= Backwards-incompatible changes =

As Django is still in pre-1.0 mode, we haven't yet committed to maintaining backwards compatibility in any APIs. Although we're keeping such changes to a minimum, Django developers should be acutely aware of these changes.

Of course, once we reach 1.0, we'll be strongly committed to backward compatibility.

This page lists all backwards-incompatible changes to Django since the 0.95 release. For changes prior to 0.95, see the OlderBackwardsIncompatibleChanges page.

== Table of Contents ==

Changes made after Django [source:/django/tags/releases/0.95 0.95]:
 * August 2, 2006: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Databaseconstraintnameschanged Database constraint names changed]
 * August 11, 2006: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Backslashescapingchanged Backslash escaping changed]
 * September 27, 2006: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#RemovedENABLE_PSYCOsetting Removed ENABLE_PSYCO setting]
 * March 14, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#EnforcingMySQLdbversion Enforcing MySQLdb version]

Changes made after Django [source:/django/tags/releases/0.96 0.96]:

 * March 31, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Changedspacelesstemplatetagtoremoveallspaces Changed 'spaceless' template tag to remove all spaces]
 * April 8, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Renamedlocalflavor.usatolocalflavor.us Renamed `localflavour.usa` to `localflavor.us`]
 * April 9, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#RemovedLazyDate Removed `LazyDate`]
 * April 20, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#MySQLIntrospectionChange MySQL Introspection change]
 * April 25, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#LOGIN_URLisnowasetting Login URL is now a setting]
 * May 5, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#TestClientloginmethodchanged Test Client `login()` method changed]
 * May 8, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Genericrelationshavemoved Generic relations have moved]
 * May 14, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#Newforms:clean_datachangedtocleaned_data Newforms: clean_data changed to cleaned_data]
 * May 20, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#RenamedFloatFieldtoDecimalField Renamed FloatField to DecimalField]
 * June 17, 2007: [http://code.djangoproject.com/wiki/BackwardsIncompatibleChanges#NodeListsmustcontainNodes NodeLists must contain Nodes]

== Database constraint names changed ==

As of [3512], the format of the constraint names Django generates for foreign key references changed slightly. These names are only used sometimes, when it is not possible to put the reference directly on the affected column, so this is not always visible.

The effect of this change is that {{{manage.py reset app_name}}} and similar commands may generate SQL with invalid constraint names and thus generate an error when run against the database (the database server will complain about the constraint not existing). To fix this, you will need to tweak the output of {{{manage.py sqlreset app_name}}} to match the correct constraint names and pass the results to the database server manually.

== Backslash escaping changed ==

As of [3552], the Django database API now escapes backslashes given as query parameters. If you have any database API code that match backslashes, and it was working before (despite the broken escaping), you'll have to change your code to "unescape" the slashes one level.

For example, this used to work:

{{{
#!python
# Code that matches a single backslash
MyModel.objects.filter(text__contains='\\\\')
}}}

But it should be rewritten as this:

{{{
#!python
# Code that matches a single backslash
MyModel.objects.filter(text__contains='\\')
}}}

== Removed ENABLE_PSYCO setting ==

As of [3877], the {{{ENABLE_PSYCO}}} setting no longer exists. If your settings file includes {{{ENABLE_PSYCO}}}, nothing will break per se, but it just won't do anything. If you want to use [http://psyco.sourceforge.net/ Psyco] with Django, write some [wiki:PsycoMiddleware custom middleware that activates Psyco].

== Enforcing MySQLdb version ==

As of [4724], Django raises an error if you try to use the MySQL backend with a {{{MySQLdb}}} (MySQL Python module) version earlier than 1.2.1p2. There were significant, production-related bugs in earlier versions, so we have upgraded the minimum requirement.

In [4767], we added a {{{mysql_old}}} backend, which is identical to the {{{mysql}}} backend prior to the change in [4724]. You can use this backend if upgrading the {{{MySQLdb}}} module is not immediately possible. However, the {{{mysql_old}}} backend is deprecated, and  we will not continue developing it.

== Changed 'spaceless' template tag to remove all spaces ==

As of [4885], the {{{spaceless}}} template tag removes *all* spaces between HTML tags instead of preserving a single space.

For example, for this template code:
{{{
{% spaceless %}
<p>
    <a href="foo/">Foo</a>
</p>
{% endspaceless %}
}}}

The old output was this:

{{{
<p> <a href="foo/">Foo</a> </p>
}}}

The new output is this:

{{{
<p><a href="foo/">Foo</a></p>
}}}

As always, {{{spaceless}}} only affects space *between* HTML tags, not space within HTML tags or space in plain text.

== Renamed `localflavor.usa` to `localflavor.us` ==

As of [4954], `localflavor.usa` has been renamed `localflavor.us`. This change was made to match the naming scheme of other local flavors. 

== Removed `LazyDate` ==

The `LazyDate` helper class was removed in [4985]. Default field values and query arguments can both be callable objects, so instances of `LazyDate` can be replaced with a reference to a callable that evaluates to the current date (i.e., `datetime.now`). For example, the following model using `LazyDate`:

{{{
#!python
class Article(models.Model):
    title = models.CharField(maxlength=100)
    published = models.DateField(default=LazyDate())
}}}

can be redefined as:

{{{
#!python
from datetime import datetime
class Article(models.Model):
    title = models.CharField(maxlength=100)
    published = models.DateField(default=datetime.now)
}}}

Note that the new model definition refers to the name of the `datetime.now()` function, but doesn't actually call the function. The call to `datetime.now()` is deferred until the value is actually required (i.e., when the model is saved).

== MySQL Introspection Change ==

In [5042] a small change was made in the `mysql` backend in how it interpreted `CHAR(n)` columns in the database. They are now mapped to Django's !CharField class, rather than the previous !TextField. See ticket #4048 for details.

This will only be apparent if you introspect a database table using 0.96 and again using [5042] or later: you will have slightly different Django models generated if there are any `CHAR(n)` columns. However, no real code changes should be necessary.

== LOGIN_URL is now a setting ==

In [5072], we moved the LOGIN_URL constant from django.contrib.auth into the settings module. This was part of a broader change to make these URLs (including logout and post-login redirect) configurable. Code that previously read
{{{
#!python
from django.contrib.auth import LOGIN_URL
}}}

should be changed to refer to `settings.LOGIN_URL` instead.

== Test Client `login()` method changed ==

The implementation of `django.test.Client.login()` operated as a wrapper around a series of GET and POST calls accessing a nominated login URL. This approach was fragile, and tightly bound to specific templates and login mechanims. 

In [5152], we changed the implementation of the `login()` method on the test Client. `login()` now accepts a list of credentials, and exercises these credentials directly on the cookies and Session objects of a site, without accessing or using a login page. This breaks the dependence on specific template formatting, and enables the login mechanism to work with any authentication backend, and any login decorator.

Existing uses of `login()`, e.g.:
{{{
#!python
c = Client()
c.login('/path/to/login','myuser','mypassword')
}}}
should be modified to read:
{{{
#!python
c = Client()
c.login(username='myuser', password='mypassword')
}}}
The keyword arguments `username` and `password` *must* be given explicitly. If an alternate authentication scheme is in use, the credentials required by that scheme can be provided instead of `username` and `password`.

== Generic relations have moved ==

In [5172], the various generic relation classes (!GenericForeignKey and !GenericRelation) have moved into the django.contrib.contenttypes module. This is because they will not work if contenttypes is not an installed app. Making the import path reflect the dependency should help remind people of this.

Code using generic relations should change from saying things like
{{{
#!python
generic_field = models.GenericRelation(SomeOtherModel)
}}}
to using
{{{
#!python
from django.contrib.contenttypes import generic
...
generic_field = generic.GenericRelation(SomeOtherModel)
}}}

No database changes or other model changes are required.

== Newforms: clean_data changed to cleaned_data ==

If you are accessing form data that has been cleaned in newforms, you could previously use the {{{clean_data}}} attribute on the form. In [5237], this was changed to {{{cleaned_data}}} to avoid a name-clash (see [5231] for why the change was necessary).

You will need to do a search-and-replace in your form code and replace clean_data with cleaned_data everywhere.

== Renamed !FloatField to !DecimalField ==

In [5302], we fixed a slight inconsistency in Django's model field system. Previously, we had a !FloatField that had a maximum number of digits and decimal places. However, although it was stored in the database as a fixed precision value, it was stored in Python as a float, so precision was lost due to rounding problems.

We now have a !DecimalField class that is the same as the old !FloatField, except that it is represented in Python by a Decimal type (and still stored as a fixed precision value in the database). We have also introduced a proper !FloatField that is represented as a float in Python and stored as a "double precision" field in the database.

To make your code compatible with these changes, you need to change all occurrences of !FloatField in your code to !DecimalField. You only need to rename the field type -- all the parameters are the same. So change
{{{
#!python
class MyModel(models.Model):
    ...
    field_name = models.FloatField(max_digits=10, decimal_places=3)    # old code!
}}}
to
{{{
#!python
class MyModel(models.Model):
    ...
    field_name = models.DecimalField(max_digits=10, decimal_places=3)    # new code!
}}}

If you forget to make this change, you will see errors about !FloatField not taking a max_digits attribute in {{{__init__}}}, since the new !FloatField takes no precision-related arguments.

If you are using MySQL or PostgreSQL, there are '''no further changes needed'''. The database column types for !DecimalField are the same as for the old !FloatField.

If you are using SQLite, you need to force the database to view the appropriate columns as decimal types, rather than floats. To do this, follow this procedure for every application you have that contains a !DecimalField. Do this ''after'' you have made the change to using !DecimalField in your code and updated the Django code.

'''Warning: Back up your database first! ''' For SQLite, this means making a copy of the single file that stores the database (the name of that file is the DATABASE_NAME in your settings.py file).

For every application using a !DecimalField, do the following. We will use applications called {{{some_app}}} and {{{another_app}}} in this example
{{{
./manage.py dumpdata --format=xml some_app another_app > data-dump.xml
./manage.py reset some_app another_app
./manage.py loaddata data-dump.xml
}}}

Notes:
 1. It is important that you remember to use XML format in the first step of this process. We are exploiting a feature of the XML data dumps that makes porting floats to decimals with SQLite possible.
 2. In the second step you will be asked to confirm that you are prepared to lose the data for the application(s) in question. We restore this data in the third step, of course.
 3. !DecimalField is not used in any of the apps shipped with Django prior to this change being made, so you do not need to worry about performing this procedure for any of the standard Django applications.

If something goes wrong in the above process, just copy your backed up database file over the top of the original file and start again.

== !NodeLists must contain Nodes ==

Django's template rendering classes includes a !NodeList class, which is a subclass of Python's list type. Previously, a !NodeList could hold objects with a {{{render()}}} method (usually Node subclasses) or strings.

In [5482], as part of an optimisation to improve template rendering speeds, this was changed so that every object in a !NodeList must be a !Node subclass (or have the same interface). Strings are no longer allowed.

This change is unlikely to affect most code. However, if you have a custom template tag that uses !NodeList and does not populate it with Nodes, you should change your code to use normal lists instead. Django's own !ForNode class (for implementing "for loops" in templates) was the only internal code affected by this change. Consult the current implementation of {{{ForNode.iter_render}}} to see how the !NodeList usage has been replaced.