Appendix A: Model Definition R

Chapter 4 explains the basics of defining models, and we use them throughout the rest of the book. There is, however, a huge range of model options available not covered elsewhere. This appendix explains each possible model definition option.

Note that although these APIs are considered stable, the Django developers consistently add new shortcuts and conveniences to the model definition. Its a good idea to always check the latest documentation online at http://docs.djangoproject.com/.

Fields

The most important part of a model and the only required part of a model is the list of database fields it defines.

Field Name Restrictions

Django places only two restrictions on model field names:

1. A field name cannot be a Python reserved word, because that would result in a Python syntax error. For example:

   class Example(models.Model):
       pass = models.IntegerField() # "pass" is a reserved word!
   

2. A field name cannot contain more than one underscore in a row, due to the way Djangos query lookup syntax works. For example:

   class Example(models.Model):
       foo__bar = models.IntegerField() # "foo__bar" has two underscores!
   

Each field in your model should be an instance of the appropriate Field class. Django uses the field class types to determine a few things:

• The database column type (e.g., INTEGER, VARCHAR).
• The widget to use in Djangos forms and admin site, if you care to use it (e.g., <input type="text">,<select>).
• The minimal validation requirements, which are used in Djangos admin interface and by forms.

A complete list of field classes follows, sorted alphabetically. Note that relationship fields (ForeignKey, etc.) are handled in the next section.

Note

Technically, these models are defined in django.db.models.fields, but for convenience theyre imported intodjango.db.models; the standard convention is to use from django.db import models and refer to fields asmodels.<Foo>Field.

AutoField

class AutoField(**options)

An IntegerField that automatically increments according to available IDs. You usually wont need to use this directly; a primary key field will automatically be added to your model if you dont specify otherwise.

BigIntegerField

class BigIntegerField([***options*])

A 64 bit integer, much like an IntegerField except that it is guaranteed to fit numbers from-9223372036854775808 to 9223372036854775807. The default form widget for this field is a TextInput.

BinaryField

class BinaryField([***options*])

A field to store raw binary data. It only supports bytes assignment. Be aware that this field has limited functionality. For example, it is not possible to filter a queryset on a BinaryField value.

Abusing BinaryField

Although you might think about storing files in the database, consider that it is bad design in 99% of the cases. This field is not a replacement for proper static files handling.

BooleanField

class BooleanField(**options)

A true/false field.

The default form widget for this field is a CheckboxInput.

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

The default value of BooleanField is None when Field.default isnt defined.

CharField

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

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

For large amounts of text, use TextField.

The default form widget for this field is a TextInput.

CharField has one extra required argument:

CharField.max_length

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

Note

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.

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 Chapter 23 for details.

CommaSeparatedIntegerField

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.

DateField

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

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

DateField.auto_now

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; its not just a default value that you can override.

DateField.auto_now_add

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; its not just a default value that you can override.

The default form widget for this field is a TextInput. The admin adds a JavaScript calendar, and a shortcut for Today. Includes an additional invalid_date error message key.

The options auto_now_add, auto_now, and default are mutually exclusive. Any combination of these options will result in an error.

Note

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

DateTimeField

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 default form widget for this field is a single TextInput. The admin uses two separate TextInput widgets with JavaScript shortcuts.

DecimalField

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

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

DecimalField.max_digits

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

DecimalField.decimal_places

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, youd 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 default form widget for this field is a TextInput.

Note

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

DurationField

class DurationField([***options*])

A field for storing periods of time – modeled in Python by timedelta. When used on PostgreSQL, the data type used is an interval and on Oracle the data type is INTERVAL DAY(9) TO SECOND(6). Otherwise a bigint of microseconds is used.

Note

Arithmetic with DurationField works in most cases. However on all databases other than PostgreSQL, comparing the value of a DurationField to arithmetic on DateTimeField instances will not work as expected.

EmailField

class EmailField([*max_length=254*, ***options*])

A CharField that checks that the value is a valid email address. It uses EmailValidator to validate the input.

FileField

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

A file-upload field.

Note

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

Has two optional arguments:

FileField.upload_to

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

This path may contain strftime() formatting, which will be replaced by the date/time of the file upload (so that uploaded files dont 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
instance

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. |

FileField.storage

A storage object, which handles the storage and retrieval of your files.

The default form widget for this field is a ClearableFileInput.

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

1. In your settings file, youll need to define MEDIA_ROOT as the full path to a directory where youd 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 servers user account.
2. Add the FileField or ImageField to your model, defining the upload_to option to specify a subdirectory ofMEDIA_ROOT to use for uploaded files.
3. All that will be stored in your database is a path to the file (relative to MEDIA_ROOT). Youll most likely want to use the convenience url attribute 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 files on-disk filename, or the files size, you could use the name andsize attributes respectively; for more information on the available attributes and methods, see the Fileclass reference and the Files topic guide.

Note

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 files 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 youre uploading them and what type of files they are, to avoid security holes. Validate all uploaded files so that youre sure the files are what you think they are. For example, if you blindly let somebody upload files, without validation, to a directory thats within your Web servers document root, then somebody could upload a CGI or PHP script and execute that script by visiting its URL on your site. Dont 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.

FileField instances are created in your database as varchar columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the max_length argument.

FILEFIELD AND FIELDFILE

class django.db.models.fields.files.FieldFile

When you access a FileField on a model, you are given an instance of FieldFile as a proxy for accessing the underlying file. In addition to the functionality inherited from django.core.files.File, this class has several attributes and methods that can be used to interact with file data:

FieldFile.url

A read-only property to access the files relative URL by calling the url() method of the underlying Storageclass.

FieldFile.open(mode=’rb’)

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

FieldFile.close()

Behaves like the standard Python file.close() method and closes the file associated with this instance.

FieldFile.save(name, 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 FileFieldinstances 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 files contents. The optional save argument controls whether or not the model instance is saved after the file associated with this field has been altered. Defaults to True.

Note that the content argument should be an instance of django.core.files.File, not Pythons 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/hello.world")
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")

FieldFile.delete(save=True)

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 model instance is saved after the file associated with this field has been deleted. Defaults to True.

Note that when a model is deleted, related files are not deleted. If you need to cleanup orphaned files, youll need to handle it yourself (for instance, with a custom management command that can be run manually or scheduled to run periodically via e.g. cron).

FilePathField

class django.db.models.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:

FilePathField.path

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

FilePathField.match

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.png.

FilePathField.recursive

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

FilePathField.allow_files

Optional. Either True or False. Default is True. Specifies whether files in the specified location should be included. Either this or allow_folders must be True.

FilePathField.allow_folders

Optional. Either True or False. Default is False. Specifies whether folders in the specified location should be included. Either this or allow_files must be True.

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

FilePathField instances are created in your database as varchar columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the max_length argument.

FloatField

class django.db.models.FloatField([***options*])

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

The default form widget for this field is a TextInput.

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 Pythons float type internally, whileDecimalField uses Pythons Decimal type. For information on the difference between the two, see Pythons documentation for the decimal module.

ImageField

class django.db.models.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 widthattributes.

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

ImageField.height_field

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

ImageField.width_field

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 Pillow library.

ImageField instances are created in your database as varchar columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the max_length argument.

The default form widget for this field is a ClearableFileInput.

IntegerField

class django.db.models.IntegerField([***options*])

An integer. Values from -2147483648 to 2147483647 are safe in all databases supported by Django. The default form widget for this field is a TextInput.

GenericIPAddressField

class django.db.models.GenericIPAddressField([*protocol=both*, *unpack_ipv4=False*, ***options*])

An IPv4 or IPv6 address, in string format (e.g. 192.0.2.30 or 2a02:42fe::4). The default form widget for this field is a TextInput.

The IPv6 address normalization follows RFC 4291#section-2.2 section 2.2, including using the IPv4 format suggested in paragraph 3 of that section, like ::ffff:192.0.2.0. For example, 2001:0::0:01 would be normalized to 2001::1, and ::ffff:0a0a:0a0a to ::ffff:10.10.10.10. All characters are converted to lowercase.

GenericIPAddressField.protocol

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

GenericIPAddressField.unpack_ipv4

Unpacks IPv4 mapped addresses like ::ffff:192.0.2.1. If this option is enabled that address would be unpacked to 192.0.2.1. Default is disabled. Can only be used when protocol is set to "both".

If you allow for blank values, you have to allow for null values since blank values are stored as null.

NullBooleanField

class django.db.models.NullBooleanField([***options*])

Like a BooleanField, but allows NULL as one of the options. Use this instead of a BooleanField with null=True. The default form widget for this field is a NullBooleanSelect.

PositiveIntegerField

class django.db.models.PositiveIntegerField([***options*])

Like an IntegerField, but must be either positive or zero (0). Values from 0 to 2147483647 are safe in all databases supported by Django. The value 0 is accepted for backward compatibility reasons.

PositiveSmallIntegerField

class django.db.models.PositiveSmallIntegerField([***options*])

Like a PositiveIntegerField, but only allows values under a certain (database-dependent) point. Values from 0 to 32767 are safe in all databases supported by Django.

SlugField

class django.db.models.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. Theyre 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.

SmallIntegerField

class django.db.models.SmallIntegerField([***options*])

Like an IntegerField, but only allows values under a certain (database-dependent) point. Values from-32768 to 32767 are safe in all databases supported by Django.

TextField

class django.db.models.TextField([***options*])

A large text field. The default form widget for this field is a Textarea.

If you specify a max_length attribute, it will be reflected in the Textarea widget of the auto-generated form field. However it is not enforced at the model or database level. Use a CharField for that.

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 Chapter 23 for details.

TimeField

class django.db.models.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 asDateField.

The default form widget for this field is a TextInput. The admin adds some JavaScript shortcuts.

URLField

class django.db.models.URLField([*max_length=200*, ***options*])

A CharField for a URL.

The default form widget for this field is a TextInput.

Like all CharField subclasses, URLField takes the optional max_length argument. If you dont specifymax_length, a default of 200 is used.

UUIDField

class django.db.models.UUIDField([***options*])

A field for storing universally unique identifiers. Uses Pythons UUID class. When used on PostgreSQL, this stores in a uuid datatype, otherwise in a char(32).

Universally unique identifiers are a good alternative to AutoField for primary_key. The database will not generate the UUID for you, so it is recommended to use default:

import uuid
from django.db import models

class MyUUIDModel(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    # other fields

Note that a callable (with the parentheses omitted) is passed to default, not an instance of UUID.

Universal Field Options

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

null

Field.null

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

Avoid using null on string-based fields such as CharField and TextField because empty string values will always be stored as empty strings, not as NULL. 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, its redundant to have two possible values for no data; the Django convention is to use the empty string, not NULL.

For both string-based and non-string-based 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).

Note

When using the Oracle database backend, the value NULL will be stored to denote the empty string regardless of this attribute.

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

blank

Field.blank

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, form validation will allow entry of an empty value. If a field has blank=False, the field will be required.

choices

Field.choices

An iterable (e.g., a list or tuple) consisting itself of iterables of exactly two items (e.g. [(A, B), (A, B)...]) to use as choices for this field. If this is given, the default form widget will be a select box with these choices instead of the standard text field.

The first element in each tuple is the actual value to be set on the model, and the second element is the human-readable name. For example:

YEAR_IN_SCHOOL_CHOICES = (
    ("FR", "Freshman"),
    ("SO", "Sophomore"),
    ("JR", "Junior"),
    ("SR", "Senior"),
)

Generally, its best to define choices inside a model class, and to define a suitably-named constant for each value:

from django.db import models

class Student(models.Model):
    FRESHMAN = "FR"
    SOPHOMORE = "SO"
    JUNIOR = "JR"
    SENIOR = "SR"
    YEAR_IN_SCHOOL_CHOICES = (
        (FRESHMAN, "Freshman"),
        (SOPHOMORE, "Sophomore"),
        (JUNIOR, "Junior"),
        (SENIOR, "Senior"),
    )
    year_in_school = models.CharField(max_length=2,
                                      choices=YEAR_IN_SCHOOL_CHOICES,
                                      default=FRESHMAN)

    def is_upperclass(self):
        return self.year_in_school in (self.JUNIOR, self.SENIOR)

Though you can define a choices list outside of a model class and then refer to it, defining the choices and names for each choice inside the model class keeps all of that information with the class that uses it, and makes the choices easy to reference (e.g, Student.SOPHOMORE will work anywhere that the Student model has been imported).

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

MEDIA_CHOICES = (
    ("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 fields current value. See get_FOO_display() in the database API documentation.

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, youre probably better off using a proper database table with a ForeignKey. choices is meant for static data that doesnt change much, if ever.

Unless blank=False is set on the field along with a default then a label containing "---------" will be rendered with the select box. To override this behavior, add a tuple to choices containing None; e.g. (None,"Your String For Display"). Alternatively, you can use an empty string instead of None where this makes sense – such as on a CharField.

db_column

Field.db_column

The name of the database column to use for this field. If this isnt given, Django will use the fields name.

If your database column name is an SQL reserved word, or contains characters that arent allowed in Python variable names notably, the hyphen thats OK. Django quotes column and table names behind the scenes.

db_index

Field.db_index

If True, a database index will be created for this field.

db_tablespace

Field.db_tablespace

The name of the database tablespace to use for this fields index, if this field is indexed. The default is the projects DEFAULT_INDEX_TABLESPACE setting, if set, or the db_tablespace of the model, if any. If the backend doesnt support tablespaces for indexes, this option is ignored.

default

Field.default

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.

The default cannot be a mutable object (model instance, list, set, etc.), as a reference to the same instance of that object would be used as the default value in all new model instances. Instead, wrap the desired default in a callable. For example, if you had a custom JSONField and wanted to specify a dictionary as the default, use a function as follows:

def contact_default():
    return {"email": "to1@example.com"}

contact_info = JSONField("ContactInfo", default=contact_default)

Note that lambdas cannot be used for field options like default because they cannot be serialized by migrations. See the Django documentation for other caveats.

The default value is used when new model instances are created and a value isnt provided for the field. When the field is a primary key, the default is also used when the field is set to None.

editable

Field.editable

If False, the field will not be displayed in the admin or any other ModelForm. They are also skipped during model validation. Default is True.

error_messages

Field.error_messages

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, unique, and unique_for_date. Additional error message keys are specified for each field in the Field types section below.

help_text

Field.help_text

Extra help text to be displayed with the form widget. Its useful for documentation even if your field isnt used on a form.

Note that this value is not HTML-escaped in automatically-generated forms. 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. Ensure that you escape any help text that may come from untrusted users to avoid a cross-site scripting attack.

primary_key

Field.primary_key

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

If you dont specify primary_key=True for any field in your model, Django will automatically add an AutoFieldto hold the primary key, so you dont need to set primary_key=True on any of your fields unless you want to override the default primary-key behavior.

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

The primary key field is read-only. If you change the value of the primary key on an existing object and then save it, a new object will be created alongside the old one.

unique

Field.unique

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

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

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

Note that when unique is True, you dont need to specify db_index, because unique implies the creation of an index.

unique_for_date

Field.unique_for_date

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 wouldnt allow the entry of two records with the same title and pub_date.

Note that if you set this to point to a DateTimeField, only the date portion of the field will be considered. Besides, when USE_TZ is True, the check will be performed in the current time zone at the time the object gets saved.

This is enforced by Model.validate_unique() during model validation but not at the database level. If anyunique_for_date constraint involves fields that are not part of a ModelForm (for example, if one of the fields is listed in exclude or has editable=False), Model.validate_unique() will skip validation for that particular constraint.

unique_for_month

Field.unique_for_month

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

unique_for_year

Field.unique_for_year

Like unique_for_date and unique_for_month.

verbose_name

Field.verbose_name

A human-readable name for the field. If the verbose name isnt given, Django will automatically create it using the fields attribute name, converting underscores to spaces. See Verbose field names.

validators

Field.validators

A list of validators to run for this field.

Field attribute reference

Every Field instance contains several attributes that allow introspecting its behavior. Use these attributes instead of isinstance checks when you need to write code that depends on a fields functionality. These attributes can be used together with the Model._meta API to narrow down a search for specific field types. Custom model fields should implement these flags.

Attributes for fields

Field.auto_created

Boolean flag that indicates if the field was automatically created, such as the OneToOneField used by model inheritance.

Field.concrete

Boolean flag that indicates if the field has a database column associated with it.

Field.hidden

Boolean flag that indicates if a field is used to back another non-hidden fields functionality (e.g. thecontent_type and object_id fields that make up a GenericForeignKey). The hidden flag is used to distinguish what constitutes the public subset of fields on the model from all the fields on the model.

Note

Options.get_fields() excludes hidden fields by default. Pass in include_hidden=True to return hidden fields in the results.

Field.is_relation

Boolean flag that indicates if a field contains references to one or more other models for its functionality (e.g. ForeignKey, ManyToManyField, OneToOneField, etc.).

Field.model

Returns the model on which the field is defined. If a field is defined on a superclass of a model, model will refer to the superclass, not the class of the instance.

Attributes for fields with relations

These attributes are used to query for the cardinality and other details of a relation. These attribute are present on all fields; however, they will only have meaningful values if the field is a relation type (Field.is_relation=True).

Field.many_to_many

Boolean flag that is True if the field has a many-to-many relation; False otherwise. The only field included with Django where this is True is ManyToManyField.

Field.many_to_one

Boolean flag that is True if the field has a many-to-one relation, such as a ForeignKey; False otherwise.

Field.one_to_many

Boolean flag that is True if the field has a one-to-many relation, such as a GenericRelation or the reverse of a ForeignKey; False otherwise.

Field.one_to_one

Boolean flag that is True if the field has a one-to-one relation, such as a OneToOneField; False otherwise.

Field.related_model

Points to the model the field relates to. For example, Author in ForeignKey(Author). If a field has a generic relation (such as a GenericForeignKey or a GenericRelation) then related_model will be None.

Relationships

ForeignKey

class django.db.models.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 usemodels.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:

from django.db import models

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

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

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 calledproduction, youd 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.

A database index is automatically created on the ForeignKey. You can disable this by setting db_index toFalse. You may want to avoid the overhead of an index if you are creating a foreign key for consistency rather than joins, or if you will be creating an alternative index like a partial or multiple column index.

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. Youll always deal with the field names of your model object.

ARGUMENTS

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

ForeignKey.limit_choices_to

Sets a limit to the available choices for this field when this field is rendered using a ModelForm or the admin (by default, all objects in the queryset are available to choose). Either a dictionary, a Q object, or a callable returning a dictionary or Q object can be used.

For example:

staff_member = models.ForeignKey(User, limit_choices_to={"is_staff": True})

causes the corresponding field on the ModelForm to list only Users that have is_staff=True. This may be helpful in the Django admin.

The callable form can be helpful, for instance, when used in conjunction with the Python datetime module to limit selections by date range. For example:

def limit_pub_date_choices():
    return {"pub_date__lte": datetime.date.utcnow()}

limit_choices_to = limit_pub_date_choices

If limit_choices_to is or returns a Q object, which is useful for complex queries , 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 ModelAdminfor the model.

Note

If a callable is used for limit_choices_to, it will be invoked every time a new form is instantiated. It may also be invoked when a model is validated, for example by management commands or the admin. The admin constructs querysets to validate its form inputs in various edge cases multiple times, so there is a possibility your callable may be invoked several times.

ForeignKey.related_name

The name to use for the relation from the related object back to this one. Its also the default value forrelated_query_name (the name to use for the reverse filter name from the target model). 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 youd 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 wont have a backwards relation to this model:

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

ForeignKey.related_query_name

The name to use for the reverse filter name from the target model. Defaults to the value of related_name if it is set, otherwise it defaults to the name of the model:

# Declare the ForeignKey with related_query_name
class Tag(models.Model):
    article = models.ForeignKey(Article, related_name="tags", related_query_name="tag")
    name = models.CharField(max_length=255)

# That"s now the name of the reverse filter
Article.objects.filter(tag__name="important")

ForeignKey.to_field

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

ForeignKey.db_constraint

Controls whether or not a constraint should be created in the database for this foreign key. The default isTrue, and thats almost certainly what you want; setting this to False can be very bad for data integrity. That said, here are some scenarios where you might want to do this:

• You have legacy data that is not valid.
• Youre sharding your database.

If this is set to False, accessing a related object that doesnt exist will raise its DoesNotExist exception.

ForeignKey.on_delete

When an object referenced by a 

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