Q Objects and F Objects

Introduction

Objects refer to the model's Manager, whose sole purpose is to handle database queries to retrieve the required data from a database. A django-objects>-tag with multiple object>-elements represents the entire collection of objects. Each of these objects has two properties: "pk" and "model".

Q Objects in Django

The Django Q object (django.db.models.Q) is a container for keyword arguments. It is primarily used for complex queries that require logical operations. The keyword arguments are encapsulated by the Q object and passed to query methods such as filter, exclude, and get. Multiple Q objects can be combined using the AND, OR, or NOT operators. The Q object is imported in the following way:

Code Example

Let us consider an example. First, we create a model.

Register the model in the theadmin.pyfile.

In the interactive console, after importing the models, let us add some records.

Now we want to use the Q object for searching a book whose name contains the letter 'y'.

Django allows the use of **OR, AND, or NOT **operators to be used along with the Django q object.

a. OR Case

Say we want to search for a book that contains the letter 'y' or a book with a price being less than 1000.

lt in the above query stands for less than. Now we want to search for a book whose name contains the letter 'd' or a book that contains pages more than 80.

gt in the above query stands for greater than.

b. AND case

Say we want to search for a book that starts with the word 'life' and contains 89 pages only.

Now we want to search for a book whose name contains the letter d and has pages less than 50.

Since there is no book matching the conditions, therefore the query set returned is empty.

c. NOT case

Say we want to find a book whose name contains the letter d and the pages are not less than 50.

F objects

An F() object represents the value of a model field, its transformed value, or an annotated column. It is primarily used to compare the model class's A and B field attributes, i.e. to operate on the value of a column in the database. The F object is imported in the following way:

The syntax for the following is given below.

Code Example of F Object

Let us consider the example we used in the above section. Say we want to increase the price of the books by 20.

In the code above we took an output variable and under F we entered the column we want to modify. Now let us view the price of the books.

In the code above we took an output variable and stored the list of the books, then we started a for loop and printed the name of the book along with their price we modified in the code earlier.

Uses of F Object

To Avoid Race Condition

If the code in the first example is executed by two Python threads, one thread could retrieve it from the database and the other would increment and save it. The value saved by the second thread will be based on the original value; the first thread's work will simply be lost. If the database is in charge of updating the field, the process is more robust: it will only update the field based on its value in the database when the save() or update() function is executed, rather than its value when the instance was retrieved.

Using F() to Sort Null Values

To control the ordering of a field's null values, we can use the F () and nulls_first or nulls_last keyword arguments to asc() or desc().

In the above code, the records with a null field price will be present at the end.

func() expression

Func() expressions are used in database functions like COALESCE and LOWER, as well as aggregates like SUM. We first import the same before using it in the command prompt.