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:
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:
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:
A local filesystem path that will be appended to your MEDIA_ROOT setting to determine the value of the django.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:
Using a FileField or an ImageField (see below) in a model takes a few steps:
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:
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:
In addition to the special attributes that are available for FileField``, an ImageField also has height and width attributes, 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, ImageField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.
An integer.
An IP address, in string format (e.g. '192.0.2.30').
Like a BooleanField, but allows NULL as one of the options. Use this instead of a BooleanField with null=True.
Like an IntegerField, but must be positive.
Like a PositiveIntegerField, but only allows values under a certain (database-dependent) point.
“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_length is not specified, Django will use a default length of 50.
Implies setting db_index to True.
Like an IntegerField, but only allows values under a certain (database-dependent) point.
A large text field.
Also see CharField for storing smaller bits of text.
A time, represented in Python by a datetime.time instance. Accepts the same auto-population options as DateField.
A CharField for a URL. Has one extra optional argument:
Like all CharField subclasses, URLField takes the optional max_length argument. If you don’t specify max_length, a default of 200 is used.
A TextField that checks that the value is valid XML that matches a given schema. Takes one required argument:
The following arguments are available to all field types. All are optional.
If True, Django will store empty values as NULL in 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=True for non-string fields such as integers, booleans and dates. For both types of fields, you will also need to set blank=True if you wish to permit empty values in forms, as the null parameter only affects database storage (see blank).
Avoid using null on string-based fields such as CharField and TextField unless you have an excellent reason. If a string-based field has null=True, that means it has two possible values for “no data”: NULL, and the empty string. In most cases, it’s redundant to have two possible values for “no data;” Django’s convention is to use the empty string, not NULL.
Note
When using the Oracle database backend, the null=True option will be coerced for string-based fields that have the empty string as a possible value, and the value NULL will be stored to denote the empty string.
For more on this, see the section “Making Date and Numeric Fields Optional” in Chapter 6.
If True, the field is allowed to be blank. Default is False.
Note that this is different than null. null is purely database-related, whereas blank is validation-related. If a field has blank=True, validation on Django’s admin site will allow entry of an empty value. If a field has blank=False, the field will be required.
An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field.
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 choices to be dynamic, you’re probably better off using a proper database table with a ForeignKey`. choices is meant for static data that doesn’t change much, if ever.
The name of the database column to use for this field. If this isn’t given, Django will use the field’s name.
If your database column name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names – notably, the hyphen – that’s OK. Django quotes column and table names behind the scenes.
If True, 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.
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.
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.
Same as limit_choices_to in ForeignKey.
limit_choices_to has no effect when used on a ManyToManyField with a custom intermediate table specified using the through paramter.
Only used in the definition of ManyToManyFields on self. Consider the following model:
class Person(models.Model):
friends = models.ManyToManyField("self")
When Django processes this model, it identifies that it has a ManyToManyField on itself, and as a result, it doesn’t add a person_set attribute to the Person class. Instead, the ManyToManyField is assumed to be symmetrical – that is, if I am your friend, then you are my friend.
If you do not want symmetry in many-to-many relationships with self, set symmetrical to False. This will force Django to add the descriptor for the reverse relationship, allowing ManyToManyField relationships to be non-symmetrical.
Django will automatically generate a table to manage many-to-many relationships. However, if you want to manually specify the intermediary table, you can use the through option to specify the Django model that represents the intermediate table that you want to use.
The most common use for this option is when you want to associate extra data with a many-to-many relationship.
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:
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 a ManyToManyField 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 the through 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".