This chapter provides additional information on each of the supported relational databases in Django, as well as notes and tips and tricks for connecting to legacy databases.
Django attempts to support as many features as possible on all database back-ends. However, not all database back-ends are alike, and the Django developers had to make design decisions on which features to support and which assumptions could be made safely.
This file describes some of the features that might be relevant to Django usage. Of course, it is not intended as a replacement for server-specific documentation or reference manuals.
Persistent connections avoid the overhead of re-establishing a connection to the database in each request. They’re controlled by the
CONN_MAX_AGE parameter which defines the maximum lifetime of a connection. It can be set independently for each database. The default value is
0, preserving the historical behavior of closing the database connection at the end of each request. To enable persistent connections, set
CONN_MAX_AGE to a positive number of seconds. For unlimited persistent connections, set it to
Django opens a connection to the database when it first makes a database query. It keeps this connection open and reuses it in subsequent requests. Django closes the connection once it exceeds the maximum age defined by
CONN_MAX_AGE or when it isn’t usable any longer.
In detail, Django automatically opens a connection to the database whenever it needs one and doesn’t have one already – either because this is the first connection, or because the previous connection was closed.
At the beginning of each request, Django closes the connection if it has reached its maximum age. If your database terminates idle connections after some time, you should set
CONN_MAX_AGE to a lower value, so that Django doesn’t attempt to use a connection that has been terminated by the database server. (This problem may only affect very low traffic sites.)
At the end of each request, Django closes the connection if it has reached its maximum age or if it is in an unrecoverable error state. If any database errors have occurred while processing the requests, Django checks whether the connection still works, and closes it if it doesn’t. Thus, database errors affect at most one request; if the connection becomes unusable, the next request gets a fresh connection.
Since each thread maintains its own connection, your database must support at least as many simultaneous connections as you have worker threads.
Sometimes a database won’t be accessed by the majority of your views, for example because it’s the database of an external system, or thanks to caching. In such cases, you should set
CONN_MAX_AGE to a low value or even
0, because it doesn’t make sense to maintain a connection that’s unlikely to be reused. This will help keep the number of simultaneous connections to this database small.
The development server creates a new thread for each request it handles, negating the effect of persistent connections. Don’t enable them during development.
When Django establishes a connection to the database, it sets up appropriate parameters, depending on the backend being used. If you enable persistent connections, this setup is no longer repeated every request. If you modify parameters such as the connection’s isolation level or time zone, you should either restore Django’s defaults at the end of each request, force an appropriate value at the beginning of each request, or disable persistent connections.
Django assumes that all databases use UTF-8 encoding. Using other encodings may result in unexpected behavior such as value too long errors from your database for data that is valid in Django. See the database specific notes below for information on how to set up your database correctly.
Django supports PostgreSQL 9.0 and higher.
It requires the use of Psycopg2 2.0.9 or higher.
Django needs the following parameters for its database connections:
'read committed'by default, or the value set in the connection options (see below)
True, value of
If these parameters already have the correct values, Django won’t set them for every new connection, which improves performance slightly. You can configure them directly in
postgresql.conf or more conveniently per database user with
Django will work just fine without this optimization, but each new connection will do some additional queries to set these parameters.
Like PostgreSQL itself, Django defaults to the
READ COMMITTED isolation level. If you need a higher isolation level such as
REPEATABLE READ or
SERIALIZABLE, set it in the
OPTIONS part of your database configuration in
Under higher isolation levels, your application should be prepared to handle exceptions raised on serialization failures. This option is designed for advanced uses.
db_index=True on your model fields, Django typically outputs a single
CREATE INDEX statement. However, if the database type for the field is either
text (e.g., used by
TextField), then Django will create an additional index that uses an appropriate PostgreSQL operator class for the column. The extra index is necessary to correctly perform lookups that use the
LIKE operator in their SQL, as is done with the
startswith lookup types.
Django supports MySQL 5.5 and higher.
inspectdb feature uses the
information_schema database, which contains detailed data on all database schemas.
Django expects the database to support Unicode (UTF-8 encoding) and delegates to it the task of enforcing transactions and referential integrity. It is important to be aware of the fact that the two latter ones aren’t actually enforced by MySQL when using the MyISAM storage engine, see the next section.
MySQL has several storage engines. You can change the default storage engine in the server configuration.
Until MySQL 5.5.4, the default engine was MyISAM. The main drawbacks of MyISAM are that it doesn’t support transactions or enforce foreign-key constraints. On the plus side, it was the only engine that supported full-text indexing and searching until MySQL 5.6.4.
Since MySQL 5.5.5, the default storage engine is InnoDB. This engine is fully transactional and supports foreign key references. It’s probably the best choice at this point. However, note that the InnoDB autoincrement counter is lost on a MySQL restart because it does not remember the
AUTO_INCREMENT value, instead recreating it as
max(id)+1. This may result in an inadvertent reuse of
If you upgrade an existing project to MySQL 5.5.5 and subsequently add some tables, ensure that your tables are using the same storage engine (i.e. MyISAM vs. InnoDB). Specifically, if tables that have a
ForeignKey between them use different storage engines, you may see an error like the following when running
The Python Database API is described in PEP 249. MySQL has three prominent drivers that implement this API:
MySQLdbwhich notably supports Python 3 and can be used as a drop-in replacement for MySQLdb. At the time of this writing, this is the recommended choice for using MySQL with Django.
All these drivers are thread-safe and provide connection pooling.
is the only one not supporting Python 3 currently.
In addition to a DB API driver, Django needs an adapter to access the database drivers from its ORM. Django provides an adapter for MySQLdb/mysqlclient while MySQL Connector/Python includes its own.
Django requires MySQLdb version 1.2.1p2 or later.
If you see
ImportError: cannot import name ImmutableSet when trying to use Django, your MySQLdb installation may contain an outdated
sets.py file that conflicts with the built-in module of the same name from Python 2.4 and later. To fix this, verify that you have installed MySQLdb version 1.2.1p2 or newer, then delete the
sets.py file in the MySQLdb directory that was left by an earlier version.
There are known issues with the way MySQLdb converts date strings into datetime objects. Specifically, date strings with value
0000-00-00 are valid for MySQL but will be converted into
None by MySQLdb.
This means you should be careful while using loaddata/dumpdata with rows that may have
0000-00-00 values, as they will be converted to
At the time of writing, the latest release of MySQLdb (1.2.4) doesn’t support Python 3. In order to use MySQLdb under Python 3, you’ll have to install
Django requires mysqlclient 1.3.3 or later.
Note that Python 3.2 is not supported. Except for the Python 3.3+ support,
mysqlclient should mostly behave the same as MySQLdb.
MySQL Connector/Python is available from the download page. The Django adapter is available in versions 1.1.X and later. It may not support the most recent releases of Django.
If you plan on using Django’s timezone support, use
mysql_tzinfo_to_sql to load time zone tables into the MySQL database. This needs to be done just once for your MySQL server, not per database.
You can create your database using the command-line tools and this SQL:
This ensures all tables and columns will use UTF-8 by default.
The collation setting for a column controls the order in which data is sorted as well as what strings compare as equal. It can be set on a database-wide level and also per-table and per-column. This is documented thoroughly in the MySQL documentation. In all cases, you set the collation by directly manipulating the database tables; Django doesn’t provide a way to set this on the model definition.
By default, with a UTF-8 database, MySQL will use the
utf8_general_ci collation. This results in all string equality comparisons being done in a case-insensitive manner. That is, “
Fred” and “
freD” are considered equal at the database level. If you have a unique constraint on a field, it would be illegal to try to insert both “
aa” and “
AA” into the same column, since they compare as equal (and, hence, non-unique) with the default collation.
In many cases, this default will not be a problem. However, if you really want case-sensitive comparisons on a particular column or table, you would change the column or table to use the
utf8_bin collation. The main thing to be aware of in this case is that if you are using MySQLdb 1.2.2, the database backend in Django will then return bytestrings (instead of Unicode strings) for any character fields it receives from the database. This is a strong variation from Django’s normal practice of always
returning Unicode strings.
It is up to you, the developer, to handle the fact that you will receive bytestrings if you configure your table(s) to use
utf8_bin collation. Django itself should mostly work smoothly with such columns (except for the
LogEntry tables described below), but your code must be prepared to call
django.utils.encoding.smart_text() at times if it really wants to work with consistent data – Django will not do this for you (the database backend layer and the model population layer are separated internally so the database layer doesn’t know it needs to make this conversion in this one particular case).
If you’re using MySQLdb 1.2.1p2, Django’s standard
CharField class will return Unicode strings even with
utf8_bin collation. However,
TextField fields will be returned as an
array.array instance (from Python’s standard
array module). There isn’t a lot Django can do about that, since, again, the information needed to make the necessary conversions isn’t available when the data is read in from the database. This problem was fixed in MySQLdb 1.2.2, so if you want to use
utf8_bin collation, upgrading to version 1.2.2 and then dealing with the bytestrings (which shouldn’t be too difficult) as described above is the recommended solution.
Should you decide to use
utf8_bin collation for some of your tables with MySQLdb 1.2.1p2 or 1.2.2, you should still use
utf8_general_ci (the default) collation for the
django.contrib.sessions.models.Session table (usually called
django_session) and the
django.contrib.admin.models.LogEntry table (usually called
django_admin_log). Please note that according to MySQL Unicode Character Sets, comparisons for the
utf8_general_ci collation are faster, but slightly less correct, than comparisons for
utf8_unicode_ci. If this is acceptable for your application, you should use
utf8_general_ci because it is faster. If this is not acceptable (for example, if you require German dictionary order), use
utf8_unicode_ci because it is more accurate.
Connection settings are used in this order:
In other words, if you set the name of the database in
OPTIONS, this will take precedence over
NAME, which would override anything in a MySQL option file. Here’s a sample configuration which uses a MySQL option file:
Several other MySQLdb connection options may be useful, such as
sql_mode. Consult the MySQLdb documentation for more details.
When Django generates the schema, it doesn’t specify a storage engine, so tables will be created with whatever default storage engine your database server is configured for.
The easiest solution is to set your database server’s default storage engine to the desired engine.
If you’re using a hosting service and can’t change your server’s default storage engine, you have a couple of options:
ALTER TABLEstatement to convert a table to a new storage engine (such as InnoDB):
This can be tedious if you have a lot of tables.
init_commandoption for MySQLdb prior to creating your tables:
This sets the default storage engine upon connecting to the database. After your tables have been created, you should remove this option as it adds a query that is only needed during table creation to each database connection.
There are known issues in even the latest versions of MySQL that can cause the case of a table name to be altered when certain SQL statements are executed under certain conditions. It is recommended that you use lowercase table names, if possible, to avoid any problems that might arise from this behavior. Django uses lowercase table names when it auto-generates table names from models, so this is mainly a consideration if you are overriding the table name via the
Both the Django ORM and MySQL (when using the InnoDB storage engine) support database savepoints.
If you use the MyISAM storage engine, please be aware of the fact that you will receive database-generated errors if you try to use the savepoint-related methods of the transactions API. The reason for this is that detecting the storage engine of a MySQL database/table is an expensive operation so it was decided it isn’t worth to dynamically convert these methods in no-op’s based in the results of such detection.
Any fields that are stored with
VARCHAR column types have their
max_length restricted to 255 characters if you are using
unique=True for the field. This affects
MySQL 5.6.4 and later can store fractional seconds, provided that the column definition includes a fractional indication (e.g.
DATETIME(6)). Earlier versions do not support them at all. In addition, versions of MySQLdb older than 1.2.5 have a bug that also prevents the use of fractional seconds with MySQL.
Django will not upgrade existing columns to include fractional seconds if the database server supports it. If you want to enable them on an existing database, it’s up to you to either manually update the column on the target database, by executing a command like:
or using a
RunSQL operation in a
By default, new
TimeField columns are now created with fractional seconds support on MySQL 5.6.4 or later with either mysqlclient or MySQLdb 1.2.5 or later.
If you are using a legacy database that contains
TIMESTAMP columns, you must set
USE_TZ = False to avoid data corruption.
inspectdb maps these columns to
DateTimeField and if you enable timezone support, both MySQL and Django will attempt to convert the values from UTC to local time.
MySQL does not support the
NOWAIT option to the
SELECT ... FOR UPDATE statement. If
select_for_update() is used with
nowait=True then a
DatabaseError will be raised.
When performing a query on a string type, but with an integer value, MySQL will coerce the types of all values in the table to an integer before performing the comparison. If your table contains the values “
def” and you query for
WHERE mycolumn=0, both rows will match. Similarly,
WHERE mycolumn=1 will match the value “
abc1”. Therefore, string type fields included in Django will always cast the value to a string before using it in a query.
If you implement custom model fields that inherit from
Field directly, are overriding
get_prep_value(), or use
raw(), you should ensure that you perform the appropriate typecasting.
SQLite provides an excellent development alternative for applications that are predominantly read-only or require a smaller installation footprint. As with all database servers, though, there are some differences that are specific to SQLite that you should be aware of.
For all SQLite versions, there is some slightly counter-intuitive behavior when attempting to match some types of strings. These are triggered when using the
contains filters in Querysets. The behavior splits into two cases:
name__contains="aa") will match a name of “Aabb”.
iexactfilter will behave exactly the same as the exact filter in these cases.
Some possible workarounds for this are documented at sqlite.org, but they aren’t utilized by the default SQLite backend in Django, as incorporating them would be fairly difficult to do robustly. Thus, Django exposes the default SQLite behavior and you should be aware of this when doing case-insensitive or substring filtering.
SQLite 188.8.131.52 and older contains a bug when handling query parameters in a
CASE expression that contains an
ELSE and arithmetic.
SQLite 184.108.40.206 was released in March 2010, and most current binary distributions for different platforms include a newer version of SQLite, with the notable exception of the Python 2.7 installers for Windows.
As of this writing, the latest release for Windows – Python 2.7.10 – includes SQLite 3.6.21. You can install
pysqlite2 or replace
sqlite3.dll (by default installed in
C:\Python27\DLLs) with a newer version from sqlite.org to remedy this issue.
Django will use a
pysqlite2 module in preference to
sqlite3 as shipped with the Python standard library if it finds one is available.
This provides the ability to upgrade both the DB-API 2.0 interface or SQLite 3 itself to versions newer than the ones included with your particular Python binary distribution, if needed.
SQLite is meant to be a lightweight database, and thus can’t support a high level of concurrency.
OperationalError: database is locked errors indicate that your application is experiencing more concurrency than
sqlite can handle in default configuration. This error means that one thread or process has an exclusive lock on the database connection and another thread timed out waiting for the lock the be released.
Python’s SQLite wrapper has a default timeout value that determines how long the second thread is allowed to wait on the lock before it times out and raises the
OperationalError: database is locked error.
If you’re getting this error, you can solve it by:
This will simply make SQLite wait a bit longer before throwing database is locked errors; it won’t really do anything to solve them.
SQLite does not support the
SELECT ... FOR UPDATE syntax. Calling it will have no effect.
For most backends, raw queries (
cursor.execute()) can use the pyformat parameter style, where placeholders in the query are given as
'%(name)s' and the parameters are passed as a dictionary rather than a list. SQLite does not support this.
sqlite3 does not provide a way to retrieve the SQL after quoting and substituting the parameters. Instead, the SQL in
connection.queries is rebuilt with a simple string interpolation. It may be incorrect. Make sure you add quotes where necessary before copying a query into an SQLite shell.
Django supports Oracle Database Server versions 11.1 and higher. Version 4.3.1 or higher of the
cx_Oracle Python driver is required, although we recommend version 5.1.3 or later as these versions support Python 3.
Note that due to a Unicode-corruption bug in
cx_Oracle 5.0, that version of the driver should not be used with Django;
cx_Oracle 5.0.1 resolved this issue, so if you’d like to use a more recent
cx_Oracle, use version 5.0.1.
cx_Oracle 5.0.1 or greater can optionally be compiled with the
WITH_UNICODE environment variable. This is recommended but not required.
In order for the
python manage.py migrate command to work, your Oracle database user must have privileges to run the following commands:
To run a project’s test suite, the user usually needs these additional privileges:
CREATE SESSION WITH ADMIN OPTION
CREATE TABLE WITH ADMIN OPTION
CREATE SEQUENCE WITH ADMIN OPTION
CREATE PROCEDURE WITH ADMIN OPTION
CREATE TRIGGER WITH ADMIN OPTION
Note that, while the
RESOURCE role has the required
CREATE PROCEDURE and
CREATE TRIGGER privileges, and a user granted
RESOURCE WITH ADMIN OPTION can grant
RESOURCE, such a user cannot grant the individual privileges (e.g.
CREATE TABLE), and thus
RESOURCE WITH ADMIN OPTION is not usually sufficient for running tests.
Some test suites also create views; to run these, the user also needs the
CREATE VIEW WITH ADMIN OPTION privilege. In particular, this is needed for Django’s own test suite.
All of these privileges are included in the DBA role, which is appropriate for use on a private developer’s database.
The Oracle database backend uses the
SYS.DBMS_LOB package, so your user will require execute permissions on it. It’s normally accessible to all users by default, but in case it is not, you’ll need to grant permissions like so:
To connect using the service name of your Oracle database, your
settings.py file should look something like this:
In this case, you should leave both
PORT empty. However, if you don’t use a
tnsnames.ora file or a similar naming method and want to connect using the SID (
xe in this example), then fill in both
PORT like so:
You should either supply both
PORT, or leave both as empty strings. Django will use a different connect descriptor depending on that choice.
If you plan to run Django in a multithreaded environment (e.g. Apache using the default MPM module on any modern operating system), then you must set the
threaded option of your Oracle database configuration to True:
Failure to do this may result in crashes and other odd behavior.
By default, the Oracle backend uses a
RETURNING INTO clause to efficiently retrieve the value of an
AutoField when inserting new rows. This behavior may result in a
DatabaseError in certain unusual setups, such as when inserting into a remote table, or into a view with an
INSTEAD OF trigger.
RETURNING INTO clause can be disabled by setting the
use_returning_into option of the database configuration to False:
In this case, the Oracle backend will use a separate
SELECT query to retrieve
Oracle imposes a name length limit of 30 characters.
To accommodate this, the backend truncates database identifiers to fit, replacing the final four characters of the truncated name with a repeatable MD5 hash value. Additionally, the backend turns database identifiers to all-uppercase.
To prevent these transformations (this is usually required only when dealing with legacy databases or accessing tables which belong to other users), use a quoted name as the value for
Quoted names can also be used with Django’s other supported database backends; except for Oracle, however, the quotes have no effect.
ORA-06552 error may be encountered if certain Oracle keywords are used as the name of a model field or the value of a
db_column option. Django quotes all identifiers used in queries to prevent most such problems, but this error can still occur when an Oracle datatype is used as a column name. In particular, take care to avoid using the names
float as a field name.
Django generally prefers to use the empty string (‘’ ”) rather than
NULL, but Oracle treats both identically. To get around this, the Oracle backend ignores an explicit
null option on fields that have the empty string as a possible value and generates DDL as if
null=True. When fetching from the database, it is assumed that a
NULL value in one of these fields really means the empty string, and the data is silently converted to reflect this assumption.
The Oracle backend stores
NCLOB columns. Oracle imposes some limitations on the usage of such LOB columns in general:
SELECT DISTINCTlist. This means that attempting to use the
QuerySet.distinctmethod on a model that includes
TextFieldcolumns will result in an error when run against Oracle. As a workaround, use the
QuerySet.defermethod in conjunction with
TextFieldcolumns from being included in the
In addition to the officially supported databases, there are backends provided by 3rd parties that allow you to use other databases with Django:
The Django versions and ORM features supported by these unofficial backends vary considerably. Queries regarding the specific capabilities of these unofficial backends, along with any support queries, should be directed to the support channels provided by each 3rd party project.
While Django is best suited for developing new applications, it’s quite possible to integrate it into legacy databases. Django includes a couple of utilities to automate as much of this process as possible.
Once you’ve got Django set up, you’ll follow this general process to integrate with an existing database.
You’ll need to tell Django what your database connection parameters are, and what the name of the database is. Do that by editing the
DATABASES setting and assigning values to the following keys for the
Django comes with a utility called
inspectdb that can create models by introspecting an existing database. You can view the output by running this command:
Save this as a file by using standard Unix output redirection (this works in Windows too):
This feature is meant as a shortcut, not as definitive model generation. See the documentation of
inspectdb for more information.
Once you’ve cleaned up your models, name the file
models.py and put it in the Python package that holds your app. Then add the app to your
inspectdb creates unmanaged models. That is,
managed = False in the model’s
Meta class tells Django not to manage each table’s creation, modification, and deletion:
If you do want to allow Django to manage the table’s lifecycle, you’ll need to change the
managed option above to
True (or simply remove it because
True is its default value).
Next, run the
migrate command to install any extra needed database records such as admin permissions and content types:
As you might expect, the database introspection isn’t perfect, and you’ll need to do some light clean-up of the resulting model code. Here are a few pointers for dealing with the generated models:
Not only are these lines redundant, but also they can cause problems if your application will be adding new records to these tables.
DateField) is determined by looking at the database column type (e.g.,
inspectdbcannot map a column’s type to a model field type, it will use
TextFieldand will insert the Python comment
'This field type is a guess.'next to the field in the generated model. Keep an eye out for that, and change the field type accordingly if needed.
inspectdbwill append “
_field” to the attribute name and set the
db_columnattribute to the real field name (e.g.,
for).For example, if a table has an
for, the generated model will have a field like this:
will insert the Python comment ‘Field renamed because it was a Python reserved word.’` next to the field.
Authorshould be defined before model
Book. If you need to create a relationship on a model that has not yet been defined, you can use a string containing the name of the model, rather than the model object itself.
inspectdbdetects primary keys for PostgreSQL, MySQL, and SQLite. That is, it inserts
primary_key=Truewhere appropriate. For other databases, you’ll need to insert
primary_key=Truefor at least one field in each model, because Django models are required to have a
IntegerField’s, assuming the foreign-key column was an
Those are the basic steps – from here you’ll want to tweak the models Django generated until they work the way you’d like. Try accessing your data via the Django database API, and try editing objects via Django’s admin site, and edit the models file accordingly.
That’s all folks!
I hope you have enjoyed reading Mastering Django: Core and have learned a lot from the book. While this book will serve you as a complete reference to Django, there is still no substitute for plain old practice – so get coding and all the best with your Django career!
The remaining chapters are purely for your reference. They include appendices and quick references for all of the functions and fields in Django.