root/django/branches/0.95-bugfixes/docs/serialization.txt

==========================
Serializing Django objects
==========================

.. note::
    
    This API is currently under heavy development and may change --
    perhaps drastically -- in the future.
    
    You have been warned.
    
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).

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 QuerySet_ to serialize.
(Actually, the second argument can be any iterator that yields Django objects,
but it'll almost always be a QuerySet).

.. _QuerySet: ../db_api/#retrieving-objects

You can also use a serializer object directly::

    xml_serializer = serializers.get_serializer("xml")
    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 a HTTPResponse_)::

    out = open("file.xml", "w")
    xml_serializer.serialize(SomeModel.objects.all(), stream=out)

.. _HTTPResponse: ../request_response/#httpresponse-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):
            obj.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
---------------------

Django "ships" with a few included serializers:

    ==========  ==============================================================
    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.
    ==========  ==============================================================

.. _json: http://json.org/
.. _simplejson: http://undefined.org/python/#simplejson

Writing custom serializers
``````````````````````````

XXX ...