testbed/django__django/docs/faq/models.txt

=========================
FAQ: Databases and models
=========================

.. _faq-see-raw-sql-queries:

How can I see the raw SQL queries Django is running?
====================================================

Make sure your Django :setting:`DEBUG` setting is set to ``True``.
Then do this:

.. code-block:: pycon

    >>> 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 :setting:`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.

If you are using :doc:`multiple databases</topics/db/multi-db>`, you can use the
same interface on each member of the ``connections`` dictionary:

.. code-block:: pycon

    >>> from django.db import connections
    >>> connections["my_db_alias"].queries

If you need to clear the query list manually at any point in your functions,
call ``reset_queries()``, like this::

    from django.db import reset_queries

    reset_queries()

Can I use Django with a preexisting database?
=============================================

Yes. See :doc:`Integrating with a legacy database </howto/legacy-databases>`.

If I make changes to a model, how do I update the database?
===========================================================

Take a look at Django's support for :mod:`schema migrations
<django.db.migrations>`.

If you don't mind clearing data, your project's ``manage.py`` utility has a
:djadmin:`flush` option to reset the database to the state it was in
immediately after :djadmin:`migrate` was executed.

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 single value to specify
an object to edit or delete.

Does Django support NoSQL databases?
====================================

NoSQL databases are not officially supported by Django itself. There are,
however, a number of side projects and forks which allow NoSQL functionality in
Django.

You can take a look on `the wiki page`_ which discusses some projects.

.. _the wiki page: https://code.djangoproject.com/wiki/NoSqlSupport

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 a migration with a
:class:`~django.db.migrations.operations.RunSQL` operation that contains
``ALTER TABLE`` statements that do what you want to do.

For example, if you're using MySQL and want your tables to use the MyISAM table
type, use the following SQL:

.. code-block:: sql

    ALTER TABLE myapp_mytable ENGINE=MyISAM;