Model field reference

This document contains all the gory details about all the field options and field types Django’s got to offer.

See also

If the built-in fields don’t do the trick, you can try django.contrib.localflavor, which contains assorted pieces of code that are useful for particular countries or cultures. Also, you can easily write your own custom model fields.


Technically, these models are defined in django.db.models.fields, but for convenience they’re imported into django.db.models; the standard convention is to use from django.db import models and refer to fields as models.<Foo>Field.

Field options

The following arguments are available to all field types. All are optional.



If True, Django will store empty values as NULL in the database. Default is False.

Note that empty string values will always get stored as empty strings, not as NULL. Only use null=True for non-string fields such as integers, booleans and dates. For both types of fields, you will also need to set blank=True if you wish to permit empty values in forms, as the null parameter only affects database storage (see blank).

Avoid using null on string-based fields such as CharField and TextField unless you have an excellent reason. If a string-based field has null=True, that means it has two possible values for “no data”: NULL, and the empty string. In most cases, it’s redundant to have two possible values for “no data;” Django convention is to use the empty string, not NULL.


When using the Oracle database backend, the null=True option will be coerced for string-based fields that have the empty string as a possible value, and the value NULL will be stored to denote the empty string.

If you want to accept null values with BooleanField, use NullBooleanField instead.



If True, the field is allowed to be blank. Default is False.

Note that this is different than null. null is purely database-related, whereas blank is validation-related. If a field has blank=True, validation on Django’s admin site will allow entry of an empty value. If a field has blank=False, the field will be required.



An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field.

If this is given, Django’s admin will use a select box instead of the standard text field and will limit choices to the choices given.

A choices list looks like this:

    ('FR', 'Freshman'),
    ('SO', 'Sophomore'),
    ('JR', 'Junior'),
    ('SR', 'Senior'),
    ('GR', 'Graduate'),

The first element in each tuple is the actual value to be stored. The second element is the human-readable name for the option.

The choices list can be defined either as part of your model class:

class Foo(models.Model):
        ('FR', 'Freshman'),
        ('SO', 'Sophomore'),
        ('JR', 'Junior'),
        ('SR', 'Senior'),
        ('GR', 'Graduate'),
    year_in_school = models.CharField(max_length=2, choices=YEAR_IN_SCHOOL_CHOICES)

or outside your model class altogether:

    ('FR', 'Freshman'),
    ('SO', 'Sophomore'),
    ('JR', 'Junior'),
    ('SR', 'Senior'),
    ('GR', 'Graduate'),
class Foo(models.Model):
    year_in_school = models.CharField(max_length=2, choices=YEAR_IN_SCHOOL_CHOICES)

You can also collect your available choices into named groups that can be used for organizational purposes:

    ('Audio', (
            ('vinyl', 'Vinyl'),
            ('cd', 'CD'),
    ('Video', (
            ('vhs', 'VHS Tape'),
            ('dvd', 'DVD'),
    ('unknown', 'Unknown'),

The first element in each tuple is the name to apply to the group. The second element is an iterable of 2-tuples, with each 2-tuple containing a value and a human-readable name for an option. Grouped options may be combined with ungrouped options within a single list (such as the unknown option in this example).

For each model field that has choices set, Django will add a method to retrieve the human-readable name for the field's current value. See get_FOO_display() in the database API documentation.

Finally, note that choices can be any iterable object -- not necessarily a list or tuple. This lets you construct choices dynamically. But if you find yourself hacking choices to be dynamic, you're probably better off using a proper database table with a ForeignKey. choices is meant for static data that doesn't change much, if ever.



The name of the database column to use for this field. If this isn't given, Django will use the field's name.

If your database column name is an SQL reserved word, or contains characters that aren't allowed in Python variable names -- notably, the hyphen -- that's OK. Django quotes column and table names behind the scenes.



If True, sqlindexes <sqlindexes> will output a CREATE INDEX statement for this field.



The name of the database tablespace to use for this field's index, if this field is indexed. The default is the project's DEFAULT_INDEX_TABLESPACE setting, if set, or the db_tablespace of the model, if any. If the backend doesn't support tablespaces for indexes, this option is ignored.



The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.



If False, the field will not be editable in the admin or via forms automatically generated from the model class. Default is True.


New in Django 1.2: Please see the release notes

The error_messages argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override.

Error message keys include null, blank, invalid, invalid_choice, and unique. Additional error message keys are specified for each field in the Field types section below.



Extra "help" text to be displayed under the field on the object's admin form. It's useful for documentation even if your object doesn't have an admin form.

Note that this value is not HTML-escaped when it's displayed in the admin interface. This lets you include HTML in help_text if you so desire. For example:

help_text="Please use the following format: <em>YYYY-MM-DD</em>."

Alternatively you can use plain text and django.utils.html.escape() to escape any HTML special characters.



If True, this field is the primary key for the model.

If you don't specify primary_key=True for any fields in your model, Django will automatically add an IntegerField to hold the primary key, so you don't need to set primary_key=True on any of your fields unless you want to override the default primary-key behavior. For more, see Automatic primary key fields.

primary_key=True implies null=False and unique=True. Only one primary key is allowed on an object.



If True, this field must be unique throughout the table.

This is enforced at the database level and at the Django admin-form level. If you try to save a model with a duplicate value in a unique field, a django.db.IntegrityError will be raised by the model's save() method.

This option is valid on all field types except ManyToManyField and FileField.



Set this to the name of a DateField or DateTimeField to require that this field be unique for the value of the date field.

For example, if you have a field title that has unique_for_date="pub_date", then Django wouldn't allow the entry of two records with the same title and pub_date.

This is enforced at the Django admin-form level but not at the database level.



Like unique_for_date, but requires the field to be unique with respect to the month.



Like unique_for_date and unique_for_month.



A human-readable name for the field. If the verbose name isn't given, Django will automatically create it using the field's attribute name, converting underscores to spaces. See Verbose field names.


New in Django 1.2: Please see the release notes

A list of validators to run for this field. See the validators documentation for more information.

Field types


class AutoField(**options)

An IntegerField that automatically increments according to available IDs. You usually won't need to use this directly; a primary key field will automatically be added to your model if you don't specify otherwise. See Automatic primary key fields.


New in Django 1.2: Please see the release notes
class BigIntegerField([**options])

A 64 bit integer, much like an IntegerField except that it is guaranteed to fit numbers from -9223372036854775808 to 9223372036854775807. The admin represents this as an <input type="text"> (a single-line input).


class BooleanField(**options)

A true/false field.

The admin represents this as a checkbox.

If you need to accept null values then use NullBooleanField instead.

Changed in Django 1.2: In previous versions of Django when running under MySQL BooleanFields would return their data as ints, instead of true bools. See the release notes for a complete description of the change.


class CharField(max_length=None[, **options])

A string field, for small- to large-sized strings.

For large amounts of text, use TextField.

The admin represents this as an <input type="text"> (a single-line input).

CharField has one extra required argument:


The maximum length (in characters) of the field. The max_length is enforced at the database level and in Django's validation.


If you are writing an application that must be portable to multiple database backends, you should be aware that there are restrictions on max_length for some backends. Refer to the database backend notes for details.

MySQL users

If you are using this field with MySQLdb 1.2.2 and the utf8_bin collation (which is not the default), there are some issues to be aware of. Refer to the MySQL database notes for details.


class CommaSeparatedIntegerField(max_length=None[, **options])

A field of integers separated by commas. As in CharField, the max_length argument is required and the note about database portability mentioned there should be heeded.


class DateField([auto_now=False, auto_now_add=False, **options])

A date, represented in Python by a instance. Has a few extra, optional arguments:


Automatically set the field to now every time the object is saved. Useful for "last-modified" timestamps. Note that the current date is always used; it's not just a default value that you can override.


Automatically set the field to now when the object is first created. Useful for creation of timestamps. Note that the current date is always used; it's not just a default value that you can override.

The admin represents this as an <input type="text"> with a JavaScript calendar, and a shortcut for "Today". Includes an additional invalid_date error message key.


As currently implemented, setting auto_now or auto_now_add to True will cause the field to have editable=False and blank=True set.


class DateTimeField([auto_now=False, auto_now_add=False, **options])

A date and time, represented in Python by a datetime.datetime instance. Takes the same extra arguments as DateField.

The admin represents this as two <input type="text"> fields, with JavaScript shortcuts.


class DecimalField(max_digits=None, decimal_places=None[, **options])

A fixed-precision decimal number, represented in Python by a Decimal instance. Has two required arguments:


The maximum number of digits allowed in the number. Note that this number must be greater than or equal to decimal_places, if it exists.


The number of decimal places to store with the number.

For example, to store numbers up to 999 with a resolution of 2 decimal places, you'd use:

models.DecimalField(..., max_digits=5, decimal_places=2)

And to store numbers up to approximately one billion with a resolution of 10 decimal places:

models.DecimalField(..., max_digits=19, decimal_places=10)

The admin represents this as an <input type="text"> (a single-line input).


For more information about the differences between the FloatField and DecimalField classes, please see FloatField vs. DecimalField.


class EmailField([max_length=75, **options])

A CharField that checks that the value is a valid email address.


class FileField(upload_to=None[, max_length=100, **options])

A file-upload field.


The primary_key and unique arguments are not supported, and will raise a TypeError if used.

Has one required argument:


A local filesystem path that will be appended to your MEDIA_ROOT setting to determine the value of the url attribute.

This path may contain strftime() formatting, which will be replaced by the date/time of the file upload (so that uploaded files don't fill up the given directory).

This may also be a callable, such as a function, which will be called to obtain the upload path, including the filename. This callable must be able to accept two arguments, and return a Unix-style path (with forward slashes) to be passed along to the storage system. The two arguments that will be passed are:

Argument Description

An instance of the model where the FileField is defined. More specifically, this is the particular instance where the current file is being attached.

In most cases, this object will not have been saved to the database yet, so if it uses the default AutoField, it might not yet have a value for its primary key field.

filename The filename that was originally given to the file. This may or may not be taken into account when determining the final destination path.

Also has one optional argument:

Optional. A storage object, which handles the storage and retrieval of your files. See Managing files for details on how to provide this object.

The admin represents this field as an <input type="file"> (a file-upload widget).

Using a FileField or an ImageField (see below) in a model takes a few steps:

  1. In your settings file, you'll need to define MEDIA_ROOT as the full path to a directory where you'd like Django to store uploaded files. (For performance, these files are not stored in the database.) Define MEDIA_URL as the base public URL of that directory. Make sure that this directory is writable by the Web server's user account.
  2. Add the FileField or ImageField to your model, making sure to define the upload_to option to tell Django to which subdirectory of MEDIA_ROOT it should upload files.
  3. All that will be stored in your database is a path to the file (relative to MEDIA_ROOT). You'll most likely want to use the convenience url function provided by Django. For example, if your ImageField is called mug_shot, you can get the absolute path to your image in a template with {{ object.mug_shot.url }}.

For example, say your MEDIA_ROOT is set to '/home/media', and upload_to is set to 'photos/%Y/%m/%d'. The '%Y/%m/%d' part of upload_to is strftime() formatting; '%Y' is the four-digit year, '%m' is the two-digit month and '%d' is the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in the directory /home/media/photos/2007/01/15.

If you wanted to retrieve the uploaded file's on-disk filename, or the file's size, you could use the name and size attributes respectively; for more information on the available attributes and methods, see the File class reference and the Managing files topic guide.


The file is saved as part of saving the model in the database, so the actual file name used on disk cannot be relied on until after the model has been saved.

The uploaded file's relative URL can be obtained using the url attribute. Internally, this calls the url() method of the underlying Storage class.

Note that whenever you deal with uploaded files, you should pay close attention to where you're uploading them and what type of files they are, to avoid security holes. Validate all uploaded files so that you're sure the files are what you think they are. For example, if you blindly let somebody upload files, without validation, to a directory that's within your Web server's document root, then somebody could upload a CGI or PHP script and execute that script by visiting its URL on your site. Don't allow that.

Also note that even an uploaded HTML file, since it can be executed by the browser (though not by the server), can pose security threats that are equivalent to XSS or CSRF attacks.

By default, FileField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.

FileField and FieldFile

When you access a FileField on a model, you are given an instance of FieldFile as a proxy for accessing the underlying file. This class has several methods that can be used to interact with file data:'rb')

Behaves like the standard Python open() method and opens the file associated with this instance in the mode specified by mode.


Behaves like the standard Python file.close() method and closes the file associated with this instance., content, save=True)

This method takes a filename and file contents and passes them to the storage class for the field, then associates the stored file with the model field. If you want to manually associate file data with FileField instances on your model, the save() method is used to persist that file data.

Takes two required arguments: name which is the name of the file, and content which is an object containing the file's contents. The optional save argument controls whether or not the instance is saved after the file has been altered. Defaults to True.

Note that the content argument should be an instance of django.core.files.File, not Python's built-in file object. You can construct a File from an existing Python file object like this:

from django.core.files import File
# Open an existing file using Python's built-in open()
f = open('/tmp/')
myfile = File(f)

Or you can construct one from a Python string like this:

from django.core.files.base import ContentFile
myfile = ContentFile("hello world")

For more information, see Managing files.


Deletes the file associated with this instance and clears all attributes on the field. Note: This method will close the file if it happens to be open when delete() is called.

The optional save argument controls whether or not the instance is saved after the file has been deleted. Defaults to True.


class FilePathField(path=None[, match=None, recursive=False, max_length=100, **options])

A CharField whose choices are limited to the filenames in a certain directory on the filesystem. Has three special arguments, of which the first is required:


Required. The absolute filesystem path to a directory from which this FilePathField should get its choices. Example: "/home/images".


Optional. A regular expression, as a string, that FilePathField will use to filter filenames. Note that the regex will be applied to the base filename, not the full path. Example: "foo.*\.txt$", which will match a file called foo23.txt but not bar.txt or foo23.gif.


Optional. Either True or False. Default is False. Specifies whether all subdirectories of path should be included

Of course, these arguments can be used together.

The one potential gotcha is that match applies to the base filename, not the full path. So, this example:

FilePathField(path="/home/images", match="foo.*", recursive=True)

...will match /home/images/foo.gif but not /home/images/foo/bar.gif because the match applies to the base filename (foo.gif and bar.gif).

By default, FilePathField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.


class FloatField([**options])

A floating-point number represented in Python by a float instance.

The admin represents this as an <input type="text"> (a single-line input).

FloatField vs. DecimalField

The FloatField class is sometimes mixed up with the DecimalField class. Although they both represent real numbers, they represent those numbers differently. FloatField uses Python's float type internally, while DecimalField uses Python's Decimal type. For information on the difference between the two, see Python's documentation for the decimal module.


class ImageField(upload_to=None[, height_field=None, width_field=None, max_length=100, **options])

Inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.

In addition to the special attributes that are available for FileField, an ImageField also has height and width attributes.

To facilitate querying on those attributes, ImageField has two extra optional arguments:


Name of a model field which will be auto-populated with the height of the image each time the model instance is saved.


Name of a model field which will be auto-populated with the width of the image each time the model instance is saved.

Requires the Python Imaging Library.

By default, ImageField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.


class IntegerField([**options])

An integer. The admin represents this as an <input type="text"> (a single-line input).


class IPAddressField([**options])

An IP address, in string format (e.g. ""). The admin represents this as an <input type="text"> (a single-line input).


class GenericIPAddressField([protocol=both, unpack_ipv4=False, **options])
New in Django 1.4: Please see the release notes

An IPv4 or IPv6 address, in string format (e.g. or 2a02:42fe::4). The admin represents this as an <input type="text"> (a single-line input).

The IPv6 address normalization follows RFC 4291 section 2.2, including using the IPv4 format suggested in paragraph 3 of that section, like ::ffff: For example, 2001:0::0:01 would be normalized to 2001::1, and ::ffff:0a0a:0a0a to ::ffff: All characters are converted to lowercase.


Limits valid inputs to the specified protocol. Accepted values are 'both' (default), 'IPv4' or 'IPv6'. Matching is case insensitive.


Unpacks IPv4 mapped addresses like ::ffff:: If this option is enabled that address would be unpacked to Default is disabled. Can only be used when protocol is set to 'both'.


class NullBooleanField([**options])

Like a BooleanField, but allows NULL as one of the options. Use this instead of a BooleanField with null=True. The admin represents this as a <select> box with "Unknown", "Yes" and "No" choices.


class PositiveIntegerField([**options])

Like an IntegerField, but must be either positive or zero (0). The value 0 is accepted for backward compatibility reasons.


class PositiveSmallIntegerField([**options])

Like a PositiveIntegerField, but only allows values under a certain (database-dependent) point.


class SlugField([max_length=50, **options])

Slug is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They're generally used in URLs.

Like a CharField, you can specify max_length (read the note about database portability and max_length in that section, too). If max_length is not specified, Django will use a default length of 50.

Implies setting Field.db_index to True.

It is often useful to automatically prepopulate a SlugField based on the value of some other value. You can do this automatically in the admin using prepopulated_fields.


class SmallIntegerField([**options])

Like an IntegerField, but only allows values under a certain (database-dependent) point.


class TextField([**options])

A large text field. The admin represents this as a <textarea> (a multi-line input).

MySQL users

If you are using this field with MySQLdb 1.2.1p2 and the utf8_bin collation (which is not the default), there are some issues to be aware of. Refer to the MySQL database notes for details.


class TimeField([auto_now=False, auto_now_add=False, **options])

A time, represented in Python by a datetime.time instance. Accepts the same auto-population options as DateField.

The admin represents this as an <input type="text"> with some JavaScript shortcuts.


class URLField([verify_exists=False, max_length=200, **options])

A CharField for a URL. Has one extra optional argument:

Deprecated in Django 1.4: verify_exists is deprecated for security reasons as of 1.4 and will be removed in Django 1.5. Prior to 1.3.1, the default value was True.

If True, the URL given will be checked for existence (i.e., the URL actually loads and doesn't give a 404 response) using a HEAD request. Redirects are allowed, but will not be followed.

Note that when you're using the single-threaded development server, validating a URL being served by the same server will hang. This should not be a problem for multithreaded servers.

The admin represents this as an <input type="text"> (a single-line input).

Like all CharField subclasses, URLField takes the optional max_length, a default of 200 is used.

Relationship fields

Django also defines a set of fields that represent relations.


class ForeignKey(othermodel[, **options])

A many-to-one relationship. Requires a positional argument: the class to which the model is related.

To create a recursive relationship -- an object that has a many-to-one relationship with itself -- use models.ForeignKey('self').

If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:

class Car(models.Model):
    manufacturer = models.ForeignKey('Manufacturer')
    # ...

class Manufacturer(models.Model):
    # ...

To refer to models defined in another application, you can explicitly specify a model with the full application label. For example, if the Manufacturer model above is defined in another application called production, you'd need to use:

class Car(models.Model):
    manufacturer = models.ForeignKey('production.Manufacturer')

This sort of reference can be useful when resolving circular import dependencies between two applications.

Database Representation

Behind the scenes, Django appends "_id" to the field name to create its database column name. In the above example, the database table for the Car model will have a manufacturer_id column. (You can change this explicitly by specifying db_column) However, your code should never have to deal with the database column name, unless you write custom SQL. You'll always deal with the field names of your model object.


ForeignKey accepts an extra set of arguments -- all optional -- that define the details of how the relation works.


A dictionary of lookup arguments and values (see Making queries) that limit the available admin choices for this object. Use this with functions from the Python datetime module to limit choices of objects by date. For example:

limit_choices_to = {'pub_date__lte':}

only allows the choice of related objects with a pub_date before the current date/time to be chosen.

Instead of a dictionary this can also be a Q object for more complex queries. However, if limit_choices_to is a Q object then it will only have an effect on the choices available in the admin when the field is not listed in raw_id_fields in the ModelAdmin for the model.


The name to use for the relation from the related object back to this one. See the related objects documentation for a full explanation and example. Note that you must set this value when defining relations on abstract models; and when you do so some special syntax is available.

If you'd prefer Django not to create a backwards relation, set related_name to '+' or end it with '+'. For example, this will ensure that the User model won't have a backwards relation to this model:

user = models.ForeignKey(User, related_name='+')

The field on the related object that the relation is to. By default, Django uses the primary key of the related object.

New in Django 1.3: Please see the release notes

When an object referenced by a ForeignKey is deleted, Django by default emulates the behavior of the SQL constraint ON DELETE CASCADE and also deletes the object containing the ForeignKey. This behavior can be overridden by specifying the on_delete argument. For example, if you have a nullable ForeignKey and you want it to be set null when the referenced object is deleted:

user = models.ForeignKey(User, blank=True, null=True, on_delete=models.SET_NULL)

The possible values for on_delete are found in django.db.models:

  • CASCADE: Cascade deletes; the default.

  • PROTECT: Prevent deletion of the referenced object by raising django.db.models.ProtectedError, a subclass of django.db.IntegrityError.

  • SET_NULL: Set the ForeignKey null; this is only possible if null is True.

  • SET_DEFAULT: Set the ForeignKey to its default value; a default for the ForeignKey must be set.

  • SET(): Set the ForeignKey to the value passed to SET(), or if a callable is passed in, the result of calling it. In most cases, passing a callable will be necessary to avoid executing queries at the time your is imported:

    def get_sentinel_user():
        return User.objects.get_or_create(username='deleted')[0]
    class MyModel(models.Model):
        user = models.ForeignKey(User, on_delete=models.SET(get_sentinel_user))
  • DO_NOTHING: Take no action. If your database backend enforces referential integrity, this will cause an IntegrityError unless you manually add a SQL ON DELETE constraint to the database field (perhaps using initial sql).


class ManyToManyField(othermodel[, **options])

A many-to-many relationship. Requires a positional argument: the class to which the model is related. This works exactly the same as it does for ForeignKey, including all the options regarding recursive and lazy relationships.

Database Representation

Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship. By default, this table name is generated using the name of the many-to-many field and the model that contains it. Since some databases don't support table names above a certain length, these table names will be automatically truncated to 64 characters and a uniqueness hash will be used. This means you might see table names like author_books_9cdf4; this is perfectly normal. You can manually provide the name of the join table using the db_table option.


ManyToManyField accepts an extra set of arguments -- all optional -- that control how the relationship functions.


Same as ForeignKey.related_name.

If you have more than one ManyToManyField pointing to the same model and want to suppress the backwards relations, set each related_name to a unique value ending with '+':

users = models.ManyToManyField(User, related_name='u+')
referents = models.ManyToManyField(User, related_name='ref+')

Same as ForeignKey.limit_choices_to.

limit_choices_to has no effect when used on a ManyToManyField with a custom intermediate table specified using the through parameter.


Only used in the definition of ManyToManyFields on self. Consider the following model:

class Person(models.Model):
    friends = models.ManyToManyField("self")

When Django processes this model, it identifies that it has a ManyToManyField on itself, and as a result, it doesn't add a person_set attribute to the Person class. Instead, the ManyToManyField is assumed to be symmetrical -- that is, if I am your friend, then you are my friend.

If you do not want symmetry in many-to-many relationships with self, set symmetrical to False. This will force Django to add the descriptor for the reverse relationship, allowing ManyToManyField relationships to be non-symmetrical.


Django will automatically generate a table to manage many-to-many relationships. However, if you want to manually specify the intermediary table, you can use the through option to specify the Django model that represents the intermediate table that you want to use.

The most common use for this option is when you want to associate extra data with a many-to-many relationship.


The name of the table to create for storing the many-to-many data. If this is not provided, Django will assume a default name based upon the names of the two tables being joined.


class OneToOneField(othermodel[, parent_link=False, **options])

A one-to-one relationship. Conceptually, this is similar to a ForeignKey with unique=True, but the "reverse" side of the relation will directly return a single object.

This is most useful as the primary key of a model which "extends" another model in some way; Multi-table inheritance is implemented by adding an implicit one-to-one relation from the child model to the parent model, for example.

One positional argument is required: the class to which the model will be related. This works exactly the same as it does for ForeignKey, including all the options regarding recursive and lazy relationships.

Additionally, OneToOneField accepts all of the extra arguments accepted by ForeignKey, plus one extra argument:

When True and used in a model which inherits from another (concrete) model, indicates that this field should be used as the link back to the parent class, rather than the extra OneToOneField which would normally be implicitly created by subclassing.