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