Reference Query Expressions Built-in Expressions

Func() expressions are the base type of all expressions that involve database functions like COALESCE and LOWER, or aggregates like SUM.

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Topic guide Performing raw SQL queries Performing raw queries

You can also execute queries containing fields that aren’t defined on the model.

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Topic guide Models Fields

Django places some restrictions on model field names: A field name cannot be a Python reserved word, because that would result in a Python syntax error.

Topic guide Creating forms from models ModelForm

As you might expect, the ForeignKey and ManyToManyField model field types are special cases: ForeignKey is represented by django.forms.ModelChoiceField, which is a ChoiceField whose choices are a model QuerySet.

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Tutorial Writing your first Django app, part 2

Now, open up mysite/settings.py. It’s a normal Python module with module-level variables representing Django settings. By default, the DATABASES configuration uses SQLite. If you’re new to databases, or you’re just interested in trying Django, this is the easiest choice.

How-to How to write custom lookups

The custom lookup above is great, but in some cases you may want to be able to chain lookups together. For example, let’s suppose we are building an application where we want to make use of the abs() operator.

How-to How to write custom lookups

The AbsoluteValue example we discussed previously is a transformation which applies to the left-hand side of the lookup. There may be some cases where you want the transformation to be applied to both the left-hand side and the right-hand side.

How-to How to create custom model fields

When planning your Field subclass, first give some thought to which existing Field class your new field is most similar to. Can you subclass an existing Django field and save yourself some work?

Tutorial Writing your first Django app, part 4

Let’s update our poll detail template (“polls/detail.html”) from the last tutorial, so that the template contains an HTML <form> element: polls/templates/polls/detail.html A quick rundown: The above template displays a radio button for each question choice.

Reference Applications Application configuration

AppConfig.name : Full Python path to the application, e.g. 'django.contrib.admin'. AppConfig.label : Short name for the application, e.g. 'admin' AppConfig.verbose_name : Human-readable name for the application, e.g. “Administration”. AppConfig.path : Filesystem path to the application directory, e.g. '/usr/lib/pythonX.Y/dist-packages/django/contrib/admin'.

Reference django-admin and manage.py Available commands

django-admin dbshell Runs the command-line client for the database engine specified in your ENGINE setting, with the connection parameters specified in your USER, PASSWORD, etc., settings. For PostgreSQL, this runs the psql command-line client.

Reference django-admin and manage.py Available commands

django-admin shell Starts the Python interactive interpreter. All models from installed apps are automatically imported into the shell environment. Models from apps listed earlier in INSTALLED_APPS take precedence.

Reference Conditional Expressions Advanced queries

What if we want to find out how many clients there are for each account_type?

Reference Model instance reference Saving objects

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.

Reference QuerySet API reference

Internally, a QuerySet can be constructed, filtered, sliced, and generally passed around without actually hitting the database. No database activity actually occurs until you do something to evaluate the queryset. You can evaluate a QuerySet in the following ways: Iteration.

Reference QuerySet API reference QuerySet APIMethods that do not return QuerySets

explain(format=None,**options) aexplain(format=None,**options) Asynchronous version: aexplain() Returns a string of the QuerySet’s execution plan, which details how the database would execute the query, including any indexes or joins that would be used.

Reference QuerySet API reference QuerySet APIAggregation functions

New in Django 6.0. classAnyValue(expression,output_field=None,filter=None,default=None,**extra)[source] : Returns an arbitrary value from the non-null input values.

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Topic guide Django’s cache framework

If you’re after even more control, you can also cache template fragments using the cache template tag. To give your template access to this tag, put {% load cache %} near the top of your template.

Topic guide Aggregation Aggregations and other QuerySet clausesvalues()

Fields that are mentioned in the order_by() part of a queryset are used when selecting the output data, even if they are not otherwise specified in the values() call.

Topic guide Aggregation Aggregations and other QuerySet clauses

When using the values() clause to group query results for annotations in MySQL with the ONLY_FULL_GROUP_BY SQL mode enabled, you may need to apply AnyValue if the annotation includes a mix of aggregate and non-aggregate expressions.

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