1 id="modelserializers">ModelSerializers</h1>
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<p>Often you'll want serializer classes that map closely to model definitions.
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The <code>ModelSerializer</code> class lets you automatically create a Serializer class with fields that corrospond to the Model fields.</p>
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<pre class="prettyprint lang-py"><code>class AccountSerializer(serializers.ModelSerializer):
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class Meta:
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model = Account
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</code></pre>
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<p><strong>[TODO: Explain model field to serializer field mapping in more detail]</strong></p>
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<h2 id="specifying-fields-explicitly">Specifying fields explicitly</h2>
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<p>You can add extra fields to a <code>ModelSerializer</code> or override the default fields by declaring fields on the class, just as you would for a <code>Serializer</code> class.</p>
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<pre class="prettyprint lang-py"><code>class AccountSerializer(serializers.ModelSerializer):
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url = CharField(source='get_absolute_url', readonly=True)
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group = NaturalKeyField()
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class Meta:
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model = Account
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</code></pre>
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<p>Extra fields can corrospond to any property or callable on the model.</p>
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<h2 id="relational-fields">Relational fields</h2>
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<p>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.</p>
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<p>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.</p>
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<p>The <code>PrimaryKeyRelatedField</code> and <code>HyperlinkedRelatedField</code> fields provide alternative flat representations.</p>
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<p>The <code>ModelSerializer</code> class can itself be used as a field, in order to serialize relationships using nested representations.</p>
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<p>The <code>RelatedField</code> class may be subclassed to create a custom represenation of a relationship. The subclass should override <code>.to_native()</code>, and optionally <code>.from_native()</code> if deserialization is supported.</p>
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<p>All the relational fields may be used for any relationship or reverse relationship on a model.</p>
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<h2 id="specifying-which-fields-should-be-included">Specifying which fields should be included</h2>
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<p>If you only want a subset of the default fields to be used in a model serializer, you can do so using <code>fields</code> or <code>exclude</code> options, just as you would with a <code>ModelForm</code>.</p>
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<p>For example:</p>
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<pre class="prettyprint lang-py"><code>class AccountSerializer(serializers.ModelSerializer):
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class Meta:
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model = Account
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exclude = ('id',)
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</code></pre>
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<h2 id="specifiying-nested-serialization">Specifiying nested serialization</h2>
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<p>The default <code>ModelSerializer</code> uses primary keys for relationships, but you can also easily generate nested representations using the <code>nested</code> option:</p>
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<pre class="prettyprint lang-py"><code>class AccountSerializer(serializers.ModelSerializer):
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class Meta:
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model = Account
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exclude = ('id',)
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nested = True
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</code></pre>
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<p>The <code>nested</code> option may be set to either <code>True</code>, <code>False</code>, or an integer value. If given an integer value it indicates the depth of relationships that should be traversed before reverting to a flat representation.</p>
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<p>When serializing objects using a nested representation any occurances of recursion will be recognised, and will fall back to using a flat representation.</p>
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<h2 id="customising-the-default-fields-used-by-a-modelserializer">Customising the default fields used by a ModelSerializer</h2>
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<pre class="prettyprint lang-py"><code>class AccountSerializer(serializers.ModelSerializer):
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class Meta:
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model = Account
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def get_pk_field(self, model_field):
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return serializers.Field(readonly=True)
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def get_nested_field(self, model_field):
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return serializers.ModelSerializer()
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def get_related_field(self, model_field, to_many=False):
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queryset = model_field.rel.to._default_manager
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if to_many:
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return serializers.ManyRelatedField(queryset=queryset)
|
|
return serializers.RelatedField(queryset=queryset)
|
|
|
|
def get_field(self, model_field):
|
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return serializers.ModelField(model_field=model_field)
|
|
</code></pre>
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