Retrieve Query

We already know how to retrieve all instances using Author.objects.all(). In this chapter, we'll learn how to obtain a specific instance or filter multiple instances based on certain attributes.

filter()

Author.objects.filter(last_name="Clear")

This query returns a new QuerySet containing objects that match the given lookup parameters. In our example, it will return only one Author with id=3.

The equivalent SQL command is SELECT * FROM Author WHERE last_name = 'Clear';

exclude()

Author.objects.exclude(last_name="Clear")

Conversely, exclude() returns a QuerySet with objects that do not match the specified parameters. Here, it will include all Authors whose last name isn't "Clear".

The equivalent SQL command is SELECT * FROM author WHERE last_name != 'Clear';

Combine filters

Author.objects.filter(last_name__startswith="C").exclude(first_name="Ronald") 

We can also combine filters. For instance, the query above demonstrates how to use multiple lookups, which we will explore more in the next sections.

The equivalent SQL command is SELECT * FROM author WHERE last_name LIKE 'C%' AND first_name != 'Ronald';

get()

Author.objects.get(pk=1)

To retrieve a specific object, we use the get method, where pk is a "primary key", and it's the same as id.

The equivalent SQL command is SELECT * FROM author WHERE id = 1;

Unlike get(), slicing with filter() doesn't raise an exception if there are no matching results; it simply returns an empty QuerySet.