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django/docs/faq/models.txt
Malcolm Tredinnick a0d4749920 Fixed #9468 -- Fixed a misleading FAQ answer.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@9428 bcc190cf-cafb-0310-a4f2-bffc1f526a37
2008-11-14 07:38:27 +00:00

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.. _faq-models:
FAQ: Databases and models
=========================
How can I see the raw SQL queries Django is running?
----------------------------------------------------
Make sure your Django ``DEBUG`` setting is set to ``True``. Then, just do
this::
>>> from django.db import connection
>>> connection.queries
[{'sql': 'SELECT polls_polls.id,polls_polls.question,polls_polls.pub_date FROM polls_polls',
'time': '0.002'}]
``connection.queries`` is only available if ``DEBUG`` is ``True``. It's a list
of dictionaries in order of query execution. Each dictionary has the following::
``sql`` -- The raw SQL statement
``time`` -- How long the statement took to execute, in seconds.
``connection.queries`` includes all SQL statements -- INSERTs, UPDATES,
SELECTs, etc. Each time your app hits the database, the query will be recorded.
Can I use Django with a pre-existing database?
----------------------------------------------
Yes. See :ref:`Integrating with a legacy database <howto-legacy-databases>`.
If I make changes to a model, how do I update the database?
-----------------------------------------------------------
If you don't mind clearing data, your project's ``manage.py`` utility has an
option to reset the SQL for a particular application::
manage.py reset appname
This drops any tables associated with ``appname`` and recreates them.
If you do care about deleting data, you'll have to execute the ``ALTER TABLE``
statements manually in your database. That's the way we've always done it,
because dealing with data is a very sensitive operation that we've wanted to
avoid automating. That said, there's some work being done to add partially
automated database-upgrade functionality.
Do Django models support multiple-column primary keys?
------------------------------------------------------
No. Only single-column primary keys are supported.
But this isn't an issue in practice, because there's nothing stopping you from
adding other constraints (using the ``unique_together`` model option or
creating the constraint directly in your database), and enforcing the
uniqueness at that level. Single-column primary keys are needed for things such
as the admin interface to work; e.g., you need a simple way of being able to
specify an object to edit or delete.
How do I add database-specific options to my CREATE TABLE statements, such as specifying MyISAM as the table type?
------------------------------------------------------------------------------------------------------------------
We try to avoid adding special cases in the Django code to accommodate all the
database-specific options such as table type, etc. If you'd like to use any of
these options, create an :ref:`SQL initial data file <initial-sql>` that
contains ``ALTER TABLE`` statements that do what you want to do. The initial
data files are executed in your database after the ``CREATE TABLE`` statements.
For example, if you're using MySQL and want your tables to use the MyISAM table
type, create an initial data file and put something like this in it::
ALTER TABLE myapp_mytable ENGINE=MyISAM;
As explained in the :ref:`SQL initial data file <initial-sql>` documentation,
this SQL file can contain arbitrary SQL, so you can make any sorts of changes
you need to make.
Why is Django leaking memory?
-----------------------------
Django isn't known to leak memory. If you find your Django processes are
allocating more and more memory, with no sign of releasing it, check to make
sure your ``DEBUG`` setting is set to ``False``. If ``DEBUG`` is ``True``, then
Django saves a copy of every SQL statement it has executed.
(The queries are saved in ``django.db.connection.queries``. See
`How can I see the raw SQL queries Django is running?`_.)
To fix the problem, set ``DEBUG`` to ``False``.
If you need to clear the query list manually at any point in your functions,
just call ``reset_queries()``, like this::
from django import db
db.reset_queries()