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 very 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://www.djangoproject.com/documentation/0.96/model-api/.
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:
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 Djangos 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 doesnt necessarily have to match
your database column name. See db_column, below. 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 widget to use in Djangos admin interface, if you care to use it (e.g., <input type="text"> ,
<select> ).
A complete list of field classes follows, sorted alphabetically. Note that relationship fields
(ForeignKey , etc.) are handled in the next section.
AutoField
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.
BooleanField
A true/false field.
CharField
A string field, for small- to large-sized strings. For large amounts of text, use TextField .
CharField has an extra required argument, maxlength , which is the maximum length (in characters) of
the field. This maximum length is enforced at the database level and in Djangos validation.
CommaSeparatedIntegerField
A field of integers separated by commas. As in CharField , the maxlength argument is required.
DateField
A date field. DateField has a few extra optional arguments, as shown in Table B-1.
Table B-1. Extra DateField Options
Argument |
Description |
auto_now |
Automatically sets the field to now every time the object is saved. Its useful for
last-modified timestamps. Note that the current date is always used; its not just a
default value that you can override. |
auto_now_add |
Automatically sets the field to now when the object is first created. Its useful for
creation of timestamps. Note that the current date is always used; its not just a
default value that you can override. |
DateTimeField
A date and time field. It takes the same extra options as DateField .
EmailField
A CharField that checks that the value is a valid email address. This doesnt accept maxlength ; its
maxlength is automatically set to 75.
FileField
A file-upload field. It has one required argument, as shown in Table B-3.
Table B-2. Extra FileField Option
Argument |
Description |
upload_to |
A local filesystem path that will be appended to your MEDIA_ROOT setting to determine
the output of the get_<fieldname>_url() helper function |
This path may contain strftime formatting (see http://www.djangoproject.com/r/python/strftime/),
which will be replaced by the date/time of the file upload (so that uploaded files dont fill up the given
directory).
Using a FileField or an ImageField in a model takes a few steps:
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.
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.
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 get_<fieldname>_url function provided by Django. For example, if
your ImageField is called mug_shot , you can get the absolute URL to your image in a template
with {{ object.get_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
January 15, 2007, it will be saved in the directory /home/media/photos/2007/01/15 .
If you want to retrieve the upload files on-disk file name, or a URL that refers to that file, or the files
size, you can use the get_FIELD_filename() , get_FIELD_url() , and get_FIELD_size() methods. See
Appendix C for a complete explanation of these methods.
Note
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 let that happen!
FilePathField
A field whose choices are limited to the file names in a certain directory on the filesystem. It has three
special arguments, as shown in Table B-4.
Table B-3. Extra FilePathField Options
Argument |
Description |
path |
Required ; the absolute filesystem path to a directory from which this FilePathField
should get its choices (e.g., "/home/images" ). |
match |
Optional; a regular expression, as a string, that FilePathField will use to filter file
names. Note that the regex will be applied to the base file name, not the full path (e.g.,
"foo.*\.txt^" , which will match a file called foo23.txt , but not bar.txt or
foo23.gif ). |
recursive |
Optional; either True or False . The default is False . It 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 file name, 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 file name (foo.gif and bar.gif ).
FloatField
A floating-pint number, represented in Python by a float instance. It has two required arguments, as
shown in Table B-2.
Table B-4. Extra FloatField Options
Argument |
Description |
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 two decimal places, youd use the following:
models.FloatField(..., max_digits=5, decimal_places=2)
And to store numbers up to approximately 1 billion with a resolution of ten decimal places, you would use
this:
models.FloatField(..., max_digits=19, decimal_places=10)
ImageField
Like FileField , but validates that the uploaded object is a valid image. It has two extra optional
arguments, height_field and width_field , which, if set, will be autopopulated with the height and
width of the image each time a model instance is saved.
In addition to the special get_FIELD_* methods that are available for FileField , an ImageField
also has get_FIELD_height() and get_FIELD_width() methods. These are documented in Appendix C.
ImageField requires the Python Imaging Library (http://www.pythonware.com/products/pil/).
IPAddressField
An IP address, in string format (e.g., "24.124.1.30" ).
NullBooleanField
Like a BooleanField , but allows None /NULL as one of the options. Use this instead of a
BooleanField with null=True .
PhoneNumberField
A CharField that checks that the value is a valid U.S.-style phone number (in the format
XXX-XXX-XXXX ).
Note
If you need to represent a phone number from another country, check the django.contrib.localflavor
package to see if field definitions for your country are included.
PositiveIntegerField
Like an IntegerField , but must be positive.
PositiveSmallIntegerField
Like a PositiveIntegerField , but only allows values under a certain point. The maximum value allowed by
these fields is database dependent, but since databases have a 2-byte small integer field, the maximum
positive small integer is usually 65,535.
SlugField
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 maxlength . If maxlength is not specified, Django will use a
default length of 50.
A SlugField implies db_index=True since slugs are primarily used for database lookups.
SlugField accepts an extra option, prepopulate_from , which is a list of fields from which to
autopopulate the slug, via JavaScript, in the objects admin form:
models.SlugField(prepopulate_fpom=("pre_name", "name"))
prepopulate_from doesnt accept DateTimeField names as arguments.
SmallIntegerField
Like an IntegerField , but only allows values in a certain database-dependent range (usually -32,768 to
+32,767).
TextField
An unlimited-length text field.
TimeField
A time of day. It accepts the same autopopulation options as DateField and DateTimeField .
URLField
A field for a URL. If the verify_exists option is True (the default), the URL given will be checked
for existence (i.e., the URL actually loads and doesnt give a 404 response).
Like other character fields, URLField takes the maxlength argument. If you dont specify
maxlength , a default of 200 is used.
USStateField
A two-letter U.S. state abbreviation.
Note
If you need to represent other countries or states, look first in the django.contrib.localflavor package
to see if Django already includes fields for your locale.
Universal Field Options
The following arguments are available to all field types. All are optional.
null
If True , Django will store empty values as NULL in the database. The default is False .
Note that empty string values will always get stored as empty strings, not as NULL . Only use
null=True for nonstring 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 the following section, titled 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, its redundant to have two possible values for no data;
Djangos convention is to use the empty string, not NULL .
blank
If True , the field is allowed to be blank. The default is False .
Note that this is different from null . null is purely database related, whereas blank is
validation related. If a field has blank=True , validation on Djangos 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, tuple, or other iterable Python object) of two tuples to use as choices for this
field.
If this is given, Djangos admin interface 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:
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(maxlength=1, choices=GENDER_CHOICES)
or outside your model class altogether:
GENDER_CHOICES = (
('M', 'Male'),
('F', 'Female'),
)
class Foo(models.Model):
gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)
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 Appendix C for more details.
db_column
The name of the database column to use for this field. If this isnt given, Django will use the fields name.
This is useful when youre defining a model around a database that already exists.
If your database column name is an SQL reserved word, or if it 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
If True , Django will create a database index on this column when creating the table (i.e., when running
manage.py syncdb ). ta default -
The default value for the field.
editable
If False , the field will not be editable in the admin interface or via form processing. The default is
True .
help_text
Extra help text to be displayed under the field on the objects admin form. Its useful for documentation even
if your object doesnt have an admin form.
primary_key
If True , this field is the primary key for the model.
If you dont specify primary_key=True for any fields in your model, Django will automatically add this
field:
id = models.AutoField('ID', primary_key=True)
Thus, 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 blank=False , null=False , and unique=True . Only one primary key
is allowed on an object.
radio_admin
By default, Djangos admin uses a select-box interface (<select>) for fields that are ForeignKey or have
choices set. If radio_admin is set to True , Django will use a radio-button interface instead.
Dont use this for a field unless its a ForeignKey or has choices set.
unique
If True , the value for this field must be unique throughout the table.
unique_for_date
Set to the name of a DateField or DateTimeField to require that this field be unique for the value
of the date field, for example:
class Story(models.Model):
pub_date = models.DateTimeField()
slug = models.SlugField(unique_for_date="pub_date")
...
In the preceding code, Django wont allow the creation of two stories with the same slug published on the
same date. This differs from using a unique_together constraint in that only the date of the
pub_date field is taken into account; the time doesnt matter.
unique_for_month
Like unique_for_date , but requires the field to be unique with respect to the month of the given field.
unique_for_year
Like unique_for_date and unique_for_month , but for an entire year.
verbose_name
Each field type, except for ForeignKey , ManyToManyField , and OneToOneField , takes an optional
first positional argument a verbose name. If the verbose name isnt given, Django will automatically create
it using the fields attribute name, converting underscores to spaces.
In this example, the verbose name is "Person's first name" :
first_name = models.CharField("Person's first name", maxlength=30)
In this example, the verbose name is "first name" :
first_name = models.CharField(maxlength=30)
ForeignKey , ManyToManyField , and OneToOneField require the first argument to be a model class,
so use the verbose_name keyword argument:
poll = models.ForeignKey(Poll, verbose_name="the related poll")
sites = models.ManyToManyField(Site, verbose_name="list of sites")
place = models.OneToOneField(Place, verbose_name="related place")
The convention is not to capitalize the first letter of the verbose_name . Django will automatically
capitalize the first letter where it needs to.
Relationships
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.
However, the semantics of one-to-one relationships are being revisited as this book goes to print, so theyre
not covered in this section. Check the online documentation for the latest information.
Many-to-One Relationships
To define a many-to-one relationship, use ForeignKey . You use it just like any other Field type: by
including it as a class attribute of your model.
ForeignKey requires a positional argument: the class to which the model is related.
For example, if a Car model has a Manufacturer that is, a Manufacturer makes multiple cars but
each Car only has one Manufacturer use the following definitions:
class Manufacturer(models.Model):
...
class Car(models.Model):
manufacturer = models.ForeignKey(Manufacturer)
...
To create a recursive relationship an object that has a many-to-one relationship with itself use
models.ForeignKey('self') :
class Employee(models.Model):
manager = 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 you can only use strings to refer to models in the same models.py file you cannot
use a string to reference a model in a different application, or to reference a model that has been imported
from elsewhere.
Behind the scenes, Django appends "_id" to the field name to create its database column name. In the
preceding example, the database table for the Car model will have a manufacturer_id column. (You can
change this explicitly by specifying db_column ; see the earlier db_column section.) 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.
Its suggested, but not required, that the name of a ForeignKey field (manufacturer in the example)
be the name of the model, in lowercase letters. You can, of course, call the field whatever you want, for
example:
class Car(models.Model):
company_that_makes_it = models.ForeignKey(Manufacturer)
# ...
ForeignKey fields take a number of extra arguments for defining how the relationship should work (see
Table B-5). All are optional.
Table B-5. ForeignKey Options
Argument |
Description |
edit_inline |
If not False , this related object is edited inline on the related objects
page. This means that the object will not have its own admin interface. Use either
models.TABULAR or models.STACKED , which, respectively, designate whether
the inline-editable objects are displayed as a table or as a stack of fieldsets. |
limit_choices_to |
A dictionary of lookup arguments and values (see Appendix C) 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, the
following:
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.
Instead of a dictionary, this can be a Q object (see Appendix C) for more
complex queries.
This is not compatible with edit_inline .
|
max_num_in_admin |
For inline-edited objects, this is the maximum number of related objects to
display in the admin interface. Thus, if a pizza could have only up to ten
toppings, max_num_in_admin=10 would ensure that a user never enters more than
ten toppings.
Note that this doesnt ensure more than ten related toppings ever get created. It
simply controls the admin interface; it doesnt enforce things at the Python API
level or database level.
|
min_num_in_admin |
The minimum number of related objects displayed in the admin interface. Normally,
at the creation stage, num_in_admin inline objects are shown, and at the edit
stage, num_extra_on_change blank objects are shown in addition to all
pre-existing related objects. However, no fewer than min_num_in_admin related
objects will ever be displayed. |
num_extra_on_change |
The number of extra blank related-object fields to show at the change stage. |
num_in_admin |
The default number of inline objects to display on the object page at the add
stage. |
raw_id_admin |
Only display a field for the integer to be entered instead of a drop-down menu.
This is useful when related to an object type that will have too many rows to make
a select box practical.
This is not used with edit_inline .
|
related_name |
The name to use for the relation from the related object back to this one. See
Appendix C for more information. |
to_field |
The field on the related object that the relation is to. By default, Django uses
the primary key of the related object. |
Many-to-Many Relationships
To define a many-to-many relationship, use ManyToManyField . Like ForeignKey , ManyToManyField
requires a positional argument: the class to which the model is related.
For example, if a Pizza has multiple Topping objects that is, a Topping can be on multiple
pizzas and each Pizza has multiple toppings heres how youd represent that:
class Topping(models.Model):
...
class Pizza(models.Model):
toppings = models.ManyToManyField(Topping)
...
As with ForeignKey , a relationship to self can be defined by using the string 'self' instead of the
model name, and you can refer to as-yet undefined models by using a string containing the model name.
However, you can only use strings to refer to models in the same models.py file you cannot use a string
to reference a model in a different application, or to reference a model that has been imported from
elsewhere.
Its suggested, but not required, that the name of a ManyToManyField (toppings in the example) be a
plural term describing the set of related model objects.
Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship.
It doesnt matter which model gets the ManyToManyField , but you need it in only one of the models not in
both.
If youre using the admin interface, ManyToManyField instances should go in the object thats going to be
edited in the admin interface. In the preceding example, toppings is in Pizza (rather than
Topping having a pizzas ManyToManyField ) because its more natural to think about a Pizza
having toppings than a topping being on multiple pizzas. The way its set up in the example, the Pizza
admin form would let users select the toppings.
ManyToManyField objects take a number of extra arguments for defining how the relationship should work
(see Table B-6). All are optional.
Table B-6. ManyToManyField Options
Argument |
Description |
related_name |
The name to use for the relation from the related object back to this one. See
Appendix C for more information. |
filter_interface |
Use a nifty, unobtrusive JavaScript filter interface instead of the
usability-challenged <select multiple> in the admin form for this object. The
value should be models.HORIZONTAL or models.VERTICAL (i.e., should the
interface be stacked horizontally or vertically). |
limit_choices_to |
See the description under ForeignKey . |
symmetrical |
Only used in the definition of ManyToManyField 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 doesnt 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 ManyToMany relationships with self , set
symmetrical to False . This will force Django to add the descriptor for the
reverse relationship, allowing ManyToMany relationships to be nonsymmetrical.
|
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. |
Model Metadata Options
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 thats 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.
db_table
The name of the database table to use for the model.
To save you time, Django automatically derives the name of the database table from the name of your model
class and the application that contains it. A models database table name is constructed by joining the
models app label the name you used in manage.py startapp to the models class name, with an underscore
between them.
For example, if you have an application books (as created by manage.py startapp books ), a model
defined as class Book will have a database table named books .
To override the database table name, use the db_table parameter in class Meta :
class Book(models.Model):
...
class Meta:
db_table = 'things_to_read'
If this isnt given, Django will use app_label + '_' + model_class_name . See the section Table Names for
more information.
If your database table name is an SQL reserved word, or it contains characters that arent allowed in Python
variable names (notably the hyphen), thats OK. Django quotes column and table names behind the scenes.
get_latest_by
The name of a DateField or DateTimeField in the model. This specifies the default field to use in
your model Manager s latest() method.
Heres an example:
class CustomerOrder(models.Model):
order_date = models.DateTimeField()
...
class Meta:
get_latest_by = "order_date"
See Appendix C for more information on the latest() method.
order_with_respect_to
Marks this object as orderable with respect to the given field. This is almost always used with related
objects to allow them to be ordered with respect to a parent object. For example, if an Answer relates
to a Question object, and a question has more than one answer, and the order of answers matters, youd do
this:
class Answer(models.Model):
question = models.ForeignKey(Question)
# ...
class Meta:
order_with_respect_to = 'question'
ordering
The default ordering for the object, for use when obtaining lists of objects:
class Book(models.Model):
title = models.CharField(maxlength=100)
class Meta:
ordering = ['title']
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.
For example, to order by a title field in ascending order (i.e., A-Z), use this:
ordering = ['title']
To order by title in descending order (i.e., Z-A), use this:
ordering = ['-title']
To order by title in descending order, and then by title in ascending order, use this:
ordering = ['-title', 'author']
Note that, regardless of how many fields are in ordering , the admin site uses only the first field.
permissions
Extra permissions to enter into the permissions table when creating this object. Add, delete, and change
permissions are automatically created for each object that has admin set. This example specifies an
extra permission, can_deliver_pizzas :
class Employee(models.Model):
...
class Meta:
permissions = (
("can_deliver_pizzas", "Can deliver pizzas"),
)
This is a list or tuple of two tuples in the format (permission_code, human_readable_permission_name) .
See Chapter 12 for more on permissions.
unique_together
Sets of field names that, taken together, must be unique:
class Employee(models.Model):
department = models.ForeignKey(Department)
extension = models.CharField(maxlength=10)
...
class Meta:
unique_together = [("department", "extension")]
This is a list of lists of fields that must be unique when considered together. Its used in the Django admin
interface and is enforced at the database level (i.e., the appropriate UNIQUE statements are included in
the CREATE TABLE statement).
verbose_name
A human-readable name for the object, singular:
class CustomerOrder(models.Model):
order_date = models.DateTimeField()
...
class Meta:
verbose_name = "order"
If this isnt given, Django will use a adapted version of the class name in which CamelCase becomes
camel case .
verbose_name_plural
The plural name for the object:
class Sphynx(models.Model):
...
class Meta:
verbose_name_plural = "sphynges"
If this isnt given, Django will add an s to the verbose_name .
Model Methods
Define custom methods on a model to add custom row-level functionality to your objects. Whereas Manager
methods are intended to do tablewide things, model methods should act on a particular model instance.
This is a valuable technique for keeping business logic in one place: the model. For example, this model has
a few custom methods:
class Person(models.Model):
first_name = models.CharField(maxlength=50)
last_name = models.CharField(maxlength=50)
birth_date = models.DateField()
address = models.CharField(maxlength=100)
city = models.CharField(maxlength=50)
state = models.USStateField() # Yes, this is America-centric...
def baby_boomer_status(self):
"""Returns the person's baby-boomer status."""
import datetime
if datetime.date(1945, 8, 1) <= self.birth_date <= datetime.date(1964, 12, 31):
return "Baby boomer"
if self.birth_date < datetime.date(1945, 8, 1):
return "Pre-boomer"
return "Post-boomer"
def is_midwestern(self):
"""Returns True if this person is from the Midwest."""
return self.state in ('IL', 'WI', 'MI', 'IN', 'OH', 'IA', 'MO')
@property
def full_name(self):
"""Returns the person's full name."""
return '%s %s' % (self.first_name, self.last_name)
The last method in this example is a property an attribute implemented by custom getter/setter user code.
Properties are a nifty trick added to Python 2.2; you can read more about them at
http://www.python.org/download/releases/2.2/descrintro/#property.
There are also a handful of model methods that have special meaning to Python or Django. These methods are
described in the sections that follow.
__str__
__str__() is a Python magic method that defines what should be returned if you call str() on the
object. Django uses str(obj) (or the related function, unicode(obj) , described shortly) in a number
of places, most notably as the value displayed to render an object in the Django admin site and as the value
inserted into a template when it displays an object. Thus, you should always return a nice, human-readable
string for the objects __str__ . Although this isnt required, its strongly encouraged.
Heres an example:
class Person(models.Model):
first_name = models.CharField(maxlength=50)
last_name = models.CharField(maxlength=50)
def __str__(self):
return '%s %s' % (self.first_name, self.last_name)
get_absolute_url
Define a get_absolute_url() method to tell Django how to calculate the URL for an object, for example:
def get_absolute_url(self):
return "/people/%i/" % self.id
Django uses this in its admin interface. If an object defines get_absolute_url() , the object-editing
page will have a View on site link that will take you directly to the objects public view, according to
get_absolute_url() .
Also, a couple of other bits of Django, such as the syndication-feed framework, use get_absolute_url()
as a convenience to reward people whove defined the method.
Its good practice to use get_absolute_url() in templates, instead of hard-coding your objects URLs. For
example, this template code is bad:
<a href="/people/{{ object.id }}/">{{ object.name }}</a>
But this template code is good:
<a href="{{ object.get_absolute_url }}">{{ object.name }}</a>
The problem with the way we just wrote get_absolute_url() is that it slightly violates the DRY
principle: the URL for this object is defined both in the URLconf file and in the model.
You can further decouple your models from the URLconf using the permalink decorator. This decorator is
passed the view function, a list of positional parameters, and (optionally) a dictionary of named
parameters. Django then works out the correct full URL path using the URLconf, substituting the parameters
you have given into the URL. For example, if your URLconf contained a line such as the following:
(r'^people/(\d+)/$', 'people.views.details'),
your model could have a get_absolute_url method that looked like this:
@models.permalink
def get_absolute_url(self):
return ('people.views.details', [str(self.id)])
Similarly, if you had a URLconf entry that looked like this:
(r'/archive/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})/$', archive_view)
you could reference this using permalink() as follows:
@models.permalink
def get_absolute_url(self):
return ('archive_view', (), {
'year': self.created.year,
'month': self.created.month,
'day': self.created.day})
Notice that we specify an empty sequence for the second argument in this case, because we want to pass only
keyword arguments, not named arguments.
In this way, youre tying the models absolute URL to the view that is used to display it, without repeating
the URL information anywhere. You can still use the get_absolute_url method in templates, as before.
Executing Custom SQL
Feel free to write custom SQL statements in custom model methods and module-level methods. The object
django.db.connection represents the current database connection. To use it, call connection.cursor()
to get a cursor object. Then, call cursor.execute(sql, [params]) to execute the SQL, and
cursor.fetchone() or cursor.fetchall() to return the resulting rows:
def my_custom_sql(self):
from django.db import connection
cursor = connection.cursor()
cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
row = cursor.fetchone()
return row
connection and cursor mostly implement the standard Python DB-API
(http://www.python.org/peps/pep-0249.html). If youre not familiar with the Python DB-API, note that the
SQL statement in cursor.execute() uses placeholders, "%s" , rather than adding parameters directly
within the SQL. If you use this technique, the underlying database library will automatically add quotes and
escaping to your parameter(s) as necessary. (Also note that Django expects the "%s" placeholder, not
the "?" placeholder, which is used by the SQLite Python bindings. This is for the sake of consistency
and sanity.)
A final note: If all you want to do is use a custom WHERE clause, you can just use the where ,
tables , and params arguments to the standard lookup API. See Appendix C.
Overriding Default Model Methods
As explained in Appendix C, each model gets a few methods automatically most notably, save() and
delete() . You can override these methods to alter behavior.
A classic use-case for overriding the built-in methods is if you want something to happen whenever you save
an object, for example:
class Blog(models.Model):
name = models.CharField(maxlength=100)
tagline = models.TextField()
def save(self):
do_something()
super(Blog, self).save() # Call the "real" save() method.
do_something_else()
You can also prevent saving:
class Blog(models.Model):
name = models.CharField(maxlength=100)
tagline = models.TextField()
def save(self):
if self.name == "Yoko Ono's blog":
return # Yoko shall never have her own blog!
else:
super(Blog, self).save() # Call the "real" save() method
Admin Options
The Admin class tells Django how to display the model in the admin site.
The following sections present a list of all possible Admin options. None of these options is required.
To use an admin interface without specifying any options, use pass , like so:
class Admin:
pass
Adding class Admin to a model is completely optional.
date_hierarchy
Set date_hierarchy to the name of a DateField or DateTimeField in your model, and the change
list page will include a date-based navigation using that field.
Heres an example:
class CustomerOrder(models.Model):
order_date = models.DateTimeField()
...
class Admin:
date_hierarchy = "order_date"
fields
Set fields to control the layout of admin interface add and change pages.
fields is a pretty complex nested data structure best demonstrated with an example. The following is
taken from the FlatPage model thats part of django.contrib.flatpages :
class FlatPage(models.Model):
...
class Admin:
fields = (
(None, {
'fields': ('url', 'title', 'content', 'sites')
}),
('Advanced options', {
'classes': 'collapse',
'fields' : ('enable_comments', 'registration_required', 'template_name')
}),
)
Formally, fields is a list of two tuples, in which each two-tuple represents a <fieldset> on the
admin form page. (A <fieldset> is a section of the form.)
The two-tuples are in the format (name, field_options) , where name is a string representing the
title of the fieldset and field_options is a dictionary of information about the fieldset, including a
list of fields to be displayed in it.
If fields isnt given, Django will default to displaying each field that isnt an AutoField and has
editable=True , in a single fieldset, in the same order as the fields are defined in the model.
The field_options dictionary can have the keys described in the sections that follow.
fields
A tuple of field names to display in this fieldset. This key is required.
To display multiple fields on the same line, wrap those fields in their own tuple. In this example, the
first_name and last_name fields will display on the same line:
'fields': (('first_name', 'last_name'), 'address', 'city', 'state'),
classes
A string containing extra CSS classes to apply to the fieldset.
Apply multiple classes by separating them with spaces:
'classes': 'wide extrapretty',
Two useful classes defined by the default admin site stylesheet are collapse and wide . Fieldsets
with the collapse style will be initially collapsed in the admin site and replaced with a small click to
expand link. Fieldsets with the wide style will be given extra horizontal space.
description
A string of optional extra text to be displayed at the top of each fieldset, under the heading of the
fieldset. Its used verbatim, so you can use any HTML and you must escape any special HTML characters (such
as ampersands) yourself.
js
A list of strings representing URLs of JavaScript files to link into the admin screen via <script
src=""> tags. This can be used to tweak a given type of admin page in JavaScript or to provide quick links
to fill in default values for certain fields.
If you use relative URLs that is, URLs that dont start with http:// or / then the admin site will
automatically prefix these links with settings.ADMIN_MEDIA_PREFIX .
list_display
Set list_display to control which fields are displayed on the change list page of the admin.
If you dont set list_display , the admin site will display a single column that displays the
__str__() representation of each object.
Here are a few special cases to note about list_display :
If the field is a ForeignKey , Django will display the __str__() of the related object.
ManyToManyField fields arent supported, because that would entail executing a separate SQL statement
for each row in the table. If you want to do this nonetheless, give your model a custom method, and add
that methods name to list_display . (More information on custom methods in list_display
shortly.)
If the field is a BooleanField or NullBooleanField , Django will display a pretty on or off icon
instead of True or False .
If the string given is a method of the model, Django will call it and display the output. This method
should have a short_description function attribute, for use as the header for the field.
Heres a full example model:
class Person(models.Model):
name = models.CharField(maxlength=50)
birthday = models.DateField()
class Admin:
list_display = ('name', 'decade_born_in')
def decade_born_in(self):
return self.birthday.strftime('%Y')[:3] + "0's"
decade_born_in.short_description = 'Birth decade'
If the string given is a method of the model, Django will HTML-escape the output by default. If youd
rather not escape the output of the method, give the method an allow_tags attribute whose value is
True .
Heres a full example model:
class Person(models.Model):
first_name = models.CharField(maxlength=50)
last_name = models.CharField(maxlength=50)
color_code = models.CharField(maxlength=6)
class Admin:
list_display = ('first_name', 'last_name', 'colored_name')
def colored_name(self):
return '<span style="color: #%s;">%s %s</span>' % (self.color_code, self.first_name, self.last_name)
colored_name.allow_tags = True
If the string given is a method of the model that returns True or False , Django will display a
pretty on or off icon if you give the method a boolean attribute whose value is True .
Heres a full example model:
class Person(models.Model):
first_name = models.CharField(maxlength=50)
birthday = models.DateField()
class Admin:
list_display = ('name', 'born_in_fifties')
def born_in_fifties(self):
return self.birthday.strftime('%Y')[:3] == 5
born_in_fifties.boolean = True
The __str__() methods are just as valid in list_display as any other model method, so its
perfectly OK to do this:
list_display = ('__str__', 'some_other_field')
Usually, elements of list_display that arent actual database fields cant be used in sorting (because
Django does all the sorting at the database level).
However, if an element of list_display represents a certain database field, you can indicate this
fact by setting the admin_order_field attribute of the item, for example:
class Person(models.Model):
first_name = models.CharField(maxlength=50)
color_code = models.CharField(maxlength=6)
class Admin:
list_display = ('first_name', 'colored_first_name')
def colored_first_name(self):
return '<span style="color: #%s;">%s</span>' % (self.color_code, self.first_name)
colored_first_name.allow_tags = True
colored_first_name.admin_order_field = 'first_name'
The preceding code will tell Django to order by the first_name field when trying to sort by
colored_first_name in the admin site.
list_display_links
Set list_display_links to control which fields in list_display should be linked to the change page
for an object.
By default, the change list page will link the first column the first field specified in list_display to
the change page for each item. But list_display_links lets you change which columns are linked. Set
list_display_links to a list or tuple of field names (in the same format as list_display ) to link.
list_display_links can specify one or many field names. As long as the field names appear in
list_display , Django doesnt care how many (or how few) fields are linked. The only requirement is that
if you want to use list_display_links , you must define list_display .
In this example, the first_name and last_name fields will be linked on the change list page:
class Person(models.Model):
...
class Admin:
list_display = ('first_name', 'last_name', 'birthday')
list_display_links = ('first_name', 'last_name')
Finally, note that in order to use list_display_links , you must define list_display , too.
list_filter
Set list_filter to activate filters in the right sidebar of the change list page of the admin interface.
This should be a list of field names, and each specified field should be either a BooleanField ,
DateField , DateTimeField , or ForeignKey .
This example, taken from the django.contrib.auth.models.User model, shows how both list_display and
list_filter work:
class User(models.Model):
...
class Admin:
list_display = ('username', 'email', 'first_name', 'last_name', 'is_staff')
list_filter = ('is_staff', 'is_superuser')
list_per_page
Set list_per_page to control how many items appear on each paginated admin change list page. By default,
this is set to 100 .
ordering
Set ordering to specify how objects on the admin change list page should be ordered. This should be a
list or tuple in the same format as a models ordering parameter.
If this isnt provided, the Django admin interface will use the models default ordering.
save_as
Set save_as to True to enable a save as feature on admin change forms.
Normally, objects have three save options: Save, Save and continue editing, and Save and add another. If
save_as is True , Save and add another will be replaced by a Save as button.
Save as means the object will be saved as a new object (with a new ID), rather than the old object.
By default, save_as is set to False .
save_on_top
Set save_on_top to add save buttons across the top of your admin change forms.
Normally, the save buttons appear only at the bottom of the forms. If you set save_on_top , the buttons
will appear both on the top and the bottom.
By default, save_on_top is set to False .
search_fields
Set search_fields to enable a search box on the admin change list page. This should be set to a list of
field names that will be searched whenever somebody submits a search query in that text box.
These fields should be some kind of text field, such as CharField or TextField . You can also
perform a related lookup on a ForeignKey with the lookup API follow notation:
class Employee(models.Model):
department = models.ForeignKey(Department)
...
class Admin:
search_fields = ['department__name']
When somebody does a search in the admin search box, Django splits the search query into words and returns
all objects that contain each of the words, case insensitive, where each word must be in at least one of
search_fields . For example, if search_fields is set to ['first_name', 'last_name'] and a user
searches for john lennon , Django will do the equivalent of this SQL WHERE clause:
WHERE (first_name ILIKE '%john%' OR last_name ILIKE '%john%')
AND (first_name ILIKE '%lennon%' OR last_name ILIKE '%lennon%')
For faster and/or more restrictive searches, prefix the field name with an operator, as shown in Table B-7.
Table B-7. Operators Allowed in search_fields
Operator |
Meaning |
^ |
Matches the beginning of the field. For example, if search_fields is set to ['^first_name',
'^last_name'] , and a user searches for john lennon , Django will do the equivalent of this
SQL WHERE clause:
WHERE (first_name ILIKE 'john%' OR last_name ILIKE 'john%')
AND (first_name ILIKE 'lennon%' OR last_name ILIKE 'lennon%')
This query is more efficient than the normal '%john%' query, because the database only needs
to check the beginning of a columns data, rather than seeking through the entire columns data.
Plus, if the column has an index on it, some databases may be able to use the index for this
query, even though its a LIKE query.
|
= |
Matches exactly, case-insensitive. For example, if search_fields is set to ['=first_name',
'=last_name'] and a user searches for john lennon , Django will do the equivalent of this
SQL WHERE clause:
WHERE (first_name ILIKE 'john' OR last_name ILIKE 'john')
AND (first_name ILIKE 'lennon' OR last_name ILIKE 'lennon')
Note that the query input is split by spaces, so, following this example, its currently not
possible to search for all records in which first_name is exactly 'john winston'
(containing a space).
|
@ |
Performs a full-text match. This is like the default search method, but it uses an index.
Currently this is available only for MySQL. |
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Many, many thanks to Jack Slocum; the inspiration and much of the code for the comment system comes from Jack's blog, and this site couldn't have been built without his wonderful
YAHOO.ext
library. Thanks also to Yahoo for YUI itself.