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256 lines
8.3 KiB
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256 lines
8.3 KiB
Plaintext
==================
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Database Functions
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==================
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.. module:: django.db.models.functions
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:synopsis: Database Functions
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.. versionadded:: 1.8
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The classes documented below provide a way for users to use functions provided
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by the underlying database as annotations, aggregations, or filters in Django.
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Functions are also :doc:`expressions <expressions>`, so they can be used and
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combined with other expressions like :ref:`aggregate functions
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<aggregation-functions>`.
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We'll be using the following model in examples of each function::
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class Author(models.Model):
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name = models.CharField(max_length=50)
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age = models.PositiveIntegerField(null=True, blank=True)
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alias = models.CharField(max_length=50, null=True, blank=True)
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goes_by = models.CharField(max_length=50, null=True, blank=True)
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We don't usually recommend allowing ``null=True`` for ``CharField`` since this
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allows the field to have two "empty values", but it's important for the
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``Coalesce`` example below.
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Coalesce
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--------
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.. class:: Coalesce(*expressions, **extra)
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Accepts a list of at least two field names or expressions and returns the
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first non-null value (note that an empty string is not considered a null
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value). Each argument must be of a similar type, so mixing text and numbers
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will result in a database error.
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Usage examples::
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>>> # Get a screen name from least to most public
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>>> from django.db.models import Sum, Value as V
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>>> from django.db.models.functions import Coalesce
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>>> Author.objects.create(name='Margaret Smith', goes_by='Maggie')
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>>> author = Author.objects.annotate(
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... screen_name=Coalesce('alias', 'goes_by', 'name')).get()
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>>> print(author.screen_name)
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Maggie
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>>> # Prevent an aggregate Sum() from returning None
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>>> aggregated = Author.objects.aggregate(
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... combined_age=Coalesce(Sum('age'), V(0)),
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... combined_age_default=Sum('age'))
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>>> print(aggregated['combined_age'])
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0
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>>> print(aggregated['combined_age_default'])
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None
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.. warning::
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A Python value passed to ``Coalesce`` on MySQL may be converted to an
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incorrect type unless explicitly cast to the correct database type:
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>>> from django.db.models.expressions import RawSQL
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>>> from django.utils import timezone
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>>> now = timezone.now()
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>>> now_sql = RawSQL("cast(%s as datetime)", (now,))
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>>> Coalesce('updated', now_sql)
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Concat
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------
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.. class:: Concat(*expressions, **extra)
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Accepts a list of at least two text fields or expressions and returns the
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concatenated text. Each argument must be of a text or char type. If you want
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to concatenate a ``TextField()`` with a ``CharField()``, then be sure to tell
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Django that the ``output_field`` should be a ``TextField()``. This is also
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required when concatenating a ``Value`` as in the example below.
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This function will never have a null result. On backends where a null argument
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results in the entire expression being null, Django will ensure that each null
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part is converted to an empty string first.
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Usage example::
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>>> # Get the display name as "name (goes_by)"
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>>> from django.db.models import CharField, Value as V
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>>> from django.db.models.functions import Concat
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>>> Author.objects.create(name='Margaret Smith', goes_by='Maggie')
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>>> author = Author.objects.annotate(
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... screen_name=Concat('name', V(' ('), 'goes_by', V(')'),
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... output_field=CharField())).get()
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>>> print(author.screen_name)
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Margaret Smith (Maggie)
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Greatest
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--------
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.. class:: Greatest(*expressions, **extra)
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.. versionadded:: 1.9
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Accepts a list of at least two field names or expressions and returns the
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greatest value. Each argument must be of a similar type, so mixing text and
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numbers will result in a database error.
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Usage example::
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class Blog(models.Model):
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body = models.TextField()
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modified = models.DateTimeField(auto_now=True)
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class Comment(models.Model):
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body = models.TextField()
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modified = models.DateTimeField(auto_now=True)
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blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
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>>> from django.db.models.functions import Greatest
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>>> blog = Blog.objects.create(body='Greatest is the best.')
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>>> comment = Comment.objects.create(body='No, Least is better.', blog=blog)
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>>> comments = Comment.objects.annotate(last_updated=Greatest('modified', 'blog__modified'))
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>>> annotated_comment = comments.get()
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``annotated_comment.last_updated`` will be the most recent of ``blog.modified``
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and ``comment.modified``.
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.. warning::
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The behavior of ``Greatest`` when one or more expression may be ``null``
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varies between databases:
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- PostgreSQL: ``Greatest`` will return the largest non-null expression,
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or ``null`` if all expressions are ``null``.
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- SQLite, Oracle, and MySQL: If any expression is ``null``, ``Greatest``
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will return ``null``.
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The PostgreSQL behavior can be emulated using ``Coalesce`` if you know
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a sensible minimum value to provide as a default.
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Least
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-----
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.. class:: Least(*expressions, **extra)
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.. versionadded:: 1.9
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Accepts a list of at least two field names or expressions and returns the
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least value. Each argument must be of a similar type, so mixing text and numbers
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will result in a database error.
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.. warning::
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The behavior of ``Least`` when one or more expression may be ``null``
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varies between databases:
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- PostgreSQL: ``Least`` will return the smallest non-null expression,
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or ``null`` if all expressions are ``null``.
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- SQLite, Oracle, and MySQL: If any expression is ``null``, ``Least``
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will return ``null``.
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The PostgreSQL behavior can be emulated using ``Coalesce`` if you know
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a sensible maximum value to provide as a default.
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Length
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------
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.. class:: Length(expression, **extra)
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Accepts a single text field or expression and returns the number of characters
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the value has. If the expression is null, then the length will also be null.
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Usage example::
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>>> # Get the length of the name and goes_by fields
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>>> from django.db.models.functions import Length
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>>> Author.objects.create(name='Margaret Smith')
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>>> author = Author.objects.annotate(
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... name_length=Length('name'),
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... goes_by_length=Length('goes_by')).get()
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>>> print(author.name_length, author.goes_by_length)
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(14, None)
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Lower
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------
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.. class:: Lower(expression, **extra)
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Accepts a single text field or expression and returns the lowercase
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representation.
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Usage example::
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>>> from django.db.models.functions import Lower
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>>> Author.objects.create(name='Margaret Smith')
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>>> author = Author.objects.annotate(name_lower=Lower('name')).get()
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>>> print(author.name_lower)
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margaret smith
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Now
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---
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.. class:: Now()
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.. versionadded:: 1.9
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Returns the database server's current date and time when the query is executed,
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typically using the SQL ``CURRENT_TIMESTAMP``.
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Usage example::
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>>> from django.db.models.functions import Now
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>>> Article.objects.filter(published__lte=Now())
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[<Article: How to Django>]
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.. admonition:: PostgreSQL considerations
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On PostgreSQL, the SQL ``CURRENT_TIMESTAMP`` returns the time that the
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current transaction started. Therefore for cross-database compatibility,
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``Now()`` uses ``STATEMENT_TIMESTAMP`` instead. If you need the transaction
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timestamp, use :class:`django.contrib.postgres.functions.TransactionNow`.
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Substr
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------
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.. class:: Substr(expression, pos, length=None, **extra)
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Returns a substring of length ``length`` from the field or expression starting
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at position ``pos``. The position is 1-indexed, so the position must be greater
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than 0. If ``length`` is ``None``, then the rest of the string will be returned.
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Usage example::
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>>> # Set the alias to the first 5 characters of the name as lowercase
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>>> from django.db.models.functions import Substr, Lower
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>>> Author.objects.create(name='Margaret Smith')
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>>> Author.objects.update(alias=Lower(Substr('name', 1, 5)))
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1
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>>> print(Author.objects.get(name='Margaret Smith').alias)
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marga
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Upper
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------
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.. class:: Upper(expression, **extra)
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Accepts a single text field or expression and returns the uppercase
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representation.
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Usage example::
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>>> from django.db.models.functions import Upper
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>>> Author.objects.create(name='Margaret Smith')
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>>> author = Author.objects.annotate(name_upper=Upper('name')).get()
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>>> print(author.name_upper)
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MARGARET SMITH
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