Writing custom model fields


The model reference documentation explains how to use Django’s standard 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 column types, such as geographic polygons or even user-created types such as PostgreSQL custom types, you can define your own Django Field subclasses.

Alternatively, you may have a complex Python object that can somehow be serialized to fit into a standard database column type. This is another case where a Field subclass will help you use your object with your models.

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: wrapping a Python object representing the deal of cards in a hand of Bridge. Don’t worry, you don’t have 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):
    """A hand of cards (bridge style)"""

    def __init__(self, north, east, south, west):
        # Input parameters are lists of cards ('Ah', '9s', etc)
        self.north = north
        self.east = east
        self.south = south
        self.west = west

    # ... (other possibly useful methods omitted) ...

This is just an ordinary Python class, with nothing Django-specific about it. We'd like to be able to do 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

new_hand = Hand(north, east, south, west)
example.hand = new_hand

We assign to and retrieve from the hand attribute in our model just like any other Python class. The trick is to tell Django how to handle saving and loading such an object.

In order to use the Hand class in our models, we do not have to change this class at all. This is ideal, because it means you can easily write model support for existing classes where you cannot change the source code.


You might only be wanting to take advantage of custom database column types and deal with the data as standard Python types in your models; strings, or floats, for example. This case is similar to our Hand example and we'll note any differences as we go along.

Background theory

Database storage

The simplest way to think of a model field is that it provides a way to take a normal Python object -- string, boolean, datetime, or something more complex like Hand -- and convert it to and from a format that is useful when dealing with the database (and serialization, but, as we'll see later, that falls out fairly naturally once you have the database side under control).

Fields in a model must somehow be converted to fit into an existing database column type. Different databases provide different sets of valid column types, but the rule is still the same: those are the only types you have to work with. Anything you want to store in the database must fit into one of those types.

Normally, you're either writing a Django field to match a particular database 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. So Hand objects can be saved to text or character columns in the database.

What does a field class do?

class Field

All of Django's fields (and when we say fields in this document, we always mean model fields and not form fields) are subclasses of django.db.models.Field. Most of the information that Django records about a field is common to all fields -- name, help text, 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 customizes key pieces of the class behavior.

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 unimportant here). This is because the field classes aren't necessary when you're just creating and modifying attributes. Instead, they provide the machinery for converting between the attribute value and what is stored in the database or sent to the serializer.

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 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 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.

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, 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

class HandField(models.Field):

    description = "A hand of cards (bridge style)"

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 104
        super(HandField, self).__init__(*args, **kwargs)

Our HandField accepts 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.


Many of Django's model fields accept options that they don't do anything with. For example, you can pass both editable and auto_now to a django.db.models.DateField and it will simply ignore the editable parameter (auto_now being set implies editable=False). No error is raised in this case.

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'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 behavior of the current fields.


The __init__() method takes the following parameters:

All of the options without an explanation in the above list have the same meaning they do for normal Django fields. See the field documentation for examples and details.

The SubfieldBase metaclass

class django.db.models.SubfieldBase

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. 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'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, but the code you need to write in your Field class is simple: make sure your field subclass uses a special metaclass:

For example:

class HandField(models.Field):

    description = "A hand of cards (bridge style)"

    __metaclass__ = models.SubfieldBase

    def __init__(self, *args, **kwargs):
        # ...

This ensures that the to_python() method, documented below, will always be called when the attribute is initialized.

ModelForms and custom fields

If you use SubfieldBase, to_python() will be called every time an instance of the field is assigned a value. This means that whenever a value may be assigned to the field, you need to ensure that it will be of the correct datatype, or that you handle any exceptions.

This is especially important if you use ModelForms. When saving a ModelForm, Django will use form values to instantiate model instances. However, if the cleaned form data can't be used as valid input to the field, the normal form validation process will break.

Therefore, you must ensure that the form field used to represent your custom field performs whatever input validation and data cleaning is necessary to convert user-provided form input into a to_python()-compatible model field value. This may require writing a custom form field, and/or implementing the formfield() method on your field to return a form field class whose to_python() returns the correct datatype.

Documenting your custom field


As always, you should document your field type, so users will know what it is. In addition to providing a docstring for it, which is useful for developers, you can also allow users of the admin app to see a short description of the field type via the django.contrib.admindocs application. To do this simply provide descriptive text in a description class attribute of your custom field. In the above example, the description displayed by the admindocs application for a HandField will be 'A hand of cards (bridge style)'.

Useful methods

Once you've created your Field subclass and set up the __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.

Custom database types

Field.db_type(self, connection)
New in Django 1.2: The connection argument was added to support multiple databases.

Returns the database column data type for the Field, taking into account the connection object, and the settings associated with it.

Say you've created a PostgreSQL custom type called mytype. You can use this field with Django by subclassing Field and implementing the db_type() method, like so:

from django.db import models

class MytypeField(models.Field):
    def db_type(self, connection):
        return 'mytype'

Once you have MytypeField, you can use it in any model, just like any other Field type:

class Person(models.Model):
    name = models.CharField(max_length=80)
    something_else = MytypeField()

If you aim to build a database-agnostic application, you should account for differences in database column types. For example, the date/time column type in PostgreSQL is called timestamp, while the same column in MySQL is called datetime. The simplest way to handle this in a db_type() method is to check the connection.settings_dict['ENGINE'] attribute.

For example:

class MyDateField(models.Field):
    def db_type(self, connection):
        if connection.settings_dict['ENGINE'] == 'django.db.backends.mysql':
            return 'datetime'
            return 'timestamp'

The db_type() method is only called by Django when the framework constructs the CREATE TABLE statements for your application -- that is, when you first create your tables. It's not called at any other time, so it can afford to execute slightly complex code, such as the connection.settings_dict check in the above example.

Some database column types accept parameters, such as CHAR(25), where the parameter 25 represents the maximum column length. In cases like these, it's more flexible if the parameter is specified in the model rather than being hard-coded in the db_type() method. For example, it wouldn't make much sense to have a CharMaxlength25Field, shown here:

# This is a silly example of hard-coded parameters.
class CharMaxlength25Field(models.Field):
    def db_type(self, connection):
        return 'char(25)'

# In the model:
class MyModel(models.Model):
    # ...
    my_field = CharMaxlength25Field()

The better way of doing this would be to make the parameter specifiable at run time -- i.e., when the class is instantiated. To do that, just implement django.db.models.Field.__init__(), like so:

# This is a much more flexible example.
class BetterCharField(models.Field):
    def __init__(self, max_length, *args, **kwargs):
        self.max_length = max_length
        super(BetterCharField, self).__init__(*args, **kwargs)

    def db_type(self, connection):
        return 'char(%s)' % self.max_length

# In the model:
class MyModel(models.Model):
    # ...
    my_field = BetterCharField(25)

Finally, if your column requires truly complex SQL setup, return None from db_type(). This will cause Django's SQL creation code to skip over this 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.

Converting database values to Python objects

Field.to_python(self, value)

Converts a value as returned by your database (or a serializer) to a Python object.

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).

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):
    # ...

    def to_python(self, value):
        if isinstance(value, Hand):
            return value

        # 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's the Python object type we want to store in the model's attribute.

Remember: If your custom field needs the to_python() method to be called when it is created, you should be using The SubfieldBase metaclass mentioned earlier. Otherwise to_python() won't be called automatically.

Converting Python objects to query values

Field.get_prep_value(self, value)
New in Django 1.2: This method was factored out of get_db_prep_value()

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 format that has been prepared for use as a parameter in a query.

This conversion should not include any database-specific conversions. If database-specific conversions are required, they should be made in the call to get_db_prep_value().

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)])

Converting query values to database values

Field.get_db_prep_value(self, value, connection, prepared=False)
New in Django 1.2: The connection and prepared arguments were added to support multiple databases.

Some data types (for example, dates) need to be in a specific format before they can be used by a database backend. 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.

The prepared argument describes whether or not the value has already been passed through get_prep_value() conversions. When prepared is False, the default implementation of get_db_prep_value() will call get_prep_value() to do initial data conversions before performing any database-specific processing.

Field.get_db_prep_save(self, value, connection)
New in Django 1.2: The connection argument was added to support multiple databases.

Same as the above, but called when the Field value must be saved to the database. As the default implementation just calls get_db_prep_value(), you shouldn't need to implement this method unless your custom field needs a special conversion when being saved that is not the same as the conversion used for normal query parameters (which is implemented by get_db_prep_value()).

Preprocessing values before saving

Field.pre_save(self, model_instance, add)

This method is called just prior to get_db_prep_save() and should return the value of the appropriate attribute from model_instance for this field. 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.

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 to the value so that code holding references to the model will always see the correct value.

Preparing values for use in database lookups

As with value conversions, preparing a value for database lookups is a two phase process.

Field.get_prep_lookup(self, lookup_type, value)
New in Django 1.2: This method was factored out of get_db_prep_lookup()

get_prep_lookup() performs the first phase of lookup preparation, performing generic data validity checks

Prepares the value for passing to the database when used in a lookup (a WHERE constraint in SQL). The lookup_type will be one of the valid Django filter lookups: exact, iexact, contains, icontains, gt, gte, lt, lte, in, startswith, istartswith, endswith, iendswith, range, year, month, day, isnull, search, regex, and iregex.

Your method must be prepared to handle all of these lookup_type values and should raise either a ValueError if the value is of the wrong sort (a list when you were expecting an object, for example) or a TypeError if your field does not support that type of lookup. For many fields, you can get by with handling the lookup types that need special handling for your field and pass the rest to the get_db_prep_lookup() method of the parent class.

If you needed to implement get_db_prep_save(), you will usually need to implement get_prep_lookup(). If you don't, get_prep_value will be called by the default implementation, to manage exact, gt, gte, lt, lte, in and range lookups.

You may also want to implement this method to limit the lookup types that could be used with your custom field type.

Note that, for range and in lookups, get_prep_lookup will receive a list of objects (presumably of the right type) and will need to convert them to a list of things of the right type for passing to the database. Most of the time, you can reuse get_prep_value(), or at least factor out some common pieces.

For example, the following code implements get_prep_lookup to limit the accepted lookup types to exact and in:

class HandField(models.Field):
    # ...

    def get_prep_lookup(self, lookup_type, value):
        # We only handle 'exact' and 'in'. All others are errors.
        if lookup_type == 'exact':
            return self.get_prep_value(value)
        elif lookup_type == 'in':
            return [self.get_prep_value(v) for v in value]
            raise TypeError('Lookup type %r not supported.' % lookup_type)
Field.get_db_prep_lookup(self, lookup_type, value, connection, prepared=False)
New in Django 1.2: The connection and prepared arguments were added to support multiple databases.

Performs any database-specific data conversions required by a lookup. As with get_db_prep_value(), the specific connection that will be used for the query is passed as the connection parameter. The prepared argument describes whether the value has already been prepared with get_prep_lookup().

Specifying the form field for a model field

Field.formfield(self, form_class=forms.CharField, **kwargs)

Returns the default form field to use when this field is displayed in a model. This method is called by the ModelForm helper.

All of the kwargs dictionary is passed directly to the form field's Field__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, 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:

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}
        return super(HandField, self).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.

Emulating built-in field types


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 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.

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 syncdb and other SQL commands create the right column type for storing a string.

If 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>.creation.DATA_TYPES -- the string will still be used by the serializer, but the default db_type() method will return None. See the documentation of 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 field data for serialization

Field.value_to_string(self, obj)

This method is used by the serializers to convert the field into a string for output. Calling Field._get_val_from_obj(obj)() is the best way to get the value to serialize. 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._get_val_from_obj(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 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.

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 file documentation.

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.