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