750 lines
31 KiB
Plaintext
750 lines
31 KiB
Plaintext
===========================
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Writing custom model fields
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===========================
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.. currentmodule:: django.db.models
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Introduction
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============
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The :doc:`model reference </topics/db/models>` documentation explains how to use
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Django's standard field classes -- :class:`~django.db.models.CharField`,
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:class:`~django.db.models.DateField`, etc. For many purposes, those classes are
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all you'll need. Sometimes, though, the Django version won't meet your precise
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requirements, or you'll want to use a field that is entirely different from
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those shipped with Django.
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Django's built-in field types don't cover every possible database column type --
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only the common types, such as ``VARCHAR`` and ``INTEGER``. For more obscure
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column types, such as geographic polygons or even user-created types such as
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`PostgreSQL custom types`_, you can define your own Django ``Field`` subclasses.
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.. _PostgreSQL custom types: https://www.postgresql.org/docs/current/static/sql-createtype.html
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Alternatively, you may have a complex Python object that can somehow be
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serialized to fit into a standard database column type. This is another case
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where a ``Field`` subclass will help you use your object with your models.
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Our example object
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------------------
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Creating custom fields requires a bit of attention to detail. To make things
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easier to follow, we'll use a consistent example throughout this document:
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wrapping a Python object representing the deal of cards in a hand of Bridge_.
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Don't worry, you don't have to know how to play Bridge to follow this example.
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You only need to know that 52 cards are dealt out equally to four players, who
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are traditionally called *north*, *east*, *south* and *west*. Our class looks
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something like this::
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class Hand(object):
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"""A hand of cards (bridge style)"""
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def __init__(self, north, east, south, west):
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# Input parameters are lists of cards ('Ah', '9s', etc.)
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self.north = north
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self.east = east
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self.south = south
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self.west = west
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# ... (other possibly useful methods omitted) ...
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.. _Bridge: https://en.wikipedia.org/wiki/Contract_bridge
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This is just an ordinary Python class, with nothing Django-specific about it.
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We'd like to be able to do things like this in our models (we assume the
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``hand`` attribute on the model is an instance of ``Hand``)::
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example = MyModel.objects.get(pk=1)
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print(example.hand.north)
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new_hand = Hand(north, east, south, west)
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example.hand = new_hand
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example.save()
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We assign to and retrieve from the ``hand`` attribute in our model just like
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any other Python class. The trick is to tell Django how to handle saving and
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loading such an object.
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In order to use the ``Hand`` class in our models, we **do not** have to change
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this class at all. This is ideal, because it means you can easily write
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model support for existing classes where you cannot change the source code.
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.. note::
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You might only be wanting to take advantage of custom database column
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types and deal with the data as standard Python types in your models;
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strings, or floats, for example. This case is similar to our ``Hand``
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example and we'll note any differences as we go along.
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Background theory
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=================
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Database storage
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----------------
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The simplest way to think of a model field is that it provides a way to take a
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normal Python object -- string, boolean, ``datetime``, or something more
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complex like ``Hand`` -- and convert it to and from a format that is useful
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when dealing with the database (and serialization, but, as we'll see later,
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that falls out fairly naturally once you have the database side under control).
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Fields in a model must somehow be converted to fit into an existing database
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column type. Different databases provide different sets of valid column types,
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but the rule is still the same: those are the only types you have to work
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with. Anything you want to store in the database must fit into one of
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those types.
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Normally, you're either writing a Django field to match a particular database
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column type, or there's a fairly straightforward way to convert your data to,
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say, a string.
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For our ``Hand`` example, we could convert the card data to a string of 104
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characters by concatenating all the cards together in a pre-determined order --
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say, all the *north* cards first, then the *east*, *south* and *west* cards. So
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``Hand`` objects can be saved to text or character columns in the database.
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What does a field class do?
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---------------------------
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All of Django's fields (and when we say *fields* in this document, we always
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mean model fields and not :doc:`form fields </ref/forms/fields>`) are subclasses
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of :class:`django.db.models.Field`. Most of the information that Django records
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about a field is common to all fields -- name, help text, uniqueness and so
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forth. Storing all that information is handled by ``Field``. We'll get into the
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precise details of what ``Field`` can do later on; for now, suffice it to say
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that everything descends from ``Field`` and then customizes key pieces of the
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class behavior.
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It's important to realize that a Django field class is not what is stored in
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your model attributes. The model attributes contain normal Python objects. The
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field classes you define in a model are actually stored in the ``Meta`` class
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when the model class is created (the precise details of how this is done are
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unimportant here). This is because the field classes aren't necessary when
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you're just creating and modifying attributes. Instead, they provide the
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machinery for converting between the attribute value and what is stored in the
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database or sent to the :doc:`serializer </topics/serialization>`.
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Keep this in mind when creating your own custom fields. The Django ``Field``
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subclass you write provides the machinery for converting between your Python
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instances and the database/serializer values in various ways (there are
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differences between storing a value and using a value for lookups, for
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example). If this sounds a bit tricky, don't worry -- it will become clearer in
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the examples below. Just remember that you will often end up creating two
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classes when you want a custom field:
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* The first class is the Python object that your users will manipulate.
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They will assign it to the model attribute, they will read from it for
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displaying purposes, things like that. This is the ``Hand`` class in our
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example.
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* The second class is the ``Field`` subclass. This is the class that knows
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how to convert your first class back and forth between its permanent
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storage form and the Python form.
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Writing a field subclass
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========================
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When planning your :class:`~django.db.models.Field` subclass, first give some
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thought to which existing :class:`~django.db.models.Field` class your new field
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is most similar to. Can you subclass an existing Django field and save yourself
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some work? If not, you should subclass the :class:`~django.db.models.Field`
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class, from which everything is descended.
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Initializing your new field is a matter of separating out any arguments that are
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specific to your case from the common arguments and passing the latter to the
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``__init__()`` method of :class:`~django.db.models.Field` (or your parent
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class).
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In our example, we'll call our field ``HandField``. (It's a good idea to call
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your :class:`~django.db.models.Field` subclass ``<Something>Field``, so it's
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easily identifiable as a :class:`~django.db.models.Field` subclass.) It doesn't
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behave like any existing field, so we'll subclass directly from
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:class:`~django.db.models.Field`::
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from django.db import models
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class HandField(models.Field):
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description = "A hand of cards (bridge style)"
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def __init__(self, *args, **kwargs):
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kwargs['max_length'] = 104
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super().__init__(*args, **kwargs)
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Our ``HandField`` accepts most of the standard field options (see the list
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below), but we ensure it has a fixed length, since it only needs to hold 52
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card values plus their suits; 104 characters in total.
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.. note::
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Many of Django's model fields accept options that they don't do anything
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with. For example, you can pass both
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:attr:`~django.db.models.Field.editable` and
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:attr:`~django.db.models.DateField.auto_now` to a
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:class:`django.db.models.DateField` and it will simply ignore the
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:attr:`~django.db.models.Field.editable` parameter
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(:attr:`~django.db.models.DateField.auto_now` being set implies
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``editable=False``). No error is raised in this case.
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This behavior simplifies the field classes, because they don't need to
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check for options that aren't necessary. They just pass all the options to
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the parent class and then don't use them later on. It's up to you whether
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you want your fields to be more strict about the options they select, or to
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use the simpler, more permissive behavior of the current fields.
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The ``Field.__init__()`` method takes the following parameters:
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* :attr:`~django.db.models.Field.verbose_name`
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* ``name``
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* :attr:`~django.db.models.Field.primary_key`
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* :attr:`~django.db.models.CharField.max_length`
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* :attr:`~django.db.models.Field.unique`
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* :attr:`~django.db.models.Field.blank`
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* :attr:`~django.db.models.Field.null`
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* :attr:`~django.db.models.Field.db_index`
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* ``rel``: Used for related fields (like :class:`ForeignKey`). For advanced
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use only.
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* :attr:`~django.db.models.Field.default`
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* :attr:`~django.db.models.Field.editable`
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* ``serialize``: If ``False``, the field will not be serialized when the model
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is passed to Django's :doc:`serializers </topics/serialization>`. Defaults to
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``True``.
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* :attr:`~django.db.models.Field.unique_for_date`
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* :attr:`~django.db.models.Field.unique_for_month`
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* :attr:`~django.db.models.Field.unique_for_year`
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* :attr:`~django.db.models.Field.choices`
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* :attr:`~django.db.models.Field.help_text`
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* :attr:`~django.db.models.Field.db_column`
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* :attr:`~django.db.models.Field.db_tablespace`: Only for index creation, if the
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backend supports :doc:`tablespaces </topics/db/tablespaces>`. You can usually
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ignore this option.
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* :attr:`~django.db.models.Field.auto_created`: ``True`` if the field was
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automatically created, as for the :class:`~django.db.models.OneToOneField`
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used by model inheritance. For advanced use only.
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All of the options without an explanation in the above list have the same
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meaning they do for normal Django fields. See the :doc:`field documentation
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</ref/models/fields>` for examples and details.
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.. _custom-field-deconstruct-method:
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Field deconstruction
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--------------------
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The counterpoint to writing your ``__init__()`` method is writing the
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``deconstruct()`` method. This method tells Django how to take an instance
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of your new field and reduce it to a serialized form - in particular, what
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arguments to pass to ``__init__()`` to re-create it.
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If you haven't added any extra options on top of the field you inherited from,
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then there's no need to write a new ``deconstruct()`` method. If, however,
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you're changing the arguments passed in ``__init__()`` (like we are in
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``HandField``), you'll need to supplement the values being passed.
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The contract of ``deconstruct()`` is simple; it returns a tuple of four items:
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the field's attribute name, the full import path of the field class, the
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positional arguments (as a list), and the keyword arguments (as a dict). Note
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this is different from the ``deconstruct()`` method :ref:`for custom classes
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<custom-deconstruct-method>` which returns a tuple of three things.
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As a custom field author, you don't need to care about the first two values;
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the base ``Field`` class has all the code to work out the field's attribute
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name and import path. You do, however, have to care about the positional
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and keyword arguments, as these are likely the things you are changing.
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For example, in our ``HandField`` class we're always forcibly setting
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max_length in ``__init__()``. The ``deconstruct()`` method on the base ``Field``
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class will see this and try to return it in the keyword arguments; thus,
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we can drop it from the keyword arguments for readability::
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from django.db import models
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class HandField(models.Field):
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def __init__(self, *args, **kwargs):
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kwargs['max_length'] = 104
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super().__init__(*args, **kwargs)
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def deconstruct(self):
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name, path, args, kwargs = super().deconstruct()
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del kwargs["max_length"]
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return name, path, args, kwargs
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If you add a new keyword argument, you need to write code to put its value
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into ``kwargs`` yourself::
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from django.db import models
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class CommaSepField(models.Field):
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"Implements comma-separated storage of lists"
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def __init__(self, separator=",", *args, **kwargs):
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self.separator = separator
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super().__init__(*args, **kwargs)
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def deconstruct(self):
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name, path, args, kwargs = super().deconstruct()
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# Only include kwarg if it's not the default
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if self.separator != ",":
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kwargs['separator'] = self.separator
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return name, path, args, kwargs
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More complex examples are beyond the scope of this document, but remember -
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for any configuration of your Field instance, ``deconstruct()`` must return
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arguments that you can pass to ``__init__`` to reconstruct that state.
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Pay extra attention if you set new default values for arguments in the
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``Field`` superclass; you want to make sure they're always included, rather
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than disappearing if they take on the old default value.
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In addition, try to avoid returning values as positional arguments; where
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possible, return values as keyword arguments for maximum future compatibility.
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Of course, if you change the names of things more often than their position
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in the constructor's argument list, you might prefer positional, but bear in
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mind that people will be reconstructing your field from the serialized version
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for quite a while (possibly years), depending how long your migrations live for.
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You can see the results of deconstruction by looking in migrations that include
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the field, and you can test deconstruction in unit tests by just deconstructing
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and reconstructing the field::
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name, path, args, kwargs = my_field_instance.deconstruct()
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new_instance = MyField(*args, **kwargs)
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self.assertEqual(my_field_instance.some_attribute, new_instance.some_attribute)
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Changing a custom field's base class
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------------------------------------
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You can't change the base class of a custom field because Django won't detect
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the change and make a migration for it. For example, if you start with::
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class CustomCharField(models.CharField):
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...
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and then decide that you want to use ``TextField`` instead, you can't change
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the subclass like this::
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class CustomCharField(models.TextField):
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...
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Instead, you must create a new custom field class and update your models to
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reference it::
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class CustomCharField(models.CharField):
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...
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class CustomTextField(models.TextField):
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...
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As discussed in :ref:`removing fields <migrations-removing-model-fields>`, you
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must retain the original ``CustomCharField`` class as long as you have
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migrations that reference it.
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Documenting your custom field
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-----------------------------
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As always, you should document your field type, so users will know what it is.
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In addition to providing a docstring for it, which is useful for developers,
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you can also allow users of the admin app to see a short description of the
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field type via the :doc:`django.contrib.admindocs
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</ref/contrib/admin/admindocs>` application. To do this simply provide
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descriptive text in a :attr:`~Field.description` class attribute of your custom
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field. In the above example, the description displayed by the ``admindocs``
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application for a ``HandField`` will be 'A hand of cards (bridge style)'.
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In the :mod:`django.contrib.admindocs` display, the field description is
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interpolated with ``field.__dict__`` which allows the description to
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incorporate arguments of the field. For example, the description for
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:class:`~django.db.models.CharField` is::
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description = _("String (up to %(max_length)s)")
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Useful methods
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--------------
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Once you've created your :class:`~django.db.models.Field` subclass, you might
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consider overriding a few standard methods, depending on your field's behavior.
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The list of methods below is in approximately decreasing order of importance,
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so start from the top.
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.. _custom-database-types:
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Custom database types
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~~~~~~~~~~~~~~~~~~~~~
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Say you've created a PostgreSQL custom type called ``mytype``. You can
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subclass ``Field`` and implement the :meth:`~Field.db_type` method, like so::
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from django.db import models
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class MytypeField(models.Field):
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def db_type(self, connection):
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return 'mytype'
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Once you have ``MytypeField``, you can use it in any model, just like any other
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``Field`` type::
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class Person(models.Model):
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name = models.CharField(max_length=80)
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something_else = MytypeField()
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If you aim to build a database-agnostic application, you should account for
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differences in database column types. For example, the date/time column type
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in PostgreSQL is called ``timestamp``, while the same column in MySQL is called
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``datetime``. The simplest way to handle this in a :meth:`~Field.db_type`
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method is to check the ``connection.settings_dict['ENGINE']`` attribute.
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For example::
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class MyDateField(models.Field):
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def db_type(self, connection):
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if connection.settings_dict['ENGINE'] == 'django.db.backends.mysql':
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return 'datetime'
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else:
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return 'timestamp'
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The :meth:`~Field.db_type` and :meth:`~Field.rel_db_type` methods are called by
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Django when the framework constructs the ``CREATE TABLE`` statements for your
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application -- that is, when you first create your tables. The methods are also
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called when constructing a ``WHERE`` clause that includes the model field --
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that is, when you retrieve data using QuerySet methods like ``get()``,
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``filter()``, and ``exclude()`` and have the model field as an argument. They
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are not called at any other time, so it can afford to execute slightly complex
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code, such as the ``connection.settings_dict`` check in the above example.
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Some database column types accept parameters, such as ``CHAR(25)``, where the
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parameter ``25`` represents the maximum column length. In cases like these,
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it's more flexible if the parameter is specified in the model rather than being
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hard-coded in the ``db_type()`` method. For example, it wouldn't make much
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sense to have a ``CharMaxlength25Field``, shown here::
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# This is a silly example of hard-coded parameters.
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class CharMaxlength25Field(models.Field):
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def db_type(self, connection):
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return 'char(25)'
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# In the model:
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class MyModel(models.Model):
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# ...
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my_field = CharMaxlength25Field()
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The better way of doing this would be to make the parameter specifiable at run
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time -- i.e., when the class is instantiated. To do that, just implement
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``Field.__init__()``, like so::
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# This is a much more flexible example.
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class BetterCharField(models.Field):
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def __init__(self, max_length, *args, **kwargs):
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self.max_length = max_length
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super().__init__(*args, **kwargs)
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def db_type(self, connection):
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return 'char(%s)' % self.max_length
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# In the model:
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class MyModel(models.Model):
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# ...
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my_field = BetterCharField(25)
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Finally, if your column requires truly complex SQL setup, return ``None`` from
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:meth:`.db_type`. This will cause Django's SQL creation code to skip
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over this field. You are then responsible for creating the column in the right
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table in some other way, of course, but this gives you a way to tell Django to
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get out of the way.
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The :meth:`~Field.rel_db_type` method is called by fields such as ``ForeignKey``
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and ``OneToOneField`` that point to another field to determine their database
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column data types. For example, if you have an ``UnsignedAutoField``, you also
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need the foreign keys that point to that field to use the same data type::
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# MySQL unsigned integer (range 0 to 4294967295).
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class UnsignedAutoField(models.AutoField):
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def db_type(self, connection):
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return 'integer UNSIGNED AUTO_INCREMENT'
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def rel_db_type(self, connection):
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|
return 'integer UNSIGNED'
|
|
|
|
.. _converting-values-to-python-objects:
|
|
|
|
Converting values to Python objects
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If your custom :class:`~Field` class deals with data structures that are more
|
|
complex than strings, dates, integers, or floats, then you may need to override
|
|
:meth:`~Field.from_db_value` and :meth:`~Field.to_python`.
|
|
|
|
If present for the field subclass, ``from_db_value()`` will be called in all
|
|
circumstances when the data is loaded from the database, including in
|
|
aggregates and :meth:`~django.db.models.query.QuerySet.values` calls.
|
|
|
|
``to_python()`` is called by deserialization and during the
|
|
:meth:`~django.db.models.Model.clean` method used from forms.
|
|
|
|
As a general rule, ``to_python()`` should deal gracefully with any of the
|
|
following arguments:
|
|
|
|
* An instance of the correct type (e.g., ``Hand`` in our ongoing example).
|
|
|
|
* A string
|
|
|
|
* ``None`` (if the field allows ``null=True``)
|
|
|
|
In our ``HandField`` class, we're storing the data as a VARCHAR field in the
|
|
database, so we need to be able to process strings and ``None`` in the
|
|
``from_db_value()``. In ``to_python()``, we need to also handle ``Hand``
|
|
instances::
|
|
|
|
import re
|
|
|
|
from django.core.exceptions import ValidationError
|
|
from django.db import models
|
|
from django.utils.translation import gettext_lazy as _
|
|
|
|
def parse_hand(hand_string):
|
|
"""Takes a string of cards and splits into a full hand."""
|
|
p1 = re.compile('.{26}')
|
|
p2 = re.compile('..')
|
|
args = [p2.findall(x) for x in p1.findall(hand_string)]
|
|
if len(args) != 4:
|
|
raise ValidationError(_("Invalid input for a Hand instance"))
|
|
return Hand(*args)
|
|
|
|
class HandField(models.Field):
|
|
# ...
|
|
|
|
def from_db_value(self, value, expression, connection, context):
|
|
if value is None:
|
|
return value
|
|
return parse_hand(value)
|
|
|
|
def to_python(self, value):
|
|
if isinstance(value, Hand):
|
|
return value
|
|
|
|
if value is None:
|
|
return value
|
|
|
|
return parse_hand(value)
|
|
|
|
Notice that we always return a ``Hand`` instance from these methods. That's the
|
|
Python object type we want to store in the model's attribute.
|
|
|
|
For ``to_python()``, if anything goes wrong during value conversion, you should
|
|
raise a :exc:`~django.core.exceptions.ValidationError` exception.
|
|
|
|
.. _converting-python-objects-to-query-values:
|
|
|
|
Converting Python objects to query values
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Since using a database requires conversion in both ways, if you override
|
|
:meth:`~Field.to_python` you also have to override :meth:`~Field.get_prep_value`
|
|
to convert Python objects back to query values.
|
|
|
|
For example::
|
|
|
|
class HandField(models.Field):
|
|
# ...
|
|
|
|
def get_prep_value(self, value):
|
|
return ''.join([''.join(l) for l in (value.north,
|
|
value.east, value.south, value.west)])
|
|
|
|
.. warning::
|
|
|
|
If your custom field uses the ``CHAR``, ``VARCHAR`` or ``TEXT``
|
|
types for MySQL, you must make sure that :meth:`.get_prep_value`
|
|
always returns a string type. MySQL performs flexible and unexpected
|
|
matching when a query is performed on these types and the provided
|
|
value is an integer, which can cause queries to include unexpected
|
|
objects in their results. This problem cannot occur if you always
|
|
return a string type from :meth:`.get_prep_value`.
|
|
|
|
.. _converting-query-values-to-database-values:
|
|
|
|
Converting query values to database values
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Some data types (for example, dates) need to be in a specific format
|
|
before they can be used by a database backend.
|
|
:meth:`~Field.get_db_prep_value` is the method where those conversions should
|
|
be made. The specific connection that will be used for the query is
|
|
passed as the ``connection`` parameter. This allows you to use
|
|
backend-specific conversion logic if it is required.
|
|
|
|
For example, Django uses the following method for its
|
|
:class:`BinaryField`::
|
|
|
|
def get_db_prep_value(self, value, connection, prepared=False):
|
|
value = super().get_db_prep_value(value, connection, prepared)
|
|
if value is not None:
|
|
return connection.Database.Binary(value)
|
|
return value
|
|
|
|
In case your custom field needs a special conversion when being saved that is
|
|
not the same as the conversion used for normal query parameters, you can
|
|
override :meth:`~Field.get_db_prep_save`.
|
|
|
|
.. _preprocessing-values-before-saving:
|
|
|
|
Preprocessing values before saving
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If you want to preprocess the value just before saving, you can use
|
|
:meth:`~Field.pre_save`. For example, Django's
|
|
:class:`~django.db.models.DateTimeField` uses this method to set the attribute
|
|
correctly in the case of :attr:`~django.db.models.DateField.auto_now` or
|
|
:attr:`~django.db.models.DateField.auto_now_add`.
|
|
|
|
If you do override this method, you must return the value of the attribute at
|
|
the end. You should also update the model's attribute if you make any changes
|
|
to the value so that code holding references to the model will always see the
|
|
correct value.
|
|
|
|
.. _specifying-form-field-for-model-field:
|
|
|
|
Specifying the form field for a model field
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
To customize the form field used by :class:`~django.forms.ModelForm`, you can
|
|
override :meth:`~Field.formfield`.
|
|
|
|
The form field class can be specified via the ``form_class`` and
|
|
``choices_form_class`` arguments; the latter is used if the field has choices
|
|
specified, the former otherwise. If these arguments are not provided,
|
|
:class:`~django.forms.CharField` or :class:`~django.forms.TypedChoiceField`
|
|
will be used.
|
|
|
|
All of the ``kwargs`` dictionary is passed directly to the form field's
|
|
``__init__()`` method. Normally, all you need to do is set up a good default
|
|
for the ``form_class`` (and maybe ``choices_form_class``) argument and then
|
|
delegate further handling to the parent class. This might require you to write
|
|
a custom form field (and even a form widget). See the :doc:`forms documentation
|
|
</topics/forms/index>` for information about this.
|
|
|
|
Continuing our ongoing example, we can write the :meth:`~Field.formfield` method
|
|
as::
|
|
|
|
class HandField(models.Field):
|
|
# ...
|
|
|
|
def formfield(self, **kwargs):
|
|
# This is a fairly standard way to set up some defaults
|
|
# while letting the caller override them.
|
|
defaults = {'form_class': MyFormField}
|
|
defaults.update(kwargs)
|
|
return super().formfield(**defaults)
|
|
|
|
This assumes we've imported a ``MyFormField`` field class (which has its own
|
|
default widget). This document doesn't cover the details of writing custom form
|
|
fields.
|
|
|
|
.. _helper functions: ../forms/#generating-forms-for-models
|
|
.. _forms documentation: ../forms/
|
|
|
|
.. _emulating-built-in-field-types:
|
|
|
|
Emulating built-in field types
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
If you have created a :meth:`.db_type` method, you don't need to worry about
|
|
:meth:`.get_internal_type` -- it won't be used much. Sometimes, though, your
|
|
database storage is similar in type to some other field, so you can use that
|
|
other field's logic to create the right column.
|
|
|
|
For example::
|
|
|
|
class HandField(models.Field):
|
|
# ...
|
|
|
|
def get_internal_type(self):
|
|
return 'CharField'
|
|
|
|
No matter which database backend we are using, this will mean that
|
|
:djadmin:`migrate` and other SQL commands create the right column type for
|
|
storing a string.
|
|
|
|
If :meth:`.get_internal_type` returns a string that is not known to Django for
|
|
the database backend you are using -- that is, it doesn't appear in
|
|
``django.db.backends.<db_name>.base.DatabaseWrapper.data_types`` -- the string
|
|
will still be used by the serializer, but the default :meth:`~Field.db_type`
|
|
method will return ``None``. See the documentation of :meth:`~Field.db_type`
|
|
for reasons why this might be useful. Putting a descriptive string in as the
|
|
type of the field for the serializer is a useful idea if you're ever going to
|
|
be using the serializer output in some other place, outside of Django.
|
|
|
|
.. _converting-model-field-to-serialization:
|
|
|
|
Converting field data for serialization
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
To customize how the values are serialized by a serializer, you can override
|
|
:meth:`~Field.value_to_string`. Using ``value_from_object()`` is the best way
|
|
to get the field's value prior to serialization. For example, since our
|
|
``HandField`` uses strings for its data storage anyway, we can reuse some
|
|
existing conversion code::
|
|
|
|
class HandField(models.Field):
|
|
# ...
|
|
|
|
def value_to_string(self, obj):
|
|
value = self.value_from_object(obj)
|
|
return self.get_prep_value(value)
|
|
|
|
Some general advice
|
|
--------------------
|
|
|
|
Writing a custom field can be a tricky process, particularly if you're doing
|
|
complex conversions between your Python types and your database and
|
|
serialization formats. Here are a couple of tips to make things go more
|
|
smoothly:
|
|
|
|
1. Look at the existing Django fields (in
|
|
:file:`django/db/models/fields/__init__.py`) for inspiration. Try to find
|
|
a field that's similar to what you want and extend it a little bit,
|
|
instead of creating an entirely new field from scratch.
|
|
|
|
2. Put a ``__str__()`` method on the class you're wrapping up as a field. There
|
|
are a lot of places where the default behavior of the field code is to call
|
|
``str()`` on the value. (In our examples in this document, ``value`` would
|
|
be a ``Hand`` instance, not a ``HandField``). So if your ``__str__()``
|
|
method automatically converts to the string form of your Python object, you
|
|
can save yourself a lot of work.
|
|
|
|
Writing a ``FileField`` subclass
|
|
================================
|
|
|
|
In addition to the above methods, fields that deal with files have a few other
|
|
special requirements which must be taken into account. The majority of the
|
|
mechanics provided by ``FileField``, such as controlling database storage and
|
|
retrieval, can remain unchanged, leaving subclasses to deal with the challenge
|
|
of supporting a particular type of file.
|
|
|
|
Django provides a ``File`` class, which is used as a proxy to the file's
|
|
contents and operations. This can be subclassed to customize how the file is
|
|
accessed, and what methods are available. It lives at
|
|
``django.db.models.fields.files``, and its default behavior is explained in the
|
|
:doc:`file documentation </ref/files/file>`.
|
|
|
|
Once a subclass of ``File`` is created, the new ``FileField`` subclass must be
|
|
told to use it. To do so, simply assign the new ``File`` subclass to the special
|
|
``attr_class`` attribute of the ``FileField`` subclass.
|
|
|
|
A few suggestions
|
|
------------------
|
|
|
|
In addition to the above details, there are a few guidelines which can greatly
|
|
improve the efficiency and readability of the field's code.
|
|
|
|
1. The source for Django's own ``ImageField`` (in
|
|
``django/db/models/fields/files.py``) is a great example of how to
|
|
subclass ``FileField`` to support a particular type of file, as it
|
|
incorporates all of the techniques described above.
|
|
|
|
2. Cache file attributes wherever possible. Since files may be stored in
|
|
remote storage systems, retrieving them may cost extra time, or even
|
|
money, that isn't always necessary. Once a file is retrieved to obtain
|
|
some data about its content, cache as much of that data as possible to
|
|
reduce the number of times the file must be retrieved on subsequent
|
|
calls for that information.
|