= Removing the magic =

The "magic-removal" branch aims to make several sweeping changes to the Django codebase. Most changes involve the database API and removing some of its unneeded magic, which confuses newbies and is a bit of a wart.

This document explains the changes in the branch.

These changes will be integrated into the next Django release (either 0.91 or 0.92, depending on whether we decide to release a 0.91 as a "middle" release for people who want new features currently in trunk but don't want to wait for magic-removal).

== Models support properties ==

'''Status: Done'''

Unlike before, properties are supported on models.

{{{
#!python
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)
}}}

== Model class and Field classes renamed/relocated ==

'''Status: Done'''

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

{{{
#!python
from django.db import models

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

== Database connection relocated/renamed ==

'''Status: Done'''

The connection is now available at {{{django.db.connection}}}. This is easier to remember, clearer and more consistent.

Old:
{{{
#!python
from django.core.db import db
cursor = db.cursor()
}}}

New:
{{{
#!python
from django.db import connection
cursor = connection.cursor()
}}}

Backend-specific functions, if you should need them, are available at {{{django.db.backend}}}.

Old:
{{{
#!python
from django.core import db
db.quote_name('foo')
}}}

New:
{{{
#!python
from django.db import backend
backend.quote_name('foo')
}}}

Also, the various backend functionality has been split into three separate modules for each backend -- {{{base.py}}}, {{{creation.py}}} and {{{introspection.py}}}. This is purely for performance and memory savings, so that basic, everyday Django usage doesn't have to load the introspective functionality into memory.

== Interact directly with model classes, not with magic modules ==

'''Status: Done'''

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

{{{
#!python
from myproject.people.models import Person
p = Person(first_name='John', last_name='Smith')
p.save()
}}}

This also removes the need for the {{{module_name}}} parameter.

== Access table-level DB API functions via model classes, not with magic modules ==

'''Status: Done'''

All "table-level" functions -- ways of retrieving records tablewide rather than performing instance-specific tasks -- are now 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.

{{{
#!python
from myproject.people.models import Person
p_list = Person.objects.get_list()
p = Person.objects.get_object()
}}}

This doesn't work from an instance.

{{{
#!python
p = Person.objects.get_object(pk=1)
p.objects.get_list() # Raises AttributeError
}}}

== Override default manager name ("objects") ==

'''Status: Done'''

If a model already has an {{{objects}}} attribute, you'll need to specify an alternate name for the magic {{{objects}}}.

{{{
#!python
class Person(models.Model):
    first_name = models.CharField(maxlength=30)
    last_name = models.CharField(maxlength=30)
    objects = models.TextField()
    people = models.Manager()

p = Person(first_name='Mary', last_name='Jones', objects='Hello there.')
p.save()
p.objects == 'Hello there.'
Person.people.get_list()
}}}

== Multiple managers ==

'''Status: Done'''

You can create as many managers as you want. When necessary (such as on the admin), Django will use the first one defined, in order.

If you define at least one custom manager, it will not get the default "objects" manager.

{{{
#!python
class Person(models.Model):
    first_name = models.CharField(maxlength=30)
    last_name = models.CharField(maxlength=30)
    people = models.Manager()
    fun_people = SomeOtherManager()
}}}

== Added a more powerful way of overriding model methods, removed hard-coded _pre_save(), _post_save(), etc. ==

'''Status: Done'''

Proper subclassing of methods now works, so you can subclass the automatic {{{save()}}} and {{{delete()}}} methods. This removes the need for the {{{_pre_save()}}}, {{{_post_save()}}}, {{{_pre_delete()}}} and {{{_post_delete()}}} hooks -- all of which have been removed. Example:

{{{
#!python
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).

{{{
#!python
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
}}}

== New API functionality: Overriding table-level functions ==

'''Status: Done'''

You can 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. The term "manager" could be replaced with some other word.

{{{
#!python
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 models.Manager.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)
    objects = PersonManager()
}}}

== Renamed DoesNotExist exception ==

'''Status: Done'''

Instead of {{{people.PersonDoesNotExist}}}, it's {{{Person.DoesNotExist}}}.

Old:
{{{
#!python
from django.models.myapp import people
try:
    people.get_object(pk=1)
except people.PersonDoesNotExist:
    print "Not there"
}}}

New:
{{{
#!python
from path.to.myapp.models import Person
try:
    Person.objects.get_object(pk=1)
except Person.DoesNotExist:
    print "Not there"
}}}

== Removed SilentVariableFailure exception ==

'''Status: Done'''

Old behavior: Any exception that subclasses {{{django.core.template.SilentVariableFailure}}} fails silently in the template system.

New behavior: Any exception that has a {{{silent_variable_failure}}} attribute fails silently in the template system. {{{django.core.template.SilentVariableFailure}}} no longer exists.

== Automatic manipulators ==

'''Status: Not yet done'''

Old:
{{{
#!python
from django.models.myapp import people
m1 = people.AddManipulator()
m2 = people.ChangeManipulator(3)
}}}

{{{
#!python
from path.to.myapp.models import Person
m1 = Person.get_add_manipulator()()
m2 = Person.get_change_manipulator()(3)
}}}

== Database lookup API changes ==

See DescriptorFields, rjwittams' proposal on the API changes.