serialization.txt

.. _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
<abstract-base-classes>`, 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
<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 <lazy-translations>` 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

.. _topics-serialization-natural-keys:

Natural keys
------------

The default serialization strategy for foreign keys and many-to-many
relations is to serialize the value of the primary key(s) of the
objects in the relation. This strategy works well for most types of
object, but it can cause difficulty in some circumstances.

Consider the case of a list of objects that have foreign key on
:class:`ContentType`. If you're going to serialize an object that
refers to a content type, you need to have a way to refer to that
content type. Content Types are automatically created by Django as
part of the database synchronization process, so you don't need to
include content types in a fixture or other serialized data. As a
result, the primary key of any given content type isn't easy to
predict - it will depend on how and when :djadmin:`syncdb` was
executed to create the content types.

There is also the matter of convenience. An integer id isn't always
the most convenient way to refer to an object; sometimes, a
more natural reference would be helpful.

Deserialization of natural keys
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is for these reasons that Django provides `natural keys`. A natural
key is a tuple of values that can be used to uniquely identify an
object instance without using the primary key value.

Consider the following two models::

    from django.db import models

    class Person(models.Model):
        first_name = models.CharField(max_length=100)
        last_name = models.CharField(max_length=100)

        birthdate = models.DateField()

    class Book(models.Model):
        name = models.CharField(max_length=100)
        author = models.ForeignKey(Person)

Ordinarily, serialized data for ``Book`` would use an integer to refer to
the author. For example, in JSON, a Book might be serialized as::

    ...
    {
        "pk": 1,
        "model": "store.book",
        "fields": {
            "name": "Mostly Harmless",
            "author": 42
        }
    }
    ...

This isn't a particularly natural way to refer to an author. It
requires that you know the primary key value for the author; it also
requires that this primary key value is stable and predictable.

However, if we add natural key handling to Person, the fixture becomes
much more humane. To add natural key handling, you define a default
Manager for Person with a ``get_by_natural_key()`` method. In the case
of a Person, a good natural key might be the pair of first and last
name::

    from django.db import models

    class PersonManager(models.Manager):
        def get_by_natural_key(self, first_name, last_name):
            return self.get(first_name=first_name, last_name=last_name)

    class Person(models.Model):
        objects = PersonManager()

        first_name = models.CharField(max_length=100)
        last_name = models.CharField(max_length=100)

        birthdate = models.DateField()

Now books can use that natural key to refer to ``Person`` objects::

    ...
    {
        "pk": 1,
        "model": "store.book",
        "fields": {
            "name": "Mostly Harmless",
            "author": ["Douglas", "Adams"]
        }
    }
    ...

When you try to load this serialized data, Django will use the
``get_by_natural_key()`` method to resolve ``["Douglas", "Adams"]``
into the primary key of an actual ``Person`` object.

Serialization of natural keys
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So how do you get Django to emit a natural key when serializing an object?
Firstly, you need to add another method -- this time to the model itself::

    class Person(models.Model):
        objects = PersonManager()

        first_name = models.CharField(max_length=100)
        last_name = models.CharField(max_length=100)

        birthdate = models.DateField()

        def natural_key(self):
            return (self.first_name, self.last_name)

Then, when you call ``serializers.serialize()``, you provide a
``use_natural_keys=True`` argument::

    >>> serializers.serialize([book1, book2], format='json', indent=2, use_natural_keys=True)

When ``use_natural_keys=True`` is specified, Django will use the
``natural_key()`` method to serialize any reference to objects of the
type that defines the method.

If you are using :djadmin:`dumpdata` to generate serialized data, you
use the `--natural` command line flag to generate natural keys.

.. note::

    You don't need to define both ``natural_key()`` and
    ``get_by_natural_key()``. If you don't want Django to output
    natural keys during serialization, but you want to retain the
    ability to load natural keys, then you can opt to not implement
    the ``natural_key()`` method.

    Conversely, if (for some strange reason) you want Django to output
    natural keys during serialization, but *not* be able to load those
    key values, just don't define the ``get_by_natural_key()`` method.

Dependencies during serialization
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since natural keys rely on database lookups to resolve references, it
is important that data exists before it is referenced. You can't make
a `forward reference` with natural keys - the data you are referencing
must exist before you include a natural key reference to that data.

To accommodate this limitation, calls to :djadmin:`dumpdata` that use
the :djadminopt:`--natural` optionwill serialize any model with a
``natural_key()`` method before it serializes normal key objects.

However, this may not always be enough. If your natural key refers to
another object (by using a foreign key or natural key to another object
as part of a natural key), then you need to be able to ensure that
the objects on which a natural key depends occur in the serialized data
before the natural key requires them.

To control this ordering, you can define dependencies on your
``natural_key()`` methods. You do this by setting a ``dependencies``
attribute on the ``natural_key()`` method itself.

For example, consider the ``Permission`` model in ``contrib.auth``.
The following is a simplified version of the ``Permission`` model::

    class Permission(models.Model):
        name = models.CharField(max_length=50)
        content_type = models.ForeignKey(ContentType)
        codename = models.CharField(max_length=100)
        # ...
        def natural_key(self):
            return (self.codename,) + self.content_type.natural_key()

The natural key for a ``Permission`` is a combination of the codename for the
``Permission``, and the ``ContentType`` to which the ``Permission`` applies. This means
that ``ContentType`` must be serialized before ``Permission``. To define this
dependency, we add one extra line::

    class Permission(models.Model):
        # ...
        def natural_key(self):
            return (self.codename,) + self.content_type.natural_key()
        natural_key.dependencies = ['contenttypes.contenttype']

This definition ensures that ``ContentType`` models are serialized before
``Permission`` models. In turn, any object referencing ``Permission`` will
be serialized after both ``ContentType`` and ``Permission``.