#

## Django documentation

# Model reference

A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you're storing. Generally, each model maps to a single database table.

The basics:

> - Each model is a Python class that subclasses `django.core.meta.Model`.
> - Each attribute of the model represents a database field.
> - Model metadata (non-field information) goes in an inner class named `META`.

A companion to this document is the official repository of model examples.

## Field objects

The most important part of a model is the list of database fields it defines. Fields are defined by class attributes. Each class attribute in a model, aside from the optional inner `class META`, should be an instance of a `meta.Field` subclass.

In this example, there are two fields, `first_name` and `last_name`

    class Person(meta.Model):
        first_name = meta.CharField(maxlength=30)
        last_name = meta.CharField(maxlength=30)

Django will use `first_name` and `last_name` as the database column names.

Each field type, except for `ForeignKey`, `ManyToManyField` and `OneToOneField`, takes an optional first positional argument -- a human-readable name. If the human-readable name isn't given, Django will automatically create the human-readable name by using the machine-readable name, converting underscores to spaces.

In this example, the human-readable name is `"Person's first name"`:

    first_name = meta.CharField("Person's first name", maxlength=30)

In this example, the human-readable name is `"first name"`:

    first_name = meta.CharField(maxlength=30)

`ForeignKey`, `ManyToManyField` and `OneToOneField` require the first argument to be a model class, so use the `verbose_name` keyword argument to specify the human-readable name:

    poll = meta.ForeignKey(Poll, verbose_name="the related poll")
    sites = meta.ManyToManyField(Site, verbose_name="list of sites")
    place = meta.OneToOneField(Place, verbose_name="related place")

Convention is not to capitalize the first letter of the `verbose_name`. Django will automatically capitalize the first letter where it needs to.

### General 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. Default is `False`.

  Note that empty string values will always get stored as empty strings, not as `NULL` -- so use `null=True` for non-string fields such as integers, booleans and dates.

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

`blank`

  If `True`, the field is allowed to be blank.

  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.

`choices`

  A list of 2-tuples to use as choices for this field.

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

  A choices list looks like this:

      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.

`core`

  For objects that are edited inline to a related object.

  In the Django admin, if all "core" fields in an inline-edited object are cleared, the object will be deleted.

  It is an error to have an inline-editable relation without at least one `core=True` field.

`db_column`

  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.

`db_index`
  If `True`, `django-admin.py sqlindexes` will output a `CREATE INDEX` statement for this field.
`default`
  The default value for the field.
`editable`
  If `False`, the field will not be editable in the admin. Default is `True`.
`help_text`
  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.
`primary_key`

  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 this field:

      id = meta.AutoField('ID', primary_key=True)

  Thus, 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 `blank=False`, `null=False` and `unique=True`. Only one primary key is allowed on an object.

`radio_admin`

  By default, Django's 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.

  Don't use this for a field unless it's a `ForeignKey` or has `choices` set.

`unique`

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

  This is enforced at the database level and at the Django admin-form level.

`unique_for_date`

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

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

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

`unique_for_month`
  Like `unique_for_date`, but requires the field to be unique with respect to the month.
`unique_for_year`
  Like `unique_for_date` and `unique_for_month`.
`validator_list`

  A list of extra validators to apply to the field. Each should be a callable that takes the parameters `field_data, all_data` and raises `django.core.validators.ValidationError` for errors. (See the validator docs.)

  Django comes with quite a few validators. They're in `django.core.validators`.

### Field types

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 admin (e.g. `<input type="text">`, `<select>`).
> - The minimal validation requirements, used in Django's admin and in manipulators.

Here are all available field types:

`AutoField`
  An `IntegerField` that automatically increments according to available IDs. You usually won't need to use this directly; a primary key field will automatically be added to your model if you don't specify otherwise. (See `primary_key` in `General field options` above.)
`BooleanField`

  A true/false field.

  The admin represents this as a checkbox.

`CharField`

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

  For large amounts of text, use `TextField`.

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

  `CharField` has an extra required argument, `maxlength`, the maximum length (in characters) of the field. The maxlength is enforced at the database level and in Django's validation.

`CommaSeparatedIntegerField`
  A field of integers separated by commas. As in `CharField`, the `maxlength` argument is required.
`DateField`

  A date field. Has a few extra optional arguments:

  > Argument | Description
  > --- | ---
  > `auto_now` | Automatically set the field to now every time the object is saved. Useful for "last-modified" timestamps.
  > `auto_now_add` | Automatically set the field to now when the object is first created. Useful for creation of timestamps.

  The admin represents this as an `<input type="text">` with a JavaScript calendar and a shortcut for "Today."

`DateTimeField`

  A date and time field. Takes the same extra options as `DateField`.

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

`EmailField`
  A `CharField` that checks that the value is a valid e-mail address. This doesn't accept `maxlength`.
`FileField`

  A file-upload field.

  Has an extra required argument, `upload_to`, a local filesystem path to which files should be upload. This path may contain strftime formatting, which will be replaced by the date/time of the file upload (so that uploaded files don't fill up the given directory).

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

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

  > 1. In your settings file, you'll need to define `MEDIA_ROOT` as the full path to a directory where you'd like Django to store uploaded files. (For performance, these files are not stored in the database.) Define `MEDIA_URL` as the base public URL of that directory. Make sure that this directory is writable by the Web server's user account.
  > 2. Add the `FileField` or `ImageField` to your model, making sure to define the `upload_to` option to tell Django to which subdirectory of `MEDIA_ROOT` it should upload files.
  > 3. All that will be stored in your database is a path to the file (relative to `MEDIA_ROOT`). You'll must 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 }}`.

`FilePathField`

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

  > Argument | Description
  > --- | ---
  > `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 called `foo23.txt` but not `bar.txt` or `foo23.gif`.
  > `recursive` | Optional. Either `True` or `False`. Default is `False`. Specifies whether all subdirectories of `path` should be included.

  Of course, these arguments can be used together.

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

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

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

`FloatField`

  A floating-point number. Has two required arguments:

  > 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 2 decimal places, you'd use:

      meta.FloatField(..., max_digits=5, decimal_places=2)

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

      meta.FloatField(..., max_digits=19, decimal_places=10)

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

`ImageField`

  Like `FileField`, but validates that the uploaded object is a valid image. Has two extra optional arguments, `height_field` and `width_field`, which, if set, will be auto-populated with the height and width of the image each time a model instance is saved.

  Requires the Python Imaging Library.

`IntegerField`

  An integer.

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

`IPAddressField`

  An IP address, in string format (i.e. "24.124.1.30").

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

`NullBooleanField`

  Like a `BooleanField`, but allows `NULL` as one of the options. Use this instead of a `BooleanField` with `null=True`.

  The admin represents this as a `<select>` box with "Unknown", "Yes" and "No" choices.

`PhoneNumberField`
  A `CharField` that checks that the value is a valid U.S.A.-style phone number (in the format `XXX-XXX-XXXX`).
`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.

  Implies `maxlength=50` and `db_index=True`.

  Accepts an extra option, `prepopulate_from`, which is a list of fields from which to auto-populate the slug, via JavaScript, in the object's admin form:

      meta.SlugField(prepopulate_from=("pre_name", "name"))

  `prepopulate_from` doesn't accept DateTimeFields.

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

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

  A large text field.

  The admin represents this as a `<textarea>` (a multi-line input).

`TimeField`

  A time. Accepts the same auto-population options as `DateField` and `DateTimeField`.

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

`URLField`

  A field for a URL. If the `verify_exists` option is `True` (default), the URL given will be checked for existence (i.e., the URL actually loads and doesn't give a 404 response).

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

`USStateField`

  A two-letter U.S. state abbreviation.

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

`XMLField`

  A `TextField` that checks that the value is valid XML that matches a given schema. Takes one required argument, `schema_path`, which is the filesystem path to a RelaxNG schema against which to validate the field.

### Relationships

Clearly, the power of relational databases lies in relating tables to each other. Django offers ways to define the most common types of database relationships: Many-to-one, many-to-many and one-to-one.

#### 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 `Place` model is in a `City` -- that is, a `City` contains multiple places but each `Place` is only in one `City` -- here's how you'd represent that:

    class City(meta.Model):
        # ...

    class Place(meta.Model):
        # ...
        city = meta.ForeignKey(City)

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

The name of a `ForeignKey` (`city` in the example above) generally should be the name of the model, singular. Behind the scenes, Django appends "_id" to the field name to create its database column name. But your code should never have to deal with the database column name, unless you write custom SQL.

See the Many-to-one relationship model example for a full example.

`ForeignKey` fields take a number of extra arguments for defining how the relationship should work. All are optional:

> Argument | Description
> --- | ---
> `edit_inline` | If not `False`, this related object is edited "inline" on the related object's page. This means that the object will not have its own admin interface. Use either `meta.TABULAR` or `meta.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 the Database API reference) that limit the available admin choices for this object. Use this with `meta.LazyDate` to limit choices of objects by date. For example:
>     limit_choices_to = {'pub_date__lte' : meta.LazyDate()}
> only allows the choice of related objects with a `pub_date` before the current date/time to be chosen. 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. Thus, if a pizza could only have up to 10 toppings, `max_num_in_admin=10` would ensure that a user never enters more than 10 toppings. Note that this doesn't ensure more than 10 related toppings ever get created. It just controls the interface.
> `min_num_in_admin` | The minimum number of related objects displayed in the admin. 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. Not used with `edit_inline`.
> `related_name` | The name to use for the relation from the related object back to this one. For example, when if `Topping` has this field:
>     meta.ForeignKey(Pizza)
> the `related_name` will be "topping" (taken from the class name), which will in turn give `Pizza` the methods `get_topping_list()` and `get_topping_count()`. If you instead were to use:
>     meta.ForeignKey(Pizza, related_name="munchie")
> then the methods would be called `get_munchie_list()`, `get_munchie_count()`, etc. This is only really useful when you have a single object that relates to the same object more than once. For example, if a `Story` object has both `primary_category` and `secondary_category` fields, to make sure that the `Category` objects have the correct methods, you'd use fields like:
>     meta.ForeignKey(Category, related_name="primary_story")
>     meta.ForeignKey(Category, related_name="secondary_story")
> ...which would give the `Category` objects methods named `get_primary_story_list()` and `get_secondary_story_list()`.
> `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`. You use it just like any other `Field` type: by including it as a class attribute of your model.

`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 -- here's how you'd represent that:

    class Topping(meta.Model):
        # ...

    class Pizza(meta.Model):
        # ...
        toppings = meta.ManyToManyField(Topping)

The name of a `ManyToManyField` (`toppings` in the example above) generally should be the name of the model, plural.

Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship.

It doesn't matter which model gets the `ManyToManyField`, but you only need it in one of the models -- not in both.

Generally, `ManyToManyField` instances should go in the object that's going to be edited in the admin. In the above example, `toppings` is in `Pizza` (rather than `Topping` having a `pizzas` `ManyToManyField` ) because it's more natural to think about a `Pizza` having toppings than a topping being on multiple pizzas. The way it's set up above, the `Pizza` admin form would let users select the toppings.

See the Many-to-many relationship model example for a full example.

`ManyToManyField` objects take a number of extra arguments for defining how the relationship should work. All are optional:

> Argument | Description
> --- | ---
> `related_name` | See the description of `related_name` in `ForeignKey` above.
> `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 `meta.HORIZONTAL` or `meta.VERTICAL` (i.e. should the interface be stacked horizontally or vertically).
> `limit_choices_to` | See the description under `ForeignKey` above.
> `singular` | The singular name of the field. Use to name the `get_*` methods: in the example above, Django gives the `Pizza` objects a `get_topping_list()` method, where `topping` is the default `singular` value derived from the lowercase version of the class being linked to. Use the singular parameter to change this, which is if you want one model to have multiple `ManyToMany` relationships to another model.

#### One-to-one relationships

To define a one-to-one relationship, use `OneToOneField`. You use it just like any other `Field` type: by including it as a class attribute of your model.

This is most useful on the primary key of an object when that object "extends" another object in some way.

`OneToOneField` requires a positional argument: The class to which the model is related.

For example, if you're building a database of "places", you would build pretty standard stuff such as address, phone number, etc. in the database. Then, if you wanted to build a database of restaurants on top of the places, instead of repeating yourself and replicating those fields in the `Restaurant` model, you could make `Restaurant` have a `OneToOneField` to `Place` (because a restaurant "is-a" place).

This `OneToOneField` will actually replace the primary key `id` field (since one-to-one relations share the same primary key), and has a few differences in the admin interface:

> - No `Place` selection interface is displayed on `Restaurant` pages. There will be one (and only one) `Restaurant` for each `Place`.
> - On the `Restaurant` change list, every `Place` -- whether it has an associated `Restaurant` or not -- will be displayed. Adding a `Restaurant` to a `Place` just means filling out the required `Restaurant` fields.

See the One-to-one relationship model example for a full example.

## META options

Give your model metadata by using an inner `"class META"`, like so:

    class Foo(meta.Model):
        bar = meta.CharField(maxlength=30)
        # ...
        class META:
            admin = meta.Admin()
            # ...

Model metadata is "anything that's not a field" -- ordering options, admin options, etc.

Here's a list of all possible `META` options. No options are required. Adding `class META` to a model is completely optional.

`admin`
  A `meta.Admin` object; see Admin options. If this field is given, the object will have an admin interface. If it isn't given, the object won't have one.
`db_table`

  The name of the database table to use for the module:

      db_table = "pizza_orders"

  If this isn't given, Django will use `app_label + '_' + module_name`.

  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.

`exceptions`

  Names of extra exception subclasses to include in the generated module. These exceptions are available from instance methods and from module-level methods:

      exceptions = ("DisgustingToppingsException", "BurntCrust")

`get_latest_by`

  The name of a `DateField` or `DateTimeField`. If given, the module will have a `get_latest()` function that fetches the "latest" object according to that field:

      get_latest_by = "order_date"

  See Getting the "latest" object for a full example.

`module_constants`

  A dictionary of names/values to use as extra module-level constants:

      module_constants = {
          'MEAT_TYPE_PEPPERONI' : 1,
          'MEAT_TYPE_SAUSAGE' : 2,
      }

`module_name`

  The name of the module:

      module_name = "pizza_orders"

  If this isn't given, Django will use a lowercased version of the class name, plus `"s"`. This "poor man's pluralization" is intentional: Any other level of magic pluralization would get confusing.

`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 a `PizzaToppping` relates to a `Pizza` object, you might use:

      order_with_respect_to = 'pizza'

  to allow the toppings to be ordered with respect to the associated pizza.

`ordering`

  The default ordering for the object, for use by `get_list` and the admin:

      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.

  See Specifying ordering for a full example.

`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`:

      permissions = (("can_deliver_pizzas", "Can deliver pizzas"),)

  This is a list or tuple of 2-tuples in the format `(permission_code, human_readable_permission_name)`.

`unique_together`

  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 in the Django admin 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:

      verbose_name = "pizza"

  If this isn't given, Django will use a munged version of the class name: `CamelCase` becomes `camel case`.

`verbose_name_plural`

  The plural name for the object:

      verbose_name_plural = "stories"

  If this isn't given, Django will use `verbose_name + "s"`.

## Admin options

The `admin` field in the model tells Django how to construct the admin interface for the object. The field is an instance of the `meta.Admin` object, which takes the following parameters. All are optional.

`date_hierarchy`

  To allow filtering of objects in the admin by date, set `date_hierarchy` to the name of the field to filter by:

      date_hierarchy = 'order_date'

`fields`

  A list of fieldsets to display on the admin page. Each fieldset is a 2-tuple: `(name, field_options)`. The `name` is a string to name the field set, and `field_options` is a dictionary of information about the fields to be displayed in that fieldset. This dictionary has the following keys:

  > `fields`

  >   A tuple of field names to display in this fieldset. To display multiple fields on the same line, wrap those fields in their own tuple.

  >   This key is required in the dictionary.

  > `classes`

  >   Extra CSS classes to apply to the fieldset. This is a simple string. You can apply multiple classes by separating them with spaces.

  >   Two useful classes defined by the default stylesheet are `collapse` and `wide`. Fieldsets with the `collapse` style will be initially collapsed in the admin and replaced with a small "click to expand" link. Fieldsets with the `wide` style will be given extra horizontal space.

  For example (taken from the `django.contrib.flatpages` model):

      fields = (
          (None, {
              'fields': ('url', 'title', 'content', 'sites')
          }),
          ('Advanced options', {
              'classes': 'collapse',
              'fields' : ('enable_comments', 'registration_required', 'template_name')
          }),
      ),

  results in an admin that looks like:

  If `fields` isn't given but a model does define `admin` as a `meta.Admin` object, Django will default to displaying each field that isn't an `AutoField` and has `editable=True`, in a single fieldset, in the same order as the fields are defined in the model.

`js`
  A list of strings representing URLs of JavaScript files to link into the admin screen. This can be used to tweak a given type of admin page in JS or to provide "quick links" to fill in default values for certain fields.
`list_display`

  List of fields to display on the list page in the admin.

  There are a few special cases that do other things besides displaying the contents of the given fields:

  > - If the field given is a `ForeignKey`, the `repr()` of the related object will be displayed.
  > - `ManyToManyField` fields aren't supported, because that would entail executing a separate SQL statement for each row in the table.
  > - If the field is a `BooleanField`, a "on" or "off" icon will be displayed instead of `True` or `False`.
  > - If the field name is a method of the model, it'll be called, and the output will be displayed. This method should have a `short_description` function attribute, for use as the header for the field.
  > - Use the string `"__repr__"` to output the representation of the object, according to your model's `__repr__()` function. If you don't define `list_display`, Django will use the `__repr__` by default.

  See the example below.

`list_filter`

  List of fields to filter by. Each field should either be a `BooleanField` or else a field with a `ManyToOne` relation.

  Here's an example of how `list_display` and `list_filter` work (taken from the `auth.user` model):

      list_display = ('username', 'email', 'first_name', 'last_name', 'is_staff'),
      list_filter = ('is_staff', 'is_superuser'),

  The above code results in an admin that looks like this:

  (This example also has `search_fields` defined; see below).

`list_select_related`

  Either `True` or `False`. Default is `False`. If `True`, the admin change list page will use the `select_related` database-API parameter in its query that retrieves the list of items.

  Note that Django will use `select_related`, regardless of this setting, if one of the `list_display` fields is a `ForeignKey`.

`ordering`
  A list or tuple (see the META options, above) that gives a different ordering for the admin change list. If this isn't given, the model's default ordering will be used.
`save_as`

  Enables a "save as" feature on object pages. 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.

`save_on_top`
  If this option is `True`, object pages will have the save buttons across the top as well as at the bottom of the page.
`search_fields`
  A list of field names to provide a text search for. These fields should, obviously, be some kind of text field, such as `CharField` or `TextField`.

## Model methods

There are a number of methods you can define on model objects to control the object's behavior. First, any methods you define will be available as methods of object instances. For example:

    class Pizza(meta.Model):
        # ...

        def is_disgusting(self):
            return "anchovies" in [topping.name for topping in self.get_topping_list()]

Now, every `Pizza` object will have a `is_disgusting()` method.

Note that the scope of custom methods is modified to be the same as the module scope. These methods do NOT have access to globals within your model's module. Additionally, custom methods have access to a few commonly-used objects for convenience:

> - The `datetime` module from Python's standard library.
> - The `db` object from `django.core.db`. This represents the database connection, so you can do custom queries via a cursor object. See "Executing custom SQL" below.

See Giving models custom methods for a full example.

A few object methods have special meaning:

`__repr__`

  Django uses `repr(obj)` in a number of places, most notably as the value inserted into a template when it displays an object. Thus, you should always return a nice, human-readable string for the object's `__repr__`.

  Although defining `__repr__()` isn't required, it's strongly encouraged.

  See Adding repr for a full example.

`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 "/pizzas/%i/" % self.id

  Django uses this in its admin interface. If an object defines `get_absolute_url`, the object detail page will have a "View on site" link that will jump you directly to the object's public view.

  It's good practice to use `get_absolute_url()` in templates, instead of hard-coding your objects' URLs.

`_pre_save`

  This method is called just before an object is saved to the database. For example, you can use it to calculate aggregate values from other fields before the object is saved.

  See Adding hooks before/after saving and deleting for a full example.

`_post_save`
  This method is called just after the object is saved to the database. This could be used to update other tables, update cached information, etc.
`_pre_delete`
  Like `_pre_save`, but for deletion.
`_post_delete`
  Like `_post_save`, but for deletion.

### Module-level methods

Since each data class effectively turns into a "magic" Python module under `django.models`, there are times you'll want to write methods that live in that module. Any model method that begins with "_module_" is turned into a module-level function:

    class Pizza(meta.Model):
        # ...

        def _module_get_pizzas_to_deliver():
            return get_list(delivered__exact=False)

This will make the top-level `pizzas` module have a `get_pizzas_to_deliver()` method:

    >>> from django.models.pizza_hut import pizzas
    >>> pizzas.get_pizzas_to_deliver()
    [ ... ]

Note that the scope of these methods is modified to be the same as the module scope. These methods do NOT have access to globals within your model's module.

### Manipulator methods

Similarly, you can add methods to the object's manipulators by defining methods that being with "_manipulator_". This is most useful for providing custom validators for certain fields, because manipulators automatically call any method that begins with "validate":

    class Pizza(meta.Model):
        # ...

        def _manipulator_validate_customer_id(self, field_data, all_data):
            from django.core import validators
            from django.conf.settings import BAD_CUSTOMER_IDS

            if int(field_data) in BAD_CUSTOMER_IDS:
                raise validators.ValidationError, "We don't deliver to this customer."

### Executing custom SQL

Feel free to write custom SQL statements in custom model methods and module-level methods. Each custom method automatically has access to the variable `db`, which is the current database connection. To use it, call `db.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. Example:

    def my_custom_sql(self):
        cursor = db.cursor()
        cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
        row = cursor.fetchone()
        return row

If your custom SQL statement alters the data in your database -- for example, via a `DELETE` or `UPDATE` -- you'll need to call `db.commit()`. Example:

    def my_custom_sql2(self):
        cursor = db.cursor()
        cursor.execute("DELETE FROM bar WHERE baz = %s", [self.baz])
        db.commit()

`db` and `cursor` simply use the standard Python DB-API. If you're 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.

## Using models

Once you've defined a model, you'll need to "enable" it in Django. This section explains how Django searches for available models.

Save your models in a normal Python module. Put this module within a package called "models", which should itself be a subpackage of some other package on your Python path. The `__init__.py` in your `models` package should contain an `__all__` variable that is set to a list of all model module names within the `models` directory.

If this sounds confusing, just use `django-admin.py startapp` -- it'll create the proper directory structure and `__init__.py` files. (See the django-admin.py documentation .)

For example, if you save your models in a module called `mymodels.py`, here's a directory layout you might use:

    myapp/
        __init__.py          # Empty file
        models/
            __init__.py      # Contains "__all__ = ['mymodels']"
            mymodels.py      # Contains your models

Then, you'll have to tell Django that the `myapp` application is installed. Do this by editing your settings file and adding `"myapp"` to the `INSTALLED_APPS` tuple.

Again, if this sounds confusing, use `django-admin.py startapp` to take care of package creation for you. This documentation exists only to explain how Django works.

Once you've added your app to `INSTALLED_APPS`, you can open a Python interactive interpreter and play with your model:

    >>> from django.models.mymodels import pizzas
    >>> pizzas.get_list()

Note that the import is from `django.models`, not `myapp.models`. Django creates a "magic" module within `django.models` for every installed application. Each of those magic modules has a dynamic API. See the database API reference for full information on how to use this API.

Why is the INSTALLED_APPS setting necessary?

Model relationships work both ways, and the dynamically-generated Django API creates API lookups in both directions. Thus, for Django to figure out all the other models related to a particular model, it has to know the complete spectrum of installed apps.

## Models across files

It's perfectly OK to relate a model to one from another module. To do this, just import the model module at the top of your model module, like so:

    from django.models import core

Make sure you're importing from `django.models`, not directly from your model module.

Then, just refer to the other model class wherever needed. For example:

    class MyModel(meta.Model):
        # ...
        sites = meta.ManyToManyField(core.Site)

## Models in multiple files

If you want to have multiple model modules in a `"models"` directory, make sure you edit `"models/__init__.py"` and add the name of your model module to the `__all__` variable. If your `models` package doesn't have your model module in `__all__`, Django won't see any of the models in that module.

## Comments

### Aaron Swartz July 15, 2005 at 9:43 p.m.

So here's a problem I have: I'm writing a db-backed web app, it's kept in subversion, and I have a couple checkouts for different installs. Often adding features requires changing the database model. Now I can manually type ALTER commands into the SQL prompt and update the Python file to match, but how do I make sure all the installs do this when they update their code?

### Jacob July 16, 2005 at 12:21 p.m.

Aaron: that's a very common problem. We've toyed with the idea of creating a function of the django-admin script that will "upgrade" a database to match the current model, but it's very difficult and prone to errors. It's not an easy problem.

### Moof July 17, 2005 at 10:36 a.m.

OK, I agree, Auto-generating ALTER statements can be quite difficult, It's not always intuitive. However, if you version your DB Schema, and keep an actual version number in the DB somewhere, you can at least add some sort of automated way whereby when it detects and old schema it'll call the (manually entered) update-to-version-x function. If it's several versions out of date, then it whould run through those update-to-version-x functions in order.

This is a similar concept to Twisted's versioned persistence framework: http://twistedmatrix.com/projects/core/d... though it would be interesting to see if you could somehow integrate subversion revision numbers into it.

### Jason Diamond July 20, 2005 at 6:15 p.m.

Documentation for ManyToOneField is missing.

Do you put ManyToManyField fields in both classes or just one? Which one?

### Adrian Holovaty July 21, 2005 at 10:18 a.m.

Jason Diamond: There's no such thing as ManyToOneField -- use ForeignKey.

OK, I lied...ManyToOneField does indeed exist, but it's only used internally. It's not documented on purpose.

### Adrian Holovaty July 21, 2005 at 10:19 a.m.

Jason Diamond: You put ManyToMany fields in just one of the models -- the one in whose admin form you want the multi-select-box displayed. If you're not using admin, it doesn't really matter.

### Dustin Sallings July 26, 2005 at 2:30 a.m.

ManyToManyField claims to only support the parameters listed, but null=True and blank=True are both accepted and required in order to remove all mappings for a for an object.

### John Fieber July 31, 2005 at 5:27 p.m.

Manually constructed ALTER statements are probably safest, but could be helped out by something along the lines of this:

http://www.stepwise.com/Articles/2005/DB...

This allows automatic application of the required manually constructed schema tweaks to make sure the database and the application are on the same page.

### dbpp August 3, 2005 at 5:04 a.m.

Please, an example of using list_display with a method

### yacc August 3, 2005 at 8:23 a.m.

I'm just wondering, if there is a way to write non-DB-based models?

Andreas

### Daniel Luz August 3, 2005 at 10:10 a.m.

Are there any plans for a way to specify that a group of fields is unique, or for compound primary keys?

Also, could there be the special methods _pre_delete and _post_delete, just like the _save counterparts?

### Daniel Luz August 3, 2005 at 2:53 p.m.

Whoops... nevermind about unique groups. I just read about the unique_together option.

### Garth T Kidd August 7, 2005 at 5:34 a.m.

The descriptions of _pre_save and _post_save should point to module_constants as being a good way to deal with NameError exceptions caused because model module globals aren't automatically copied to the virtual modules for each model object.

### Adrian Holovaty August 7, 2005 at 3:23 p.m.

Garth: A cleaner way to handle that, instead of messing with module_constants, would be to put your import statements within your custom functions.

### Garth T Kidd August 7, 2005 at 5:25 p.m.

I'm sure the performance hit isn't that drastic, but importing within a function runs pretty hard against my normal practice. The taboo is so strong I was actually thinking of using decorators to pass model module globals into module_constants. For whatever reason, @needs('md5') in front of the class definition feels cleaner than importing inside the function itself.

### ianm August 11, 2005 at 9:25 a.m.

I am concerned about the size of this model module. With the number of classes I have for my app and my custom functions I, could see this one file getting pretty big.

Is there a way to break up an model module into multiple files? Or is there another recommended approach? (other than breaking the app into multiple smaller apps, which I would do if I could)

Nice job with the project and thanks...

### imaurer August 11, 2005 at 3:17 p.m.

Ignore my last post, I was checking the sql and not just doing an install. Someone on google groups was having the same problem and figured out my problem for me.

### syn August 16, 2005 at 2:55 p.m.

How about adding a new table? I have a feeling that it is not as simple as adding in the new Model subclass and running django-admin install, obviously. I have a app I am designing which will, most likely, have additional tables added to it over time to cover additional topics. Since they would be identical in layout (forum boards) I could just add another field (call it 'forum_name') and store all data in the one DB -- I'd rather not do that, however, since in the current php version, I add/remove as needed.

### syn August 18, 2005 at 1:04 p.m.

I think I have figured it out, actually. I could still add the new table to the models file, but I'd have to manually create the table myself within the DB. Or, alternatively, manually create the DB (which in my case, is just a schema copy of another able, sans data, with a different table name) and then use 'django-admin.py inspectdb' to have it create the model for me, mostly, then edit and paste it into the models file.

### hollerith August 21, 2005 at 5:31 a.m.

Can I add myCustomPhoneNumberfield or would that be bad?

### John August 22, 2005 at 12:39 p.m.

How do you add an additional field to the cross-reference table generated by a ManyToManyField? For example, if I have an xref table that looks like:

`id` mediumint(9) unsigned NOT NULL auto_increment,
`foo_id` int(11) NOT NULL default '0',
`bar_id` int(11) NOT NULL default '0',
`baz` varchar(255) NOT NULL

...how do I enable my Foo and/or Bar objects to have access to variable "baz"? Reworking my schema to store "baz" elsewhere is not an option :)

### John August 24, 2005 at 10:28 a.m.

The above question was answered for me on the mailing list: use a cross-reference model as per http://www.djangoproject.com/documentati...

### hollerith August 28, 2005 at 7:27 a.m.

I hate to criticise when so much is good, and it is very early days too (I hope!). But... :)

Is the suffix 'Field' not redundant? It certainly feels it after even a small-sized schema.

PhoneNumberField should be USPhoneNumberField. An international format should be the base class. +1 ... ... ... &c.

On this again - Extensibility? Is it envisaged that core classes would be extended by application developers? This could get messy as people add their own 'definitive' versions. Field masks might obviate the need to modify validators and formfields to provide non-US defaults, such as UKPostCode but project level classes which extend core classes would be better.

Or am I missing something?

### Michael September 18, 2005 at 12:06 p.m.

Aren't the field types a bit heterogeneous? Many of them correspond to common database field types, which you would expect when building models for a database backend. However, things such as EmailField, XMLField, FileField and ImageField are different - they represent data + associated validation schemes, blurring the distinction between model and user interface. You will wind up with

<exaggeration>

No. of field types=SQL field types * HTML form field types * data formats

</exaggeration>

- cruft alert. I think something like

email = meta.CharField(maxLength=50, validator=validators.Email(check='DNS'))

image = meta.CharField(maxLength=100, validator=validators.Image(allowedTypes=['eps','png']))

would be better. If required (FileField or similar), the representation in the standard admin UI could also be overridden in the Validator class.
In this way, proliferation of field types would be limited, and if we want to change the validation procedure or the standard UI representation we still can be sure that the underlying database field type will stay the same.

Best, Michael

### metastan October 20, 2005 at 11:21 a.m.

The preset upload_to in ImageField isn't useful for building a photo-sharing site. Suppose I'd like each user to be able to upload images to a personal directory (path defined when user added), which could be secured? Rest seems pretty cool, though.

### padraig November 16, 2005 at 5:25 p.m.

It would be very useful to specify exactly what characters a SlugField will contain when generated by 'prepopulate_from'. As far as I can figure out it will only contain lowercase letters, hyphens and dashes...

### padraig November 16, 2005 at 5:28 p.m.

Actually after inspecting the JS in the admin code a Slugfield will contain lowercase letters, numbers, and dashes when generated by 'prepopulate_from', i.e. [a-z0-9-]

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