root/django/trunk/docs/db-api.txt

======================
Database API reference
======================

Once you've created your `data models`_, Django automatically gives you a
database-abstraction API that lets you create, retrieve, update and delete
objects. This document explains that API.

.. _`data models`: ../model-api/

Throughout this reference, we'll refer to the following models, which comprise
a weblog application::

    class Blog(models.Model):
        name = models.CharField(max_length=100)
        tagline = models.TextField()

        def __unicode__(self):
            return self.name

    class Author(models.Model):
        name = models.CharField(max_length=50)
        email = models.EmailField()

        def __unicode__(self):
            return self.name

    class Entry(models.Model):
        blog = models.ForeignKey(Blog)
        headline = models.CharField(max_length=255)
        body_text = models.TextField()
        pub_date = models.DateTimeField()
        authors = models.ManyToManyField(Author)

        def __unicode__(self):
            return self.headline

Creating objects
================

To represent database-table data in Python objects, Django uses an intuitive
system: A model class represents a database table, and an instance of that
class represents a particular record in the database table.

To create an object, instantiate it using keyword arguments to the model class,
then call ``save()`` to save it to the database.

You import the model class from wherever it lives on the Python path, as you
may expect. (We point this out here because previous Django versions required
funky model importing.)

Assuming models live in a file ``mysite/blog/models.py``, here's an example::

    from mysite.blog.models import Blog
    b = Blog(name='Beatles Blog', tagline='All the latest Beatles news.')
    b.save()

This performs an ``INSERT`` SQL statement behind the scenes. Django doesn't hit
the database until you explicitly call ``save()``.

The ``save()`` method has no return value.

To create an object and save it all in one step see the `create`__ method.

__ `create(**kwargs)`_

Auto-incrementing primary keys
------------------------------

If a model has an ``AutoField`` -- an auto-incrementing primary key -- then
that auto-incremented value will be calculated and saved as an attribute on
your object the first time you call ``save()``.

Example::

    b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
    b2.id     # Returns None, because b doesn't have an ID yet.
    b2.save()
    b2.id     # Returns the ID of your new object.

There's no way to tell what the value of an ID will be before you call
``save()``, because that value is calculated by your database, not by Django.

(For convenience, each model has an ``AutoField`` named ``id`` by default
unless you explicitly specify ``primary_key=True`` on a field. See the
`AutoField documentation`_.)

.. _AutoField documentation: ../model-api/#autofield

Explicitly specifying auto-primary-key values
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If a model has an ``AutoField`` but you want to define a new object's ID
explicitly when saving, just define it explicitly before saving, rather than
relying on the auto-assignment of the ID.

Example::

    b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
    b3.id     # Returns 3.
    b3.save()
    b3.id     # Returns 3.

If you assign auto-primary-key values manually, make sure not to use an
already-existing primary-key value! If you create a new object with an explicit
primary-key value that already exists in the database, Django will assume
you're changing the existing record rather than creating a new one.

Given the above ``'Cheddar Talk'`` blog example, this example would override
the previous record in the database::

    b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
    b4.save()  # Overrides the previous blog with ID=3!

See `How Django knows to UPDATE vs. INSERT`_, below, for the reason this
happens.

Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
objects, when you're confident you won't have primary-key collision.

What happens when you save?
---------------------------

When you save an object, Django performs the following steps:

    1. **Emit a ``pre_save`` signal.** This provides a notification that
       an object is about to be saved. You can register a listener that
       will be invoked whenever this signal is emitted. (These signals are
       not yet documented.)

    2. **Pre-process the data.** Each field on the object is asked to
       perform any automated data modification that the field may need
       to perform.

       Most fields do *no* pre-processing -- the field data is kept as-is.
       Pre-processing is only used on fields that have special behavior.
       For example, if your model has a ``DateField`` with ``auto_now=True``,
       the pre-save phase will alter the data in the object to ensure that
       the date field contains the current date stamp. (Our documentation
       doesn't yet include a list of all the fields with this "special
       behavior.")

    3. **Prepare the data for the database.** Each field is asked to provide
       its current value in a data type that can be written to the database.

       Most fields require *no* data preparation. Simple data types, such as
       integers and strings, are 'ready to write' as a Python object. However,
       more complex data types often require some modification.

       For example, ``DateFields`` use a Python ``datetime`` object to store
       data. Databases don't store ``datetime`` objects, so the field value
       must be converted into an ISO-compliant date string for insertion
       into the database.

    4. **Insert the data into the database.** The pre-processed, prepared
       data is then composed into an SQL statement for insertion into the
       database.

    5. **Emit a ``post_save`` signal.** As with the ``pre_save`` signal, this
       is used to provide notification that an object has been successfully
       saved. (These signals are not yet documented.)

Saving changes to objects
=========================

To save changes to an object that's already in the database, use ``save()``.

Given a ``Blog`` instance ``b5`` that has already been saved to the database,
this example changes its name and updates its record in the database::

    b5.name = 'New name'
    b5.save()

This performs an ``UPDATE`` SQL statement behind the scenes. Django doesn't hit
the database until you explicitly call ``save()``.

The ``save()`` method has no return value.

Saving ForeignKey and ManyToManyField fields
--------------------------------------------

Updating ``ForeignKey`` fields works exactly the same way as saving a normal
field; simply assign an object of the right type to the field in question::

    cheese_blog = Blog.objects.get(name="Cheddar Talk")
    entry.blog = cheese_blog
    entry.save()

Updating a ``ManyToManyField`` works a little differently; use the ``add()``
method on the field to add a record to the relation::

    joe = Author.objects.create(name="Joe")
    entry.authors.add(joe)

Django will complain if you try to assign or add an object of the wrong type.

How Django knows to UPDATE vs. INSERT
-------------------------------------

You may have noticed Django database objects use the same ``save()`` method
for creating and changing objects. Django abstracts the need to use ``INSERT``
or ``UPDATE`` SQL statements. Specifically, when you call ``save()``, Django
follows this algorithm:

    * If the object's primary key attribute is set to a value that evaluates to
      ``True`` (i.e., a value other than ``None`` or the empty string), Django
      executes a ``SELECT`` query to determine whether a record with the given
      primary key already exists.
    * If the record with the given primary key does already exist, Django
      executes an ``UPDATE`` query.
    * If the object's primary key attribute is *not* set, or if it's set but a
      record doesn't exist, Django executes an ``INSERT``.

The one gotcha here is that you should be careful not to specify a primary-key
value explicitly when saving new objects, if you cannot guarantee the
primary-key value is unused. For more on this nuance, see
"Explicitly specifying auto-primary-key values" above.

Retrieving objects
==================

To retrieve objects from your database, you construct a ``QuerySet`` via a
``Manager`` on your model class.

A ``QuerySet`` represents a collection of objects from your database. It can
have zero, one or many *filters* -- criteria that narrow down the collection
based on given parameters. In SQL terms, a ``QuerySet`` equates to a ``SELECT``
statement, and a filter is a limiting clause such as ``WHERE`` or ``LIMIT``.

You get a ``QuerySet`` by using your model's ``Manager``. Each model has at
least one ``Manager``, and it's called ``objects`` by default. Access it
directly via the model class, like so::

    Blog.objects  # <django.db.models.manager.Manager object at ...>
    b = Blog(name='Foo', tagline='Bar')
    b.objects     # AttributeError: "Manager isn't accessible via Blog instances."

(``Managers`` are accessible only via model classes, rather than from model
instances, to enforce a separation between "table-level" operations and
"record-level" operations.)

The ``Manager`` is the main source of ``QuerySets`` for a model. It acts as a
"root" ``QuerySet`` that describes all objects in the model's database table.
For example, ``Blog.objects`` is the initial ``QuerySet`` that contains all
``Blog`` objects in the database.

Retrieving all objects
----------------------

The simplest way to retrieve objects from a table is to get all of them.
To do this, use the ``all()`` method on a ``Manager``.

Example::

    all_entries = Entry.objects.all()

The ``all()`` method returns a ``QuerySet`` of all the objects in the database.

(If ``Entry.objects`` is a ``QuerySet``, why can't we just do ``Entry.objects``?
That's because ``Entry.objects``, the root ``QuerySet``, is a special case
that cannot be evaluated. The ``all()`` method returns a ``QuerySet`` that
*can* be evaluated.)

Filtering objects
-----------------

The root ``QuerySet`` provided by the ``Manager`` describes all objects in the
database table. Usually, though, you'll need to select only a subset of the
complete set of objects.

To create such a subset, you refine the initial ``QuerySet``, adding filter
conditions. The two most common ways to refine a ``QuerySet`` are:

``filter(**kwargs)``
    Returns a new ``QuerySet`` containing objects that match the given lookup
    parameters.

``exclude(**kwargs)``
    Returns a new ``QuerySet`` containing objects that do *not* match the given
    lookup parameters.

The lookup parameters (``**kwargs`` in the above function definitions) should
be in the format described in `Field lookups`_ below.

For example, to get a ``QuerySet`` of blog entries from the year 2006, use
``filter()`` like so::

    Entry.objects.filter(pub_date__year=2006)

(Note we don't have to add an ``all()`` -- ``Entry.objects.all().filter(...)``.
That would still work, but you only need ``all()`` when you want all objects
from the root ``QuerySet``.)

Chaining filters
~~~~~~~~~~~~~~~~

The result of refining a ``QuerySet`` is itself a ``QuerySet``, so it's
possible to chain refinements together. For example::

    Entry.objects.filter(
        headline__startswith='What').exclude(
            pub_date__gte=datetime.now()).filter(
                pub_date__gte=datetime(2005, 1, 1))

...takes the initial ``QuerySet`` of all entries in the database, adds a
filter, then an exclusion, then another filter. The final result is a
``QuerySet`` containing all entries with a headline that starts with "What",
that were published between January 1, 2005, and the current day.

Filtered QuerySets are unique
-----------------------------

Each time you refine a ``QuerySet``, you get a brand-new ``QuerySet`` that is
in no way bound to the previous ``QuerySet``. Each refinement creates a
separate and distinct ``QuerySet`` that can be stored, used and reused.

Example::

    q1 = Entry.objects.filter(headline__startswith="What")
    q2 = q1.exclude(pub_date__gte=datetime.now())
    q3 = q1.filter(pub_date__gte=datetime.now())

These three ``QuerySets`` are separate. The first is a base ``QuerySet``
containing all entries that contain a headline starting with "What". The second
is a subset of the first, with an additional criteria that excludes records
whose ``pub_date`` is greater than now. The third is a subset of the first,
with an additional criteria that selects only the records whose ``pub_date`` is
greater than now. The initial ``QuerySet`` (``q1``) is unaffected by the
refinement process.

QuerySets are lazy
------------------

``QuerySets`` are lazy -- the act of creating a ``QuerySet`` doesn't involve
any database activity. You can stack filters together all day long, and Django
won't actually run the query until the ``QuerySet`` is *evaluated*.

When QuerySets are evaluated
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can evaluate a ``QuerySet`` in the following ways:

    * **Iteration.** A ``QuerySet`` is iterable, and it executes its database
      query the first time you iterate over it. For example, this will print
      the headline of all entries in the database::

          for e in Entry.objects.all():
              print e.headline

    * **Slicing.** As explained in `Limiting QuerySets`_ below, a ``QuerySet``
      can be sliced, using Python's array-slicing syntax. Usually slicing a
      ``QuerySet`` returns another (unevaluated )``QuerySet``, but Django will
      execute the database query if you use the "step" parameter of slice
      syntax.

    * **repr().** A ``QuerySet`` is evaluated when you call ``repr()`` on it.
      This is for convenience in the Python interactive interpreter, so you can
      immediately see your results when using the API interactively.

    * **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it.
      This, as you might expect, returns the length of the result list.

      Note: *Don't* use ``len()`` on ``QuerySet``\s if all you want to do is
      determine the number of records in the set. It's much more efficient to
      handle a count at the database level, using SQL's ``SELECT COUNT(*)``,
      and Django provides a ``count()`` method for precisely this reason. See
      ``count()`` below.

    * **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on
      it. For example::

          entry_list = list(Entry.objects.all())

      Be warned, though, that this could have a large memory overhead, because
      Django will load each element of the list into memory. In contrast,
      iterating over a ``QuerySet`` will take advantage of your database to
      load data and instantiate objects only as you need them.


Pickling QuerySets
~~~~~~~~~~~~~~~~~~

If you pickle_ a ``QuerySet``, this will also force all the results to be
loaded into memory prior to pickling. This is because pickling is usually used
as a precursor to caching and when the cached queryset is reloaded, you want
the results to already be present. This means that when you unpickle a
``QuerySet``, it contains the results at the moment it was pickled, rather
than the results that are currently in the database.

If you only want to pickle the necessary information to recreate the
``Queryset`` from the database at a later time, pickle the ``query`` attribute
of the ``QuerySet``. You can then recreate the original ``QuerySet`` (without
any results loaded) using some code like this::

    >>> import pickle
    >>> query = pickle.loads(s)     # Assuming 's' is the pickled string.
    >>> qs = MyModel.objects.all()
    >>> qs.query = query            # Restore the original 'query'.

.. _pickle: http://docs.python.org/lib/module-pickle.html

Limiting QuerySets
------------------

Use Python's array-slicing syntax to limit your ``QuerySet`` to a certain
number of results. This is the equivalent of SQL's ``LIMIT`` and ``OFFSET``
clauses.

For example, this returns the first 5 objects (``LIMIT 5``)::

    Entry.objects.all()[:5]

This returns the sixth through tenth objects (``OFFSET 5 LIMIT 5``)::

    Entry.objects.all()[5:10]

You can also slice from the item ''N'' to the end of the queryset. For
example, to return everything from the sixth item onwards::

    Entry.objects.all()[5:]

How this last example is implemented in SQL varies depending upon the database
used, but it is supported in all cases.

Generally, slicing a ``QuerySet`` returns a new ``QuerySet`` -- it doesn't
evaluate the query. An exception is if you use the "step" parameter of Python
slice syntax. For example, this would actually execute the query in order to
return a list of every *second* object of the first 10::

    Entry.objects.all()[:10:2]

To retrieve a *single* object rather than a list
(e.g. ``SELECT foo FROM bar LIMIT 1``), use a simple index instead of a
slice. For example, this returns the first ``Entry`` in the database, after
ordering entries alphabetically by headline::

    Entry.objects.order_by('headline')[0]

This is roughly equivalent to::

    Entry.objects.order_by('headline')[0:1].get()

Note, however, that the first of these will raise ``IndexError`` while the
second will raise ``DoesNotExist`` if no objects match the given criteria.

QuerySet methods that return new QuerySets
------------------------------------------

Django provides a range of ``QuerySet`` refinement methods that modify either
the types of results returned by the ``QuerySet`` or the way its SQL query is
executed.

``filter(**kwargs)``
~~~~~~~~~~~~~~~~~~~~

Returns a new ``QuerySet`` containing objects that match the given lookup
parameters.

The lookup parameters (``**kwargs``) should be in the format described in
`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
underlying SQL statement.

``exclude(**kwargs)``
~~~~~~~~~~~~~~~~~~~~~

Returns a new ``QuerySet`` containing objects that do *not* match the given
lookup parameters.

The lookup parameters (``**kwargs``) should be in the format described in
`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
underlying SQL statement, and the whole thing is enclosed in a ``NOT()``.

This example excludes all entries whose ``pub_date`` is later than 2005-1-3
AND whose ``headline`` is "Hello"::

    Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3), headline='Hello')

In SQL terms, that evaluates to::

    SELECT ...
    WHERE NOT (pub_date > '2005-1-3' AND headline = 'Hello')

This example excludes all entries whose ``pub_date`` is later than 2005-1-3
OR whose headline is "Hello"::

    Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3)).exclude(headline='Hello')

In SQL terms, that evaluates to::

    SELECT ...
    WHERE NOT pub_date > '2005-1-3'
    AND NOT headline = 'Hello'

Note the second example is more restrictive.

``order_by(*fields)``
~~~~~~~~~~~~~~~~~~~~~

By default, results returned by a ``QuerySet`` are ordered by the ordering
tuple given by the ``ordering`` option in the model's ``Meta``. You can
override this on a per-``QuerySet`` basis by using the ``order_by`` method.

Example::

    Entry.objects.filter(pub_date__year=2005).order_by('-pub_date', 'headline')

The result above will be ordered by ``pub_date`` descending, then by
``headline`` ascending. The negative sign in front of ``"-pub_date"`` indicates
*descending* order. Ascending order is implied. To order randomly, use ``"?"``,
like so::

    Entry.objects.order_by('?')

Note: ``order_by('?')`` queries may be expensive and slow, depending on the
database backend you're using.

To order by a field in a different model, use the same syntax as when you are
querying across model relations. That is, the name of the field, followed by a
double underscore (``__``), followed by the name of the field in the new model,
and so on for as many models as you want to join. For example::

    Entry.objects.order_by('blog__name', 'headline')

If you try to order by a field that is a relation to another model, Django will
use the default ordering on the related model (or order by the related model's
primary key if there is no ``Meta.ordering`` specified. For example::

    Entry.objects.order_by('blog')

...is identical to::

    Entry.objects.order_by('blog__id')

...since the ``Blog`` model has no default ordering specified.

Be cautious when ordering by fields in related models if you are also using
``distinct()``. See the note in the `distinct()`_ section for an explanation
of how related model ordering can change the expected results.

It is permissible to specify a multi-valued field to order the results by (for
example, a ``ManyToMany`` field). Normally this won't be a sensible thing to
do and it's really an advanced usage feature. However, if you know that your
queryset's filtering or available data implies that there will only be one
ordering piece of data for each of the main items you are selecting, the
ordering may well be exactly what you want to do. Use ordering on multi-valued
fields with care and make sure the results are what you expect.

**New in Django development version:** If you don't want any ordering to be
applied to a query, not even the default ordering, call ``order_by()`` with no
parameters.

**New in Django development version:** The syntax for ordering across related
models has changed. See the `Django 0.96 documentation`_ for the old behavior.

.. _Django 0.96 documentation: http://www.djangoproject.com/documentation/0.96/model-api/#floatfield

There's no way to specify whether ordering should be case sensitive. With
respect to case-sensitivity, Django will order results however your database
backend normally orders them.

``reverse()``
~~~~~~~~~~~~~

**New in Django development version**

Use the ``reverse()`` method to reverse the order in which a queryset's
elements are returned. Calling ``reverse()`` a second time restores the
ordering back to the normal direction.

To retrieve the ''last'' five items in a queryset, you could do this::

    my_queryset.reverse()[:5]

Note that this is not quite the same as slicing from the end of a sequence in
Python. The above example will return the last item first, then the
penultimate item and so on. If we had a Python sequence and looked at
``seq[:-5]``, we would see the fifth-last item first. Django doesn't support
that mode of access (slicing from the end), because it's not possible to do it
efficiently in SQL.

``distinct()``
~~~~~~~~~~~~~~

Returns a new ``QuerySet`` that uses ``SELECT DISTINCT`` in its SQL query. This
eliminates duplicate rows from the query results.

By default, a ``QuerySet`` will not eliminate duplicate rows. In practice, this
is rarely a problem, because simple queries such as ``Blog.objects.all()``
don't introduce the possibility of duplicate result rows. However, if your
query spans multiple tables, it's possible to get duplicate results when a
``QuerySet`` is evaluated. That's when you'd use ``distinct()``.

.. note::
    Any fields used in an ``order_by()`` call are included in the SQL
    ``SELECT`` columns. This can sometimes lead to unexpected results when
    used in conjunction with ``distinct()``. If you order by fields from a
    related model, those fields will be added to the selected columns and they
    may make otherwise duplicate rows appear to be distinct. Since the extra
    columns don't appear in the returned results (they are only there to
    support ordering), it sometimes looks like non-distinct results are being
    returned.

    Similarly, if you use a ``values()`` query to restrict the columns
    selected, the columns used in any ``order_by()`` (or default model
    ordering) will still be involved and may affect uniqueness of the results.

    The moral here is that if you are using ``distinct()`` be careful about
    ordering by related models. Similarly, when using ``distinct()`` and
    ``values()`` together, be careful when ordering by fields not in the
    ``values()`` call.

``values(*fields)``
~~~~~~~~~~~~~~~~~~~

Returns a ``ValuesQuerySet`` -- a ``QuerySet`` that evaluates to a list of
dictionaries instead of model-instance objects.

Each of those dictionaries represents an object, with the keys corresponding to
the attribute names of model objects.

This example compares the dictionaries of ``values()`` with the normal model
objects::

    # This list contains a Blog object.
    >>> Blog.objects.filter(name__startswith='Beatles')
    [Beatles Blog]

    # This list contains a dictionary.
    >>> Blog.objects.filter(name__startswith='Beatles').values()
    [{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]

``values()`` takes optional positional arguments, ``*fields``, which specify
field names to which the ``SELECT`` should be limited. If you specify the
fields, each dictionary will contain only the field keys/values for the fields
you specify. If you don't specify the fields, each dictionary will contain a
key and value for every field in the database table.

Example::

    >>> Blog.objects.values()
    [{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}],
    >>> Blog.objects.values('id', 'name')
    [{'id': 1, 'name': 'Beatles Blog'}]

You can also retrieve values from across ``ForeignKey`` relations by using
double underscores to separate the field names, just as when calling the
``filter()`` command. For example::

    >>> Entry.objects.values('blog__name').distinct()
    [{'name': 'Beatles Blog'}]

A couple of subtleties that are worth mentioning:

    * The ``values()`` method does not return anything for ``ManyToManyField``
      attributes and will raise an error if you try to pass in this type of
      field to it.
    * If you have a field called ``foo`` that is a ``ForeignKey``, the default
      ``values()`` call will return a dictionary key called ``foo_id``, since
      this is the name of the hidden model attribute that stores the actual
      value (the ``foo`` attribute refers to the related model). When you are
      calling ``values()`` and passing in field names, you can pass in either
      ``foo`` or ``foo_id`` and you will get back the same thing (the
      dictionary key will match the field name you passed in).

      For example::

        >>> Entry.objects.values()
        [{'blog_id: 1, 'headline': u'First Entry', ...}, ...]

        >>> Entry.objects.values('blog')
        [{'blog': 1}, ...]

        >>> Entry.objects.values('blog_id')
        [{'blog_id': 1}, ...]
    * When using ``values()`` together with ``distinct()``, be aware that
      ordering can affect the results. See the note in the `distinct()`_
      section, above, for details.

**New in Django development version:** Previously, it was not possible to pass
``blog_id`` to ``values()`` in the above example, only ``blog``.

A ``ValuesQuerySet`` is useful when you know you're only going to need values
from a small number of the available fields and you won't need the
functionality of a model instance object. It's more efficient to select only
the fields you need to use.

Finally, note a ``ValuesQuerySet`` is a subclass of ``QuerySet``, so it has all
methods of ``QuerySet``. You can call ``filter()`` on it, or ``order_by()``, or
whatever. Yes, that means these two calls are identical::

    Blog.objects.values().order_by('id')
    Blog.objects.order_by('id').values()

The people who made Django prefer to put all the SQL-affecting methods first,
followed (optionally) by any output-affecting methods (such as ``values()``),
but it doesn't really matter. This is your chance to really flaunt your
individualism.

``values_list(*fields)``
~~~~~~~~~~~~~~~~~~~~~~~~

**New in Django development version**

This is similar to ``values()`` except that instead of returning a list of
dictionaries, it returns a list of tuples. Each tuple contains the value from
the respective field passed into the ``values_list()`` call -- so the first
item is the first field, etc. For example::

    >>> Entry.objects.values_list('id', 'headline')
    [(1, u'First entry'), ...]

If you only pass in a single field, you can also pass in the ``flat``
parameter. If ``True``, this will mean the returned results are single values,
rather than one-tuples. An example should make the difference clearer::

    >>> Entry.objects.values_list('id').order_by('id')
    [(1,), (2,), (3,), ...]

    >>> Entry.objects.values_list('id', flat=True).order_by('id')
    [1, 2, 3, ...]

It is an error to pass in ``flat`` when there is more than one field.

If you don't pass any values to ``values_list()``, it will return all the
fields in the model, in the order they were declared.

``dates(field, kind, order='ASC')``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Returns a ``DateQuerySet`` -- a ``QuerySet`` that evaluates to a list of
``datetime.datetime`` objects representing all available dates of a particular
kind within the contents of the ``QuerySet``.

``field`` should be the name of a ``DateField`` or ``DateTimeField`` of your
model.

``kind`` should be either ``"year"``, ``"month"`` or ``"day"``. Each
``datetime.datetime`` object in the result list is "truncated" to the given
``type``.

    * ``"year"`` returns a list of all distinct year values for the field.
    * ``"month"`` returns a list of all distinct year/month values for the field.
    * ``"day"`` returns a list of all distinct year/month/day values for the field.

``order``, which defaults to ``'ASC'``, should be either ``'ASC'`` or
``'DESC'``. This specifies how to order the results.

Examples::

    >>> Entry.objects.dates('pub_date', 'year')
    [datetime.datetime(2005, 1, 1)]
    >>> Entry.objects.dates('pub_date', 'month')
    [datetime.datetime(2005, 2, 1), datetime.datetime(2005, 3, 1)]
    >>> Entry.objects.dates('pub_date', 'day')
    [datetime.datetime(2005, 2, 20), datetime.datetime(2005, 3, 20)]
    >>> Entry.objects.dates('pub_date', 'day', order='DESC')
    [datetime.datetime(2005, 3, 20), datetime.datetime(2005, 2, 20)]
    >>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
    [datetime.datetime(2005, 3, 20)]

``none()``
~~~~~~~~~~

**New in Django development version**

Returns an ``EmptyQuerySet`` -- a ``QuerySet`` that always evaluates to
an empty list. This can be used in cases where you know that you should
return an empty result set and your caller is expecting a ``QuerySet``
object (instead of returning an empty list, for example.)

Examples::

    >>> Entry.objects.none()
    []

``all()``
~~~~~~~~~~

**New in Django development version**

Returns a ''copy'' of the current ``QuerySet`` (or ``QuerySet`` subclass you
pass in). This can be useful in some situations where you might want to pass
in either a model manager or a ``QuerySet`` and do further filtering on the
result. You can safely call ``all()`` on either object and then you'll
definitely have a ``QuerySet`` to work with.

``select_related()``
~~~~~~~~~~~~~~~~~~~~

Returns a ``QuerySet`` that will automatically "follow" foreign-key
relationships, selecting that additional related-object data when it executes
its query. This is a performance booster which results in (sometimes much)
larger queries but means later use of foreign-key relationships won't require
database queries.

The following examples illustrate the difference between plain lookups and
``select_related()`` lookups. Here's standard lookup::

    # Hits the database.
    e = Entry.objects.get(id=5)

    # Hits the database again to get the related Blog object.
    b = e.blog

And here's ``select_related`` lookup::

    # Hits the database.
    e = Entry.objects.select_related().get(id=5)

    # Doesn't hit the database, because e.blog has been prepopulated
    # in the previous query.
    b = e.blog

``select_related()`` follows foreign keys as far as possible. If you have the
following models::

    class City(models.Model):
        # ...

    class Person(models.Model):
        # ...
        hometown = models.ForeignKey(City)

    class Book(models.Model):
        # ...
        author = models.ForeignKey(Person)

...then a call to ``Book.objects.select_related().get(id=4)`` will cache the
related ``Person`` *and* the related ``City``::

    b = Book.objects.select_related().get(id=4)
    p = b.author         # Doesn't hit the database.
    c = p.hometown       # Doesn't hit the database.

    b = Book.objects.get(id=4) # No select_related() in this example.
    p = b.author         # Hits the database.
    c = p.hometown       # Hits the database.

Note that, by default, ``select_related()`` does not follow foreign keys that
have ``null=True``.

Usually, using ``select_related()`` can vastly improve performance because your
app can avoid many database calls. However, in situations with deeply nested
sets of relationships ``select_related()`` can sometimes end up following "too
many" relations, and can generate queries so large that they end up being slow.

In these situations, you can use the ``depth`` argument to ``select_related()``
to control how many "levels" of relations ``select_related()`` will actually
follow::

    b = Book.objects.select_related(depth=1).get(id=4)
    p = b.author         # Doesn't hit the database.
    c = p.hometown       # Requires a database call.

The ``depth`` argument is new in the Django development version.

**New in Django development version:** Sometimes you only need to access
specific models that are related to your root model, not all of the related
models. In these cases, you can pass the related field names to
``select_related()`` and it will only follow those relations. You can even do
this for models that are more than one relation away by separating the field
names with double underscores, just as for filters. For example, if we have
this model::

    class Room(models.Model):
        # ...
        building = models.ForeignKey(...)

    class Group(models.Model):
        # ...
        teacher = models.ForeignKey(...)
        room = models.ForeignKey(Room)
        subject = models.ForeignKey(...)

...and we only needed to work with the ``room`` and ``subject`` attributes, we
could write this::

    g = Group.objects.select_related('room', 'subject')

This is also valid::

    g = Group.objects.select_related('room__building', 'subject')

...and would also pull in the ``building`` relation.

You can only refer to ``ForeignKey`` relations in the list of fields passed to
``select_related``. You *can* refer to foreign keys that have ``null=True``
(unlike the default ``select_related()`` call). It's an error to use both a
list of fields and the ``depth`` parameter in the same ``select_related()``
call, since they are conflicting options.

``extra(select=None, where=None, params=None, tables=None, order_by=None, select_params=None)``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes, the Django query syntax by itself can't easily express a complex
``WHERE`` clause. For these edge cases, Django provides the ``extra()``
``QuerySet`` modifier -- a hook for injecting specific clauses into the SQL
generated by a ``QuerySet``.

By definition, these extra lookups may not be portable to different database
engines (because you're explicitly writing SQL code) and violate the DRY
principle, so you should avoid them if possible.

Specify one or more of ``params``, ``select``, ``where`` or ``tables``. None
of the arguments is required, but you should use at least one of them.

``select``
    The ``select`` argument lets you put extra fields in the ``SELECT`` clause.
    It should be a dictionary mapping attribute names to SQL clauses to use to
    calculate that attribute.

    Example::

        Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})

    As a result, each ``Entry`` object will have an extra attribute,
    ``is_recent``, a boolean representing whether the entry's ``pub_date`` is
    greater than Jan. 1, 2006.

    Django inserts the given SQL snippet directly into the ``SELECT``
    statement, so the resulting SQL of the above example would be::

        SELECT blog_entry.*, (pub_date > '2006-01-01')
        FROM blog_entry;


    The next example is more advanced; it does a subquery to give each
    resulting ``Blog`` object an ``entry_count`` attribute, an integer count
    of associated ``Entry`` objects::

        Blog.objects.extra(
            select={
                'entry_count': 'SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id'
            },
        )

    (In this particular case, we're exploiting the fact that the query will
    already contain the ``blog_blog`` table in its ``FROM`` clause.)

    The resulting SQL of the above example would be::

        SELECT blog_blog.*, (SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id)
        FROM blog_blog;

    Note that the parenthesis required by most database engines around
    subqueries are not required in Django's ``select`` clauses. Also note that
    some database backends, such as some MySQL versions, don't support
    subqueries.

    **New in Django development version**
    In some rare cases, you might wish to pass parameters to the SQL fragments
    in ``extra(select=...)```. For this purpose, use the ``select_params``
    parameter. Since ``select_params`` is a sequence and the ``select``
    attribute is a dictionary, some care is required so that the parameters
    are matched up correctly with the extra select pieces.  In this situation,
    you should use a ``django.utils.datastructures.SortedDict`` for the
    ``select`` value, not just a normal Python dictionary.

    This will work, for example::

        Blog.objects.extra(
            select=SortedDict(('a', '%s'), ('b', '%s')),
            select_params=('one', 'two'))

``where`` / ``tables``
    You can define explicit SQL ``WHERE`` clauses -- perhaps to perform
    non-explicit joins -- by using ``where``. You can manually add tables to
    the SQL ``FROM`` clause by using ``tables``.

    ``where`` and ``tables`` both take a list of strings. All ``where``
    parameters are "AND"ed to any other search criteria.

    Example::

        Entry.objects.extra(where=['id IN (3, 4, 5, 20)'])

    ...translates (roughly) into the following SQL::

        SELECT * FROM blog_entry WHERE id IN (3, 4, 5, 20);

    Be careful when using the ``tables`` parameter if you're specifying
    tables that are already used in the query. When you add extra tables
    via the ``tables`` parameter, Django assumes you want that table included
    an extra time, if it is already included. That creates a problem,
    since the table name will then be given an alias. If a table appears
    multiple times in an SQL statement, the second and subsequent occurrences
    must use aliases so the database can tell them apart. If you're
    referring to the extra table you added in the extra ``where`` parameter
    this is going to cause errors.

    Normally you'll only be adding extra tables that don't already appear in
    the query. However, if the case outlined above does occur, there are a few
    solutions. First, see if you can get by without including the extra table
    and use the one already in the query. If that isn't possible, put your
    ``extra()`` call at the front of the queryset construction so that your
    table is the first use of that table. Finally, if all else fails, look at
    the query produced and rewrite your ``where`` addition to use the alias
    given to your extra table. The alias will be the same each time you
    construct the queryset in the same way, so you can rely upon the alias
    name to not change.

``order_by``
    If you need to order the resulting queryset using some of the new fields
    or tables you have included via ``extra()`` use the ``order_by`` parameter
    to ``extra()`` and pass in a sequence of strings. These strings should
    either be model fields (as in the normal ``order_by()`` method on
    querysets), of the form ``table_name.column_name`` or an alias for a column
    that you specified in the ``select`` parameter to ``extra()``.

    For example::

        q = Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})
        q = q.extra(order_by = ['-is_recent'])

    This would sort all the items for which ``is_recent`` is true to the front
    of the result set (``True`` sorts before ``False`` in a descending
    ordering).

    This shows, by the way, that you can make multiple calls to
    ``extra()`` and it will behave as you expect (adding new constraints each
    time).

``params``
    The ``where`` parameter described above may use standard Python database
    string placeholders -- ``'%s'`` to indicate parameters the database engine
    should automatically quote. The ``params`` argument is a list of any extra
    parameters to be substituted.

    Example::

        Entry.objects.extra(where=['headline=%s'], params=['Lennon'])

    Always use ``params`` instead of embedding values directly into ``where``
    because ``params`` will ensure values are quoted correctly according to
    your particular backend. (For example, quotes will be escaped correctly.)

    Bad::

        Entry.objects.extra(where=["headline='Lennon'"])

    Good::

        Entry.objects.extra(where=['headline=%s'], params=['Lennon'])

**New in Django development version** The ``select_params`` argument to
``extra()`` is new. Previously, you could attempt to pass parameters for
``select`` in the ``params`` argument, but it worked very unreliably.

QuerySet methods that do not return QuerySets
---------------------------------------------

The following ``QuerySet`` methods evaluate the ``QuerySet`` and return
something *other than* a ``QuerySet``.

These methods do not use a cache (see `Caching and QuerySets`_ below). Rather,
they query the database each time they're called.

``get(**kwargs)``
~~~~~~~~~~~~~~~~~

Returns the object matching the given lookup parameters, which should be in
the format described in `Field lookups`_.

``get()`` raises ``AssertionError`` if more than one object was found.

``get()`` raises a ``DoesNotExist`` exception if an object wasn't found for the
given parameters. The ``DoesNotExist`` exception is an attribute of the model
class. Example::

    Entry.objects.get(id='foo') # raises Entry.DoesNotExist

The ``DoesNotExist`` exception inherits from
``django.core.exceptions.ObjectDoesNotExist``, so you can target multiple
``DoesNotExist`` exceptions. Example::

    from django.core.exceptions import ObjectDoesNotExist
    try:
        e = Entry.objects.get(id=3)
        b = Blog.objects.get(id=1)
    except ObjectDoesNotExist:
        print "Either the entry or blog doesn't exist."

``create(**kwargs)``
~~~~~~~~~~~~~~~~~~~~

A convenience method for creating an object and saving it all in one step.  Thus::

    p = Person.objects.create(first_name="Bruce", last_name="Springsteen")

and::

    p = Person(first_name="Bruce", last_name="Springsteen")
    p.save()

are equivalent.

``get_or_create(**kwargs)``
~~~~~~~~~~~~~~~~~~~~~~~~~~~

A convenience method for looking up an object with the given kwargs, creating
one if necessary.

Returns a tuple of ``(object, created)``, where ``object`` is the retrieved or
created object and ``created`` is a boolean specifying whether a new object was
created.

This is meant as a shortcut to boilerplatish code and is mostly useful for
data-import scripts. For example::

    try:
        obj = Person.objects.get(first_name='John', last_name='Lennon')
    except Person.DoesNotExist:
        obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
        obj.save()

This pattern gets quite unwieldy as the number of fields in a model goes up.
The above example can be rewritten using ``get_or_create()`` like so::

    obj, created = Person.objects.get_or_create(first_name='John', last_name='Lennon',
                      defaults={'birthday': date(1940, 10, 9)})

Any keyword arguments passed to ``get_or_create()`` -- *except* an optional one
called ``defaults`` -- will be used in a ``get()`` call. If an object is found,
``get_or_create()`` returns a tuple of that object and ``False``. If an object
is *not* found, ``get_or_create()`` will instantiate and save a new object,
returning a tuple of the new object and ``True``. The new object will be
created according to this algorithm::

    defaults = kwargs.pop('defaults', {})
    params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
    params.update(defaults)
    obj = self.model(**params)
  &nbs