188 lines
7.2 KiB
Plaintext
188 lines
7.2 KiB
Plaintext
.. _topics-serialization:
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==========================
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Serializing Django objects
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==========================
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Django's serialization framework provides a mechanism for "translating" Django
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objects into other formats. Usually these other formats will be text-based and
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used for sending Django objects over a wire, but it's possible for a
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serializer to handle any format (text-based or not).
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Serializing data
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----------------
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At the highest level, serializing data is a very simple operation::
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from django.core import serializers
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data = serializers.serialize("xml", SomeModel.objects.all())
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The arguments to the ``serialize`` function are the format to serialize the data
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to (see `Serialization formats`_) and a :class:`~django.db.models.QuerySet` to
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serialize. (Actually, the second argument can be any iterator that yields Django
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objects, but it'll almost always be a QuerySet).
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You can also use a serializer object directly::
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XMLSerializer = serializers.get_serializer("xml")
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xml_serializer = XMLSerializer()
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xml_serializer.serialize(queryset)
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data = xml_serializer.getvalue()
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This is useful if you want to serialize data directly to a file-like object
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(which includes an :class:`~django.http.HttpResponse`)::
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out = open("file.xml", "w")
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xml_serializer.serialize(SomeModel.objects.all(), stream=out)
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Subset of fields
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~~~~~~~~~~~~~~~~
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If you only want a subset of fields to be serialized, you can
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specify a ``fields`` argument to the serializer::
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from django.core import serializers
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data = serializers.serialize('xml', SomeModel.objects.all(), fields=('name','size'))
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In this example, only the ``name`` and ``size`` attributes of each model will
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be serialized.
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.. note::
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Depending on your model, you may find that it is not possible to
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deserialize a model that only serializes a subset of its fields. If a
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serialized object doesn't specify all the fields that are required by a
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model, the deserializer will not be able to save deserialized instances.
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Inherited Models
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~~~~~~~~~~~~~~~~
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If you have a model that is defined using an :ref:`abstract base class
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<abstract-base-classes>`, you don't have to do anything special to serialize
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that model. Just call the serializer on the object (or objects) that you want to
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serialize, and the output will be a complete representation of the serialized
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object.
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However, if you have a model that uses :ref:`multi-table inheritance
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<multi-table-inheritance>`, you also need to serialize all of the base classes
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for the model. This is because only the fields that are locally defined on the
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model will be serialized. For example, consider the following models::
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class Place(models.Model):
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name = models.CharField(max_length=50)
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class Restaurant(Place):
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serves_hot_dogs = models.BooleanField()
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If you only serialize the Restaurant model::
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data = serializers.serialize('xml', Restaurant.objects.all())
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the fields on the serialized output will only contain the `serves_hot_dogs`
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attribute. The `name` attribute of the base class will be ignored.
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In order to fully serialize your Restaurant instances, you will need to
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serialize the Place models as well::
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all_objects = list(Restaurant.objects.all()) + list(Place.objects.all())
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data = serializers.serialize('xml', all_objects)
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Deserializing data
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------------------
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Deserializing data is also a fairly simple operation::
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for obj in serializers.deserialize("xml", data):
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do_something_with(obj)
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As you can see, the ``deserialize`` function takes the same format argument as
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``serialize``, a string or stream of data, and returns an iterator.
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However, here it gets slightly complicated. The objects returned by the
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``deserialize`` iterator *aren't* simple Django objects. Instead, they are
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special ``DeserializedObject`` instances that wrap a created -- but unsaved --
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object and any associated relationship data.
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Calling ``DeserializedObject.save()`` saves the object to the database.
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This ensures that deserializing is a non-destructive operation even if the
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data in your serialized representation doesn't match what's currently in the
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database. Usually, working with these ``DeserializedObject`` instances looks
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something like::
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for deserialized_object in serializers.deserialize("xml", data):
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if object_should_be_saved(deserialized_object):
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deserialized_object.save()
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In other words, the usual use is to examine the deserialized objects to make
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sure that they are "appropriate" for saving before doing so. Of course, if you
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trust your data source you could just save the object and move on.
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The Django object itself can be inspected as ``deserialized_object.object``.
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.. _serialization-formats:
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Serialization formats
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---------------------
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Django "ships" with a few included serializers:
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========== ==============================================================
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Identifier Information
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========== ==============================================================
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``xml`` Serializes to and from a simple XML dialect.
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``json`` Serializes to and from JSON_ (using a version of simplejson_
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bundled with Django).
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``python`` Translates to and from "simple" Python objects (lists, dicts,
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strings, etc.). Not really all that useful on its own, but
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used as a base for other serializers.
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``yaml`` Serializes to YAML (YAML Ain't a Markup Language). This
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serializer is only available if PyYAML_ is installed.
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========== ==============================================================
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.. _json: http://json.org/
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.. _simplejson: http://undefined.org/python/#simplejson
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.. _PyYAML: http://www.pyyaml.org/
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Notes for specific serialization formats
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----------------------------------------
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json
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~~~~
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If you're using UTF-8 (or any other non-ASCII encoding) data with the JSON
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serializer, you must pass ``ensure_ascii=False`` as a parameter to the
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``serialize()`` call. Otherwise, the output won't be encoded correctly.
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For example::
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json_serializer = serializers.get_serializer("json")()
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json_serializer.serialize(queryset, ensure_ascii=False, stream=response)
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The Django source code includes the simplejson_ module. However, if you're
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using Python 2.6 (which includes a builtin version of the module), Django will
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use the builtin ``json`` module automatically. If you have a system installed
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version that includes the C-based speedup extension, or your system version is
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more recent than the version shipped with Django (currently, 2.0.7), the
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system version will be used instead of the version included with Django.
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Be aware that if you're serializing using that module directly, not all Django
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output can be passed unmodified to simplejson. In particular, :ref:`lazy
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translation objects <lazy-translations>` need a `special encoder`_ written for
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them. Something like this will work::
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from django.utils.functional import Promise
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from django.utils.encoding import force_unicode
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class LazyEncoder(simplejson.JSONEncoder):
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def default(self, obj):
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if isinstance(obj, Promise):
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return force_unicode(obj)
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return obj
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.. _special encoder: http://svn.red-bean.com/bob/simplejson/tags/simplejson-1.7/docs/index.html
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