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@ -1,5 +1,5 @@
===================
Custom Model Fields
Custom model fields
===================
**New in Django development version**
@ -8,9 +8,10 @@ Introduction
============
The `model reference`_ documentation explains how to use Django's standard
field classes. For many purposes, those classes are all you'll need. Sometimes,
though, the Django version won't meet your precise requirements, or you'll want
to use a field that is entirely different from those shipped with Django.
field classes -- ``CharField``, ``DateField``, etc. For many purposes, those
classes are all you'll need. Sometimes, though, the Django version won't meet
your precise requirements, or you'll want to use a field that is entirely
different from those shipped with Django.
Django's built-in field types don't cover every possible database column type --
only the common types, such as ``VARCHAR`` and ``INTEGER``. For more obscure
@ -27,10 +28,10 @@ Our example object
Creating custom fields requires a bit of attention to detail. To make things
easier to follow, we'll use a consistent example throughout this document.
Suppose you have a Python object representing the deal of cards in a hand of
Bridge_. It doesn't matter if you don't know how to play Bridge. You only need
to know that 52 cards are dealt out equally to four players, who are
traditionally called *north*, *east*, *south* and *west*. Our class looks
something like this::
Bridge_. (Don't worry, you don't know how to play Bridge to follow this
example. You only need to know that 52 cards are dealt out equally to four
players, who are traditionally called *north*, *east*, *south* and *west*.)
Our class looks something like this::
class Hand(object):
def __init__(self, north, east, south, west):
@ -42,10 +43,9 @@ something like this::
# ... (other possibly useful methods omitted) ...
This is just an ordinary Python class, nothing Django-specific about it. We
would like to be able to things like this in our models (we assume the
``hand`` attribute on the model is an instance of ``Hand``)::
This is just an ordinary Python class, with nothing Django-specific about it.
We'd like to be able to things like this in our models (we assume the ``hand``
attribute on the model is an instance of ``Hand``)::
example = MyModel.objects.get(pk=1)
print example.hand.north
@ -72,7 +72,7 @@ model support for existing classes where you cannot change the source code.
.. _PostgreSQL custom types: http://www.postgresql.org/docs/8.2/interactive/sql-createtype.html
.. _Bridge: http://en.wikipedia.org/wiki/Contract_bridge
Background Theory
Background theory
=================
Database storage
@ -95,10 +95,9 @@ column type, or there's a fairly straightforward way to convert your data to,
say, a string.
For our ``Hand`` example, we could convert the card data to a string of 104
characters by concatenating all the cards together in a pre-determined order.
Say, all the *north* cards first, then the *east*, *south* and *west* cards, in
that order. So ``Hand`` objects can be saved to text or character columns in
the database.
characters by concatenating all the cards together in a pre-determined order --
say, all the *north* cards first, then the *east*, *south* and *west* cards. So
``Hand`` objects can be saved to text or character columns in the database.
What does a field class do?
---------------------------
@ -109,12 +108,12 @@ mean model fields and not `form fields`_) are subclasses of
field is common to all fields -- name, help text, validator lists, uniqueness
and so forth. Storing all that information is handled by ``Field``. We'll get
into the precise details of what ``Field`` can do later on; for now, suffice it
to say that everything descends from ``Field`` and then customises key pieces
of the class behaviour.
to say that everything descends from ``Field`` and then customizes key pieces
of the class behavior.
.. _form fields: ../newforms/#fields
It's important to realise that a Django field class is not what is stored in
It's important to realize that a Django field class is not what is stored in
your model attributes. The model attributes contain normal Python objects. The
field classes you define in a model are actually stored in the ``Meta`` class
when the model class is created (the precise details of how this is done are
@ -127,31 +126,35 @@ Keep this in mind when creating your own custom fields. The Django ``Field``
subclass you write provides the machinery for converting between your Python
instances and the database/serializer values in various ways (there are
differences between storing a value and using a value for lookups, for
example). If this sounds a bit tricky, don't worry. It will hopefully become
clearer in the examples below. Just remember that you will often end up
creating two classes when you want a custom field. The first class is the
Python object that your users will manipulate. They will assign it to the model
attribute, they will read from it for displaying purposes, things like that.
This is the ``Hand`` class in our example. The second class is the ``Field``
subclass. This is the class that knows how to convert your first class back and
forth between its permanent storage form and the Python form.
example). If this sounds a bit tricky, don't worry -- it will become clearer in
the examples below. Just remember that you will often end up creating two
classes when you want a custom field:
* The first class is the Python object that your users will manipulate.
They will assign it to the model attribute, they will read from it for
displaying purposes, things like that. This is the ``Hand`` class in our
example.
* The second class is the ``Field`` subclass. This is the class that knows
how to convert your first class back and forth between its permanent
storage form and the Python form.
Writing a ``Field`` subclass
=============================
When you are planning your ``Field`` subclass, first give some thought to
which existing field your new field is most similar to. Can you subclass an
existing Django field and save yourself some work? If not, you should subclass the ``Field`` class, from which everything is descended.
When planning your ``Field`` subclass, first give some thought to which
existing ``Field`` class your new field is most similar to. Can you subclass an
existing Django field and save yourself some work? If not, you should subclass
the ``Field`` class, from which everything is descended.
Initialising your new field is a matter of separating out any arguments that
Initializing your new field is a matter of separating out any arguments that
are specific to your case from the common arguments and passing the latter to
the ``__init__()`` method of ``Field`` (or your parent class).
In our example, the Django field we create is going to be called
``HandField``. It's not a bad idea to use a similar naming scheme to Django's
fields so that our new class is identifiable and yet clearly related to the
``Hand`` class it is wrapping. It doesn't behave like any existing field, so
we'll subclass directly from ``Field``::
In our example, we'll call our field ``HandField``. (It's a good idea to call
your ``Field`` subclass ``(Something)Field``, so it's easily identifiable as a
``Field`` subclass.) It doesn't behave like any existing field, so we'll
subclass directly from ``Field``::
from django.db import models
@ -160,7 +163,7 @@ we'll subclass directly from ``Field``::
kwargs['max_length'] = 104
super(HandField, self).__init__(*args, **kwargs)
Our ``HandField`` will accept most of the standard field options (see the list
Our ``HandField`` accept most of the standard field options (see the list
below), but we ensure it has a fixed length, since it only needs to hold 52
card values plus their suits; 104 characters in total.
@ -171,40 +174,40 @@ card values plus their suits; 104 characters in total.
(``auto_now`` being set implies ``editable=False``). No error is raised in
this case.
This behaviour simplifies the field classes, because they don't need to
This behavior simplifies the field classes, because they don't need to
check for options that aren't necessary. They just pass all the options to
the parent class and then don't use them later on. It is up to you whether
the parent class and then don't use them later on. It's up to you whether
you want your fields to be more strict about the options they select, or
to use the simpler, more permissive behaviour of the current fields.
to use the simpler, more permissive behavior of the current fields.
The ``Field.__init__()`` method takes the following parameters, in this
order:
- ``verbose_name``
- ``name``
- ``primary_key``
- ``max_length``
- ``unique``
- ``blank``
- ``null``
- ``db_index``
- ``core``
- ``rel``: Used for related fields (like ``ForeignKey``). For advanced use
* ``verbose_name``
* ``name``
* ``primary_key``
* ``max_length``
* ``unique``
* ``blank``
* ``null``
* ``db_index``
* ``core``
* ``rel``: Used for related fields (like ``ForeignKey``). For advanced use
only.
- ``default``
- ``editable``
- ``serialize``: If ``False``, the field will not be serialized when the
* ``default``
* ``editable``
* ``serialize``: If ``False``, the field will not be serialized when the
model is passed to Django's serializers_. Defaults to ``True``.
- ``prepopulate_from``
- ``unique_for_date``
- ``unique_for_month``
- ``unique_for_year``
- ``validator_list``
- ``choices``
- ``radio_admin``
- ``help_text``
- ``db_column``
- ``db_tablespace``: Currently only used with the Oracle backend and only
* ``prepopulate_from``
* ``unique_for_date``
* ``unique_for_month``
* ``unique_for_year``
* ``validator_list``
* ``choices``
* ``radio_admin``
* ``help_text``
* ``db_column``
* ``db_tablespace``: Currently only used with the Oracle backend and only
for index creation. You can usually ignore this option.
All of the options without an explanation in the above list have the same
@ -218,22 +221,19 @@ The ``SubfieldBase`` metaclass
------------------------------
As we indicated in the introduction_, field subclasses are often needed for
two reasons. Either to take advantage of a custom database column type, or to
handle complex Python types. A combination of the two is obviously also
possible. If you are only working with custom database column types and your
two reasons: either to take advantage of a custom database column type, or to
handle complex Python types. Obviously, a combination of the two is also
possible. If you're only working with custom database column types and your
model fields appear in Python as standard Python types direct from the
database backend, you don't need to worry about this section.
If you are handling custom Python types, such as our ``Hand`` class, we need
to make sure that when Django initialises an instance of our model and assigns
a database value to our custom field attribute we convert that value into the
If you're handling custom Python types, such as our ``Hand`` class, we need
to make sure that when Django initializes an instance of our model and assigns
a database value to our custom field attribute, we convert that value into the
appropriate Python object. The details of how this happens internally are a
little complex. For the field writer, though, things are fairly simple. Make
sure your field subclass uses ``django.db.models.SubfieldBase`` as its
metaclass. This ensures that the ``to_python()`` method, documented below_,
will always be called when the attribute is initialised.
Our ``HandField`` class now looks like this::
little complex, but the code you need to write in your ``Field`` class is
simple: make sure your field subclass uses ``django.db.models.SubfieldBase`` as
its metaclass::
class HandField(models.Field):
__metaclass__ = models.SubfieldBase
@ -241,16 +241,18 @@ Our ``HandField`` class now looks like this::
def __init__(self, *args, **kwargs):
# ...
This ensures that the ``to_python()`` method, documented below_, will always be
called when the attribute is initialized.
.. _below: #to-python-self-value
Useful methods
--------------
Once you've created your ``Field`` subclass and set up up the
``__metaclass__``, if necessary, there are a few standard methods you need to
consider overriding. Which of these you need to implement will depend on you
particular field behaviour. The list below is in approximately decreasing
order of importance, so start from the top.
``__metaclass__``, you might consider overriding a few standard methods,
depending on your field's behavior. The list of methods below is in
approximately decreasing order of importance, so start from the top.
``db_type(self)``
~~~~~~~~~~~~~~~~~
@ -337,23 +339,32 @@ field. You are then responsible for creating the column in the right table in
some other way, of course, but this gives you a way to tell Django to get out
of the way.
``to_python(self, value)``
~~~~~~~~~~~~~~~~~~~~~~~~~~
Converts between all the ways your field can receive its initial value and the
Python object you want to end up with. The default version just returns
``value``, so is useful is the database backend returns the data already in
the correct form (a Python string, for example).
Converts a value as returned by your database (or a serializer) to a Python
object.
Normally, you will need to override this method. As a general rule, be
prepared to accept an instance of the right type (e.g. ``Hand`` in our ongoing
example), a string (from a deserializer, for example), and whatever the
database wrapper returns for the column type you are using.
The default implementation simply returns ``value``, for the common case in
which the database backend already returns data in the correct format (as a
Python string, for example).
In our ``HandField`` class, we are storing the data in a character field in
the database, so we need to be able to process strings and ``Hand`` instances
in ``to_python()``::
If your custom ``Field`` class deals with data structures that are more complex
than strings, dates, integers or floats, then you'll need to override this
method. As a general rule, the method should deal gracefully with any of the
following arguments:
* An instance of the correct type (e.g., ``Hand`` in our ongoing example).
* A string (e.g., from a deserializer).
* Whatever the database returns for the column type you're using.
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 ``Hand`` instances in
``to_python()``::
import re
class HandField(models.Field):
# ...
@ -362,14 +373,14 @@ in ``to_python()``::
if isinstance(value, Hand):
return value
# The string case
# The string case.
p1 = re.compile('.{26}')
p2 = re.compile('..')
args = [p2.findall(x) for x in p1.findall(value)]
return Hand(*args)
Notice that we always return a ``Hand`` instance from this method. That is the
Python object we want to store in the model's attribute.
Notice that we always return a ``Hand`` instance from this method. That's the
Python object type we want to store in the model's attribute.
``get_db_prep_save(self, value)``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -377,7 +388,7 @@ Python object we want to store in the model's attribute.
This is the reverse of ``to_python()`` when working with the database backends
(as opposed to serialization). The ``value`` parameter is the current value of
the model's attribute (a field has no reference to its containing model, so it
cannot retrieve the value itself) and the method should return data in a
cannot retrieve the value itself), and the method should return data in a
format that can be used as a parameter in a query for the database backend.
For example::
@ -389,7 +400,6 @@ For example::
return ''.join([''.join(l) for l in (self.north,
self.east, self.south, self.west)])
``pre_save(self, model_instance, add)``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -399,10 +409,10 @@ The attribute name is in ``self.attname`` (this is set up by ``Field``). If
the model is being saved to the database for the first time, the ``add``
parameter will be ``True``, otherwise it will be ``False``.
Often you won't need to override this method. However, at times it can be very
useful. For example, the Django ``DateTimeField`` uses this method to set the
attribute to the correct value before returning it in the cases when
``auto_now`` or ``auto_now_add`` are set on the field.
You only need to override this method if you want to preprocess the value
somehow, just before saving. For example, Django's ``DateTimeField`` uses this
method to set the attribute correctly in the case of ``auto_now`` or
``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
@ -460,9 +470,9 @@ 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`` 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 `forms documentation`_ for information about this. Also
have a look at ``django.contrib.localflavor`` for some examples of custom
widgets.
form widget). See the `forms documentation`_ for information about this, and
take a look at the code in ``django.contrib.localflavor`` for some examples of
custom widgets.
Continuing our ongoing example, we can write the ``formfield()`` method as::
@ -471,14 +481,14 @@ Continuing our ongoing example, we can write the ``formfield()`` method as::
def formfield(self, **kwargs):
# This is a fairly standard way to set up some defaults
# whilst letting the caller override them.
# while letting the caller override them.
defaults = {'form_class': MyFormField}
defaults.update(kwargs)
return super(HandField, self).formfield(**defaults)
This assumes we have some ``MyFormField`` field class (which has its own
default widget) imported. This document doesn't cover the details of writing
custom form fields.
This assumes we're 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: ../newforms/#generating-forms-for-models
.. _forms documentation: ../newforms/
@ -490,7 +500,7 @@ Returns a string giving the name of the ``Field`` subclass we are emulating at
the database level. This is used to determine the type of database column for
simple cases.
If you have created a ``db_type()`` method, you do not need to worry about
If you have created a ``db_type()`` method, you don't need to worry about
``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.
@ -512,7 +522,7 @@ the database backend you are using -- that is, it doesn't appear in
be used by the serializer, but the default ``db_type()`` method will return
``None``. See the documentation of ``db_type()`` above_ 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 are ever going to be using the
the serializer is a useful idea if you're ever going to be using the
serializer output in some other place, outside of Django.
.. _above: #db-type-self
@ -528,7 +538,7 @@ serializer output in some other place, outside of Django.
Returns a dictionary, mapping the field's attribute name to a flattened string
version of the data. This method has some internal uses that aren't of
interest to use here (mostly having to do with manipulators). For our
purposes, it is sufficient to return a one item dictionary that maps the
purposes, it's sufficient to return a one item dictionary that maps the
attribute name to a string.
This method is used by the serializers to convert the field into a string for
@ -549,19 +559,20 @@ we can reuse some existing conversion code::
Some general advice
--------------------
Writing a custom field can be a tricky process sometimes, particularly if you
are doing complex conversions between your Python types and your database and
serialization formats. A couple of tips to make things go more smoothly:
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
``django/db/models/fields/__init__.py``) for inspiration. Try to find a field
that is already close to what you want and extend it a little bit, in
preference to creating an entirely new field from scratch.
2. Put a ``__str__()`` or ``__unicode__()`` method on the class you are
wrapping up as a field. There are a lot of places where the default behaviour
of the field code is to call ``force_unicode()`` on the value (in our
examples in this document, ``value`` would be a ``Hand`` instance, not a
``HandField``). So if your ``__unicode__()`` method automatically converts to
the string form of your Python object, you can save yourself a lot of work.
``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__()`` or ``__unicode__()`` 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 ``force_unicode()`` on the value.
(In our examples in this document, ``value`` would be a ``Hand``
instance, not a ``HandField``). So if your ``__unicode__()`` method
automatically converts to the string form of your Python object, you can
save yourself a lot of work.