When the 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 Manager.raw() to perform raw queries and return model instances, or you can avoid the model layer entirely and execute custom SQL directly.
The raw() manager method can be used to perform raw SQL queries that return model instances:
This method method takes a raw SQL query, executes it, and returns a RawQuerySet instance. This 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.
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 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.
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)
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]
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 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
... 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.
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.
...
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.
Sometimes even 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}]
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 the notes on the requirements of Django’s transaction handling for more details.
connection and cursor mostly implement the standard Python DB-API described in PEP 249 (except when it comes to 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.)
Dec 23, 2012