Code


Version 4 (modified by adrian, 8 years ago) (diff)

Clarification

Removing the magic

This document proposes a new, cleaner and less magical Django database API.

Model definition

Difference: Import is from django.db.models instead of django.core.meta. This is easier to remember. models may not be the best name for it.

from django.db import models

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

Properties are allowed.

from django.db import models

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

def _get_full_name(self):
    return "%s %s" % (self.first_name, self.last_name)
full_name = property(_get_full_name)

Database connection

Old:

from django.core.db import db
cursor = db.cursor()

New:

from django.db import connection
cursor = connection.cursor()

This is easier to remember, clearer and more consistent.

API usage: Object creation

Import the model class directly from the module in which it was defined. No more django.models.* magic.

from myproject.people.models import Person
p = Person(first_name='John', last_name='Smith')
p.save()

API usage: Table-level functions

All "table-level" functions -- ways of retrieving records tablewide rather than performing instance-specific tasks -- are accessed via a model class's objects attribute. They aren't direct methods of a model instance object because we want to keep the "table-wide" and "row-specific" namespaces separate.

from myproject.people.models import Person
p_list = Person.objects.get_list()
p = Person.objects.get_object()

This doesn't work from an instance.

p = Person.objects.get_object(pk=1)
p.objects.get_list() # Will raise an exception.

If a model already has an objects attribute, the field-accessor attribute is renamed to objects_.

class Person(models.Model):
    first_name = models.CharField(maxlength=30)
    last_name = models.CharField(maxlength=30)
    objects = models.TextField()

p = Person(first_name='Mary', last_name='Jones', objects_='Hello there.')
p.save()
p.objects_ == 'Hello there.'

API usage: Overridding model methods (and pre- and post-save hooks)

Proper subclassing of methods will now work, so you can subclass the automatic save() and delete() methods. This removes the need for the _pre_save() and _post_save() hooks. Example:

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

def save(self):
    self.do_something()
    super(Person, self).save(self) # Call the "real" save() method.
    self.do_something_else()

You can even skip saving (as requested in #1014).

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

def save(self):
    if datetime.date.today() > datetime.date(2005, 1, 1):
        super(Person, self).save(self) # Call the "real" save() method.
    else:
        # Don't save.
        pass

API usage: Overridding table-level functions

You can also override any table-level functions, such as get_list() or get_object(). Do this by creating a custom models.Manager subclass and passing it to your model.

from django.db import models
class PersonManager(models.Manager):
    def get_list(self, **kwargs):
        # Changes get_list() to hard-code a limit=10.
        kwargs['limit'] = 10
        return super(PersonManager, self).get_list(self, **kwargs) # Call the "real" get_list() method.

class Person(models.Model):
    first_name = models.CharField(maxlength=30)
    last_name = models.CharField(maxlength=30)
    class META:
        manager = PersonManager()

API usage: Specifying lookups

Old:

people.get_list(first_name__exact='Adrian')

New:

Person.objects.get_list(Person.q.first_name == 'Adrian')

The old syntax will still be supported and documented, indefinitely. The new syntax will be implemented as a thin wrapper around the old syntax. See patch on #851 for implementation.

Other "module"-level members: Automatic manipulators and ObjectDoesNotExist exception

Person.get_add_manipulator()
Person.get_change_manipulator()
Person.DoesNotExist