Chapter 5 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. It's a good idea to always check the latest documentation online at http://docs.djangoproject.com/.
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:
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!
A field name cannot contain more than one underscore in a row, due to the way Django's query lookup syntax works. For example:
class Example(models.Model): foo__bar = models.IntegerField() # 'foo__bar' has two underscores!
These limitations can be worked around, though, because your field name doesn't necessarily have to match your database column name. See "db_column", below.
SQL reserved words, such as join
, where
, or select
, are allowed as model field names, because Django escapes all database table names and column names in every underlying SQL query. It uses the quoting syntax of your particular database engine.
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 Django's forms and admin site, if you care to use it (e.g.,
<input type="text">
,<select>
). - The minimal validation requirements, which are used in Django's 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.
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.
A true/false field.
MySQL users...
A boolean field in MySQL is stored as a TINYINT
column with a value of either 0 or 1 (most databases have a proper BOOLEAN
type instead). So, for MySQL, only, when a BooleanField
is retrieved from the database and stored on a model attribute, it will have the values 1 or 0, rather than True
or False
. Normally, this shouldn't be a problem, since Python guarantees that 1 == True
and 0 == False
are both true. Just be careful if you're writing something like obj is True
when obj
is a value from a boolean attribute on a model. If that model was constructed using the mysql
backend, the "is
" test will fail. Prefer an equality test (using "==
") in cases like this.
A string field, for small- to large-sized strings.
For very large amounts of text, use TextField
.
CharField
has one extra required argument: max_length
. This is the maximum length (in characters) of the field. The max_length
is enforced at the database level and in Django's validation.
A field of integers separated by commas. As in CharField
, the max_length
argument is required.
A date, represented in Python by a datetime.date
instance.
A date and time, represented in Python by a datetime.datetime
instance.
A fixed-precision decimal number, represented in Python by a decimal.Decimal
instance. Has two required arguments:
max_digits
The maximum number of digits allowed in the number
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, 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)
When assigning to a DecimalField
, use either a decimal.Decimal
object or a string -- not a Python float.
A CharField
that checks that the value is a valid e-mail address.
A file-upload field.
Note
The primary_key
and unique
arguments are not supported, and will raise a TypeError
if used.
Has one required argument:
upload_to
A local filesystem path that will be appended to your
MEDIA_ROOT
setting to determine the value of thedjango.core.files.File.url
attribute.This path may contain "strftime formatting" (see the Python docs for the
time
standard library module), which will be replaced using 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 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.
Also has one optional argument:
storage
Optional. A storage object, which handles the storage and retrieval of your files.
Using a FileField
or an ImageField
(see below) in a model takes a few steps:
- 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.) DefineMEDIA_URL
as the base public URL of that directory. Make sure that this directory is writable by the Web server's user account. - Add the
FileField
orImageField
to your model, making sure to define theupload_to
option to tell Django to which subdirectory ofMEDIA_ROOT
it should upload files. - 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 convenienceurl
function provided by Django. For example, if yourImageField
is calledmug_shot
, you can get the absolute URL 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 want to retrieve the upload file's on-disk filename, or a URL that refers to that file, or the file's size, you can use the name
, url
and size
attributes.
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.
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.
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:
path
Required. The absolute filesystem path to a directory from which this
FilePathField
should get its choices. Example:"/home/images"
.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 calledfoo23.txt
but notbar.txt
orfoo23.gif
.recursive
Optional. Either
True
orFalse
. Default isFalse
. Specifies whether all subdirectories ofpath
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/bar/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.
A floating-point number represented in Python by a float
instance.
Like FileField
, but validates that the uploaded object is a valid image. Has two extra optional arguments:
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.
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.
In addition to the special attributes that are available for FileField, an
ImageFieldalso has
heightand
widthattributes, both of which correspond to the image's height and width in pixels. Requires the Python Imaging Library, available at http://www.pythonware.com/products/pil/. By default,
ImageFieldinstances are created as
varchar(100)columns in your database. As with other fields, you can change the maximum length using the
max_lengthargument. IntegerField ------------ An integer. IPAddressField -------------- An IP address, in string format (e.g.
'192.0.2.30'). NullBooleanField ---------------- Like a
BooleanField, but allows
NULLas one of the options. Use this instead of a
BooleanFieldwith
null=True. PositiveIntegerField -------------------- Like an
IntegerField, but must be positive. PositiveSmallIntegerField ------------------------- Like a
PositiveIntegerField, but only allows values under a certain (database-dependent) point. SlugField --------- "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. If
max_lengthis not specified, Django will use a default length of 50. Implies setting
db_indexto
True. SmallIntegerField ----------------- Like an
IntegerField, but only allows values under a certain (database-dependent) point. TextField --------- A large text field. Also see
CharFieldfor storing smaller bits of text. TimeField --------- A time, represented in Python by a
datetime.timeinstance. Accepts the same auto-population options as
DateField. URLField -------- A
CharFieldfor a URL. Has one extra optional argument:
verify_existsIf
True(the default), the URL given will be checked for existence (i.e., the URL actually loads and doesn't give a 404 response). It should be noted that when 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. Like all
CharFieldsubclasses,
URLFieldtakes the optional
max_lengthargument. If you don't specify
max_length, a default of 200 is used. XMLField -------- A
TextFieldthat checks that the value is valid XML that matches a given schema. Takes one required argument:
schema_pathThe filesystem path to a RelaxNG schema against which to validate the field. For more on RelaxNG, see http://www.relaxng.org/. Universal Field Options ======================= The following arguments are available to all field types. All are optional. null ---- If
True, Django will store empty values as
NULLin the database. If
False, saving empty values will likely result in a database error. Default is
False. Note that empty string values will always get stored as empty strings, not as
NULL. Only use
null=Truefor non-string fields such as integers, booleans and dates. For both types of fields, you will also need to set
blank=Trueif you wish to permit empty values in forms, as the
nullparameter only affects database storage (see
blank). Avoid using
nullon string-based fields such as
CharFieldand
TextFieldunless 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's convention is to use the empty string, not
NULL. .. note:: When using the Oracle database backend, the
null=Trueoption will be coerced for string-based fields that have the empty string as a possible value, and the value
NULLwill be stored to denote the empty string. For more on this, see the section "Making Date and Numeric Fields Optional" in Chapter 6. blank ----- If
True, the field is allowed to be blank. Default is
False. Note that this is different than
null.
nullis purely database-related, whereas
blankis 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. choices ------- An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. A choices list looks like this:: YEAR_IN_SCHOOL_CHOICES = ( ('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): GENDER_CHOICES = ( ('M', 'Male'), ('F', 'Female'), ) gender = models.CharField(max_length=1, choices=GENDER_CHOICES) or outside your model class altogether:: GENDER_CHOICES = ( ('M', 'Male'), ('F', 'Female'), ) class Foo(models.Model): gender = models.CharField(max_length=1, choices=GENDER_CHOICES) You can also collect your available choices into named groups that can be used for organizational purposes in a form:: 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). 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
choicesto 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
, django-admin.py 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, 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
.
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 AutoField
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.
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 level of forms created with ModelForm
(including forms in the Django admin site). If you try to save a model with a duplicate value in a unique
field, an IntegrityError
will be raised by the model's save
method.
This option is valid on all field types except ManyToManyField
, FileField
and ImageField
.
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 level of forms created with ModelForm
(including forms in the Django admin site) 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.
Clearly, the power of relational databases lies in relating tables to each other. Django offers ways to define the three most common types of database relationships: many-to-one, many-to-many, and one-to-one.
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):
# ...
Note, however, that this only refers to models in the same models.py
file.
To refer to models defined in another application, you must instead explicitly specify the 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')
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.
limit_choices_to
A dictionary of lookup arguments and values 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': datetime.now}
only allows the choice of related objects with a
pub_date
before the current date/time to be chosen.limit_choices_to
has no effect on the inline FormSets that are created to display related objects in the admin.related_name
The name to use for the relation from the related object back to this one.
to_field
The field on the related object that the relation is to. By default, Django uses the primary key of the related object.
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 relationships and lazy relationships.
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 names of the two tables being joined. 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.
related_name
Same as
related_name
inForeignKey
.limit_choices_to
Same as
limit_choices_to
inForeignKey
.limit_choices_to
has no effect when used on aManyToManyField
with a custom intermediate table specified using thethrough
paramter.symmetrical
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 aperson_set
attribute to thePerson
class. Instead, theManyToManyField
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
, setsymmetrical
toFalse
. This will force Django to add the descriptor for the reverse relationship, allowingManyToManyField
relationships to be non-symmetrical.through
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.
db_table
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.
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 relationships and lazy relationships.
Additionally, OneToOneField
accepts all of the extra arguments accepted by ForeignKey
, plus one extra argument:
parent_link
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 extraOneToOneField
which would normally be implicitly created by subclassing.
Model-specific metadata lives in a class Meta
defined in the body of your model class:
class Book(models.Model):
title = models.CharField(maxlength=100)
class Meta:
# model metadata options go here
...
Model metadata is "anything that's not a field," such as ordering options and so forth.
The sections that follow present a list of all possible Meta
options. No options are required. Adding class Meta
to a model is completely optional.
If True
, this model will be an abstract base class. See the Django documentation for more on abstract base classes.
The name of the database table to use for the model:
db_table = 'music_album'
To save you time, Django automatically derives the name of the database table from the name of your model class and the app that contains it. A model's database table name is constructed by joining the model's "app label" -- the name you used in manage.py startapp
-- to the model's class name, with an underscore between them.
For example, if you have an app bookstore
(as created by manage.py startapp bookstore
), a model defined as class Book
will have a database table named bookstore_book
.
To override the database table name, use the db_table
parameter in class Meta
.
If your database table 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.
The name of the database tablespace to use for the model. If the backend doesn't support tablespaces, this option is ignored.
The name of a DateField
or DateTimeField
in the model. This specifies the default field to use in your model Manager
's latest
method.
Example:
get_latest_by = "order_date"
Defaults to True
, meaning Django will create the appropriate database tables in django-admin.py syncdb
and remove them as part of a reset
management command. That is, Django manages the database tables' lifecycles.
If False
, no database table creation or deletion operations will be performed for this model. This is useful if the model represents an existing table or a database view that has been created by some other means. This is the only difference when managed
is False
. All other aspects of model handling are exactly the same as normal. This includes
- Adding an automatic primary key field to the model if you don't declare it. To avoid confusion for later code readers, it's recommended to specify all the columns from the database table you are modeling when using unmanaged models.
If a model with
managed=False
contains aManyToManyField
that points to another unmanaged model, then the intermediary table for the many-to-many join will also not be created. However, the intermediary table between one managed and one unmanaged model will be created.If you need to change this default behavior, create the intermediary table as an explicit model (with
managed
set as needed) and use thethrough
attribute to make the relation use your custom model.
For tests involving models with managed=False
, it's up to you to ensure the correct tables are created as part of the test setup.
If you're interested in changing the Python-level behavior of a model class, you could use managed=False
and create a copy of an existing model. However, there's a better approach for that situation: proxy-models.
The default ordering for the object, for use when obtaining lists of objects:
ordering = ['-order_date']
This is a tuple or list of strings. Each string is a field name with an optional "-" prefix, which indicates descending order. Fields without a leading "-" will be ordered ascending. Use the string "?" to order randomly.
Note
Regardless of how many fields are in ordering
, the admin site uses only the first field.
For example, to order by a pub_date
field ascending, use this:
ordering = ['pub_date']
To order by pub_date
descending, use this:
ordering = ['-pub_date']
To order by pub_date
descending, then by author
ascending, use this:
ordering = ['-pub_date', 'author']
If set to True
, a model which subclasses another model will be treated as a proxy model. For more on proxy models, see the Django documentation.
Sets of field names that, taken together, must be unique:
unique_together = (("driver", "restaurant"),)
This is a list of lists of fields that must be unique when considered together. It's used by ModelForm
forms (including forms in the Django admin site) and is enforced at the database level (i.e., the appropriate UNIQUE
statements are included in the CREATE TABLE
statement).
For convenience, unique_together can be a single sequence when dealing with a single set of fields:
unique_together = ("driver", "restaurant")
A human-readable name for the object, singular:
verbose_name = "pizza"
If this isn't given, Django will use a munged version of the class name: CamelCase
becomes camel case
.
The plural name for the object:
verbose_name_plural = "stories"
If this isn't given, Django will use verbose_name
+ "s"
.