========================== Performing raw SQL queries ========================== .. currentmodule:: django.db.models When the :doc:`model query APIs ` don't go far enough, you can fall back to writing raw SQL. Django gives you two ways of performing raw SQL queries: you can use :meth:`Manager.raw()` to `perform raw queries and return model instances`__, or you can avoid the model layer entirely and `execute custom SQL directly`__. __ `performing raw queries`_ __ `executing custom SQL directly`_ .. _executing-raw-queries: Performing raw queries ====================== .. versionadded:: 1.2 The ``raw()`` manager method can be used to perform raw SQL queries that return model instances: .. method:: Manager.raw(raw_query, params=None, translations=None) This method method takes a raw SQL query, executes it, and returns a :class:`~django.db.models.query.RawQuerySet` instance. This :class:`~django.db.models.query.RawQuerySet` instance can be iterated over just like an normal QuerySet to provide object instances. This is best illustrated with an example. Suppose you've got the following model:: class Person(models.Model): first_name = models.CharField(...) last_name = models.CharField(...) birth_date = models.DateField(...) You could then execute custom SQL like so:: >>> for p in Person.objects.raw('SELECT * FROM myapp_person'): ... print p John Smith Jane Jones Of course, this example isn't very exciting -- it's exactly the same as running ``Person.objects.all()``. However, ``raw()`` has a bunch of other options that make it very powerful. .. admonition:: Model table names Where'd the name of the ``Person`` table come from in that example? By default, Django figures out a database table name by joining the model's "app label" -- the name you used in ``manage.py startapp`` -- to the model's class name, with an underscore between them. In the example we've assumed that the ``Person`` model lives in an app named ``myapp``, so its table would be ``myapp_person``. For more details check out the documentation for the :attr:`~Options.db_table` option, which also lets you manually set the database table name. .. warning:: No checking is done on the SQL statement that is passed in to ``.raw()``. Django expects that the statement will return a set of rows from the database, but does nothing to enforce that. If the query does not return rows, a (possibly cryptic) error will result. Mapping query fields to model fields ------------------------------------ ``raw()`` automatically maps fields in the query to fields on the model. The order of fields in your query doesn't matter. In other words, both of the following queries work identically:: >>> Person.objects.raw('SELECT id, first_name, last_name, birth_date FROM myapp_person') ... >>> Person.objects.raw('SELECT last_name, birth_date, first_name, id FROM myapp_person') ... Matching is done by name. This means that you can use SQL's ``AS`` clauses to map fields in the query to model fields. So if you had some other table that had ``Person`` data in it, you could easily map it into ``Person`` instances:: >>> Person.objects.raw('''SELECT first AS first_name, ... last AS last_name, ... bd AS birth_date, ... pk as id, ... FROM some_other_table''') As long as the names match, the model instances will be created correctly. Alternatively, you can map fields in the query to model fields using the ``translations`` argument to ``raw()``. This is a dictionary mapping names of fields in the query to names of fields on the model. For example, the above query could also be written:: >>> name_map = {'first': 'first_name', 'last': 'last_name', 'bd': 'birth_date', 'pk': 'id'} >>> Person.objects.raw('SELECT * FROM some_other_table', translations=name_map) Index lookups ------------- ``raw()`` supports indexing, so if you need only the first result you can write:: >>> first_person = Person.objects.raw('SELECT * from myapp_person')[0] However, the indexing and slicing are not performed at the database level. If you have a big amount of ``Person`` objects in your database, it is more efficient to limit the query at the SQL level:: >>> first_person = Person.objects.raw('SELECT * from myapp_person LIMIT 1')[0] Deferring model fields ---------------------- Fields may also be left out:: >>> people = Person.objects.raw('SELECT id, first_name FROM myapp_person') The ``Person`` objects returned by this query will be deferred model instances (see :meth:`~django.db.models.query.QuerySet.defer()`). This means that the fields that are omitted from the query will be loaded on demand. For example:: >>> for p in Person.objects.raw('SELECT id, first_name FROM myapp_person'): ... print p.first_name, # This will be retrieved by the original query ... print p.last_name # This will be retrieved on demand ... John Smith Jane Jones From outward appearances, this looks like the query has retrieved both the first name and last name. However, this example actually issued 3 queries. Only the first names were retrieved by the raw() query -- the last names were both retrieved on demand when they were printed. There is only one field that you can't leave out - the primary key field. Django uses the primary key to identify model instances, so it must always be included in a raw query. An ``InvalidQuery`` exception will be raised if you forget to include the primary key. Adding annotations ------------------ You can also execute queries containing fields that aren't defined on the model. For example, we could use `PostgreSQL's age() function`__ to get a list of people with their ages calculated by the database:: >>> people = Person.objects.raw('SELECT *, age(birth_date) AS age FROM myapp_person') >>> for p in people: ... print "%s is %s." % (p.first_name, p.age) John is 37. Jane is 42. ... __ http://www.postgresql.org/docs/8.4/static/functions-datetime.html Passing parameters into ``raw()`` --------------------------------- If you need to perform parameterized queries, you can use the ``params`` argument to ``raw()``:: >>> lname = 'Doe' >>> Person.objects.raw('SELECT * FROM myapp_person WHERE last_name = %s', [lname]) ``params`` is a list of parameters. You'll use ``%s`` placeholders in the query string (regardless of your database engine); they'll be replaced with parameters from the ``params`` list. .. warning:: **Do not use string formatting on raw queries!** It's tempting to write the above query as:: >>> query = 'SELECT * FROM myapp_person WHERE last_name = %s' % lname >>> Person.objects.raw(query) **Don't.** Using the ``params`` list completely protects you from `SQL injection attacks`__, a common exploit where attackers inject arbitrary SQL into your database. If you use string interpolation, sooner or later you'll fall victim to SQL injection. As long as you remember to always use the ``params`` list you'll be protected. __ http://en.wikipedia.org/wiki/SQL_injection .. _executing-custom-sql: Executing custom SQL directly ============================= Sometimes even :meth:`Manager.raw` isn't quite enough: you might need to perform queries that don't map cleanly to models, or directly execute ``UPDATE``, ``INSERT``, or ``DELETE`` queries. In these cases, you can always access the database directly, routing around the model layer entirely. The object ``django.db.connection`` represents the default database connection, and ``django.db.transaction`` represents the default database transaction. To use the database connection, call ``connection.cursor()`` to get a cursor object. Then, call ``cursor.execute(sql, [params])`` to execute the SQL and ``cursor.fetchone()`` or ``cursor.fetchall()`` to return the resulting rows. After performing a data changing operation, you should then call ``transaction.commit_unless_managed()`` to ensure your changes are committed to the database. If your query is purely a data retrieval operation, no commit is required. For example:: def my_custom_sql(): from django.db import connection, transaction cursor = connection.cursor() # Data modifying operation - commit required cursor.execute("UPDATE bar SET foo = 1 WHERE baz = %s", [self.baz]) transaction.commit_unless_managed() # Data retrieval operation - no commit required cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz]) row = cursor.fetchone() return row If you are using more than one database you can use ``django.db.connections`` to obtain the connection (and cursor) for a specific database. ``django.db.connections`` is a dictionary-like object that allows you to retrieve a specific connection using its alias:: from django.db import connections cursor = connections['my_db_alias'].cursor() # Your code here... transaction.commit_unless_managed(using='my_db_alias') By default, the Python DB API will return results without their field names, which means you end up with a ``list`` of values, rather than a ``dict``. At a small performance cost, you can return results as a ``dict`` by using something like this:: def dictfetchall(cursor): "Returns all rows from a cursor as a dict" desc = cursor.description return [ dict(zip([col[0] for col in desc], row)) for row in cursor.fetchall() ] Here is an example of the difference between the two:: >>> cursor.execute("SELECT id, parent_id from test LIMIT 2"); >>> cursor.fetchall() ((54360982L, None), (54360880L, None)) >>> cursor.execute("SELECT id, parent_id from test LIMIT 2"); >>> dictfetchall(cursor) [{'parent_id': None, 'id': 54360982L}, {'parent_id': None, 'id': 54360880L}] .. _transactions-and-raw-sql: Transactions and raw SQL ------------------------ When you make a raw SQL call, Django will automatically mark the current transaction as dirty. You must then ensure that the transaction containing those calls is closed correctly. See :ref:`the notes on the requirements of Django's transaction handling ` for more details. .. versionchanged:: 1.3 Prior to Django 1.3, it was necessary to manually mark a transaction as dirty using ``transaction.set_dirty()`` when using raw SQL calls. Connections and cursors ----------------------- ``connection`` and ``cursor`` mostly implement the standard Python DB-API described in :pep:`249` (except when it comes to :doc:`transaction handling `). If you're not familiar with the Python DB-API, note that the SQL statement in ``cursor.execute()`` uses placeholders, ``"%s"``, rather than adding parameters directly within the SQL. If you use this technique, the underlying database library will automatically add quotes and escaping to your parameter(s) as necessary. (Also note that Django expects the ``"%s"`` placeholder, *not* the ``"?"`` placeholder, which is used by the SQLite Python bindings. This is for the sake of consistency and sanity.)