Topic guide Working with forms

In HTML, a form is a collection of elements inside <form>...</form> that allow a visitor to do things like enter text, select options, manipulate objects or controls, and so on, and then send that information back to the server.

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Reference Databases Oracle notes

The Oracle backend stores TextFields as NCLOB columns. Oracle imposes some limitations on the usage of such LOB columns in general: LOB columns may not be used as primary keys. LOB columns may not be used in indexes.

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Reference Form fields Built-in Field classes

classNullBooleanField(**kwargs)[source] : - Default widget: NullBooleanSelect - Empty value: None - Normalizes to: A Python True, False or None value. - Validates nothing (i.e., it never raises a ValidationError).

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Reference Query Expressions Built-in Expressions

classRawSQL(sql,params,output_field=None)[source] Sometimes database expressions can’t easily express a complex WHERE clause. In these edge cases, use the RawSQL expression.

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Reference Model field reference Field options

Field.choices[source] A mapping or iterable in the format described below to use as choices for this field.

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How-to How to write custom lookups

When using the above written abs lookup, the SQL produced will not use indexes efficiently in some cases. In particular, when we use change__abs__lt=27, this is equivalent to change__gt=-27 AND change__lt=27.

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Internals Organization of the Django Project

The steering council selects Mergers as necessary to maintain their number at a minimum of three, in order to spread the workload and avoid over-burdening or burning out any individual Merger.

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Reference GeoDjango Installation

The OSGeo4W installer helps to install the PROJ, GDAL, and GEOS libraries required by GeoDjango. First, download the OSGeo4W installer, and run it. Select Express Web-GIS Install and click next. In the ‘Select Packages’ list, ensure that GDAL is selected.

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Reference GeoDjango Tutorial

When doing spatial queries, GeoDjango automatically transforms geometries if they’re in a different coordinate system.

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Reference The messages framework

The messages framework can use different backends to store temporary messages. Django provides three built-in storage classes in django.contrib.messages: classstorage.session.SessionStorage : This class stores all messages inside of the request’s session. Therefore it requires Django’s contrib.sessions application.

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Reference Databases

By default, the Oracle backend uses a RETURNING INTO clause to efficiently retrieve the value of an AutoField when inserting new rows.

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Reference django-admin and manage.py

The colors used for syntax highlighting can be customized. Django ships with three color palettes: dark, suited to terminals that show white text on a black background. This is the default palette.

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Reference Form fields

classNullBooleanField(**kwargs)[source] : - Default widget: NullBooleanSelect - Empty value: None - Normalizes to: A Python True, False or None value. - Validates nothing (i.e., it never raises a ValidationError).

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Reference Form fields

Two fields are available for representing relationships between models: ModelChoiceField and ModelMultipleChoiceField. Both of these fields require a single queryset parameter that is used to create the choices for the field.

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Reference Form fields

By default, ModelChoiceField and ModelMultipleChoiceField use ModelChoiceIterator to generate their field choices. When iterated, ModelChoiceIterator yields 2-tuple choices containing ModelChoiceIteratorValue instances as the first value element in each choice.

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Reference Widgets

These widgets make use of the HTML elements <select>, <input type="checkbox">, and <input type="radio">. Widgets that render multiple choices have an option_template_name attribute that specifies the template used to render each choice.

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Reference Query 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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Reference Query Expressions

classSubquery(queryset,output_field=None)[source] You can add an explicit subquery to a QuerySet using the Subquery expression.

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Reference Query Expressions

classExists(queryset)[source] Exists is a Subquery subclass that uses an SQL EXISTS statement. In many cases it will perform better than a subquery since the database is able to stop evaluation of the subquery when a first matching row is found.

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Reference Query Expressions

Subquery() that returns a boolean value and Exists() may be used as a condition in When expressions, or to directly filter a queryset: This will ensure that the subquery will not be added to the SELECT columns, which may result …

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