Reference Databases MySQL notesConnecting to the database

When running concurrent loads, database transactions from different sessions (say, separate threads handling different requests) may interact with each other. These interactions are affected by each session’s transaction isolation level.

Reference Databases MySQL notes

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.

Reference Databases Oracle notes

LOB columns may not be used in a SELECT DISTINCT list. This means that attempting to use the QuerySet.distinct method on a model that includes TextField columns will result in an ORA-00932 error when run against Oracle.

Reference Query Expressions Built-in ExpressionsSubquery() expressions

There are times when a single column must be returned from a Subquery, for instance, to use a Subquery as the target of an __in lookup.

Reference Query Expressions Built-in Expressions

You can specify multiple windows in the same query which in Django ORM would be equivalent to including multiple expressions in a QuerySet.annotate() call. The ORM doesn’t make use of named windows, instead they are part of the selected columns.

Reference Query Expressions Technical Information

Query expressions implement the query expression API, but also expose a number of extra methods and attributes listed below. All query expressions must inherit from Expression() or a relevant subclass.

Reference Query Expressions Technical Information

As an example, an expression implementing `NULLS LAST` would change its value to be `NULLS FIRST`. Modifications are only required for expressions that implement sort order like `OrderBy`. This method is called when [`reverse()`](querysets.md#django.db.models.query.QuerySet.reverse) is called on a queryset.

Reference Model field reference Field types

classDateField(auto_now=False,auto_now_add=False,**options) A date, represented in Python by a datetime.date instance. Has a few extra, optional arguments: DateField.auto_now : Automatically set the field to now every time the object is saved. Useful for “last-modified” timestamps.

Reference Model field reference Relationship fieldsManyToManyField

ForeignKey.limit_choices_to : Sets a limit to the available choices for this field when this field is rendered using a ModelForm or the admin (by default, all objects in the queryset are available to choose).

Reference Model field reference

classField : Field is an abstract class that represents a database table column. Django uses fields to create the database table (db_type()), to map Python types to database (get_prep_value()) and vice-versa (from_db_value()).

Reference

Model field reference Field attribute reference Model index reference Constraints reference Model _meta API Related objects reference Model class reference Model Meta options Model instance reference QuerySet API reference Lookup API reference Query Expressions Conditional Expressions Database Functions

Reference

This document describes the details of the Model API. It builds on the material presented in the model and database query guides, so you’ll probably want to read and understand those documents before reading this one.

Reference Model instance reference Saving objects

If save() is passed a list of field names in keyword argument update_fields, only the fields named in that list will be updated.

Reference Model instance reference Other attributes

Model._state : The _state attribute refers to a ModelState object that tracks the lifecycle of the model instance.

Reference Lookup API reference

classLookup : A Lookup is a generic class to implement lookups.

Reference Model Meta options Available Meta options

Options.get_latest_by : The name of a field or a list of field names in the model, typically DateField, DateTimeField, or IntegerField. This specifies the default field(s) to use in your model Manager’s latest() and earliest() methods.

Reference

classRelatedManager : A “related manager” is a manager used in a one-to-many or many-to-many related context. This happens in two cases:

Reference

Using `remove()` with a many-to-many relationship, however, will delete the relationships using [`QuerySet.delete()`](querysets.md#django.db.models.query.QuerySet.delete) which means no model `save()` methods are called; listen to the [`m2m_changed`](../signals.md#django.db.models.signals.m2m_changed) signal if you wish to execute custom code when a relationship is deleted.

Reference Paginator Page class

Paginator.ELLIPSIS : A translatable string used as a substitute for elided page numbers in the page range returned by get_elided_page_range(). Default is '…'. Paginator.count : The total number of objects, across all pages. Paginator.num_pages : The total number of pages.

Reference Signals Model signals

WARNING: For performance reasons, you shouldn’t perform queries in receivers of > pre_init or post_init signals because they would be executed for > each instance returned during queryset iteration.