ORM Queries - Advanced Django Concepts

What are ORM Queries?

ORM stands for Object Relational Mapping, which is a programming technique that allows developers to interact with databases using object-oriented programming languages like Python instead of SQL. Django ORM is a powerful tool that allows developers to interact with databases without needing to write SQL queries.

In Django, ORM queries involve retrieving data from the database, filtering it, and selecting certain fields to be returned. These are done using Django querysets.

Syntax

The syntax for using Django ORM queries involves chaining methods together to retrieve data from the database. Here is a basic syntax for a query:

queryset = Model.objects.filter(fieldname__condition=value).values('fieldname')

Example

Let's suppose we have a database that contains information about employees, including their names, salaries and job titles. Here is an example of a query that retrieves the names and salaries of all employees who have a job title of 'Software Engineer' and whose salary is greater than $100,000:

queryset = Employee.objects.filter(job_title='Software Engineer', salary__gt=100000).values('name', 'salary')

Output

The output of the above query would be a queryset object containing a list of dictionaries, each representing an employee with their name and salary:

<QuerySet [{'name': 'John Doe', 'salary': 125000.00}, {'name': 'Mary Smith', 'salary': 120000.00}]>

Explanation

Let's break down the query above to understand how it works:

• We start by calling the Employee.objects manager to access all employees in the Employee model.
• We then filter the queryset to only include employees with a job title of 'Software Engineer' and whose salary is greater than $100,000 using filter(job_title='Software Engineer', salary__gt=100000) method.
• Finally, we call the values() method to define the fields we want to retrieve ('name' and 'salary') and return a list of dictionaries containing those fields.

Use

ORM queries are used to retrieve data from databases without having to write SQL queries directly. They simplify data access and management and make it easier for developers to interact with databases.

Important Points

• Django querysets allow developers to interact with databases using Python instead of SQL.
• Querysets can be filtered, ordered, sliced, and annotated to retrieve specific data from databases.
• Querysets are only evaluated when data is required, making them efficient for handling large datasets.

Summary

Django ORM queries are a powerful tool for developers to interact with databases using Python. They simplify data access and management and make it easier for developers to interact with databases without needing to write SQL queries. Using querysets, developers can filter, slice, and order data to retrieve specific information from databases.