How to fetch single column from database in Django | Django values_list querysets
How to Fetch Single or Column Data from Database in Django ORM
In Django, with the help of the values_list() method, you can fetch single or multiple column data from your database efficiently.
Introduction
The values_list() method is similar to values(), except that instead of returning dictionaries, it returns tuples when iterated over. Each tuple contains the value from the respective field passed into the values_list() call.
Example Model
Your Posts model is like this:
class Posts(models.Model):
title = models.CharField(max_length=100)
description = models.TextField(max_length=155)
class Meta:
verbose_name_plural = "Posts"
def __str__(self):
return self.titleBasic Usage
# Fetch single column data
Posts.objects.values_list('id',)
# Fetch more than one column data
Posts.objects.values_list('id', 'title',)
# You can also fetch in order_by
Posts.objects.values_list('id', 'title',).order_by('?')Note: You can also pass in a flat parameter if you only pass in a single field. It will return true/false values.
Understanding flat=True
If flat=True, the returned results are single values, rather than one-tuples. Let's look at an example to make the difference clear:
# Without flat=True
Posts.objects.values_list('id')
# Output: [(1,), (2,), (3,), ...]
# With flat=True
Posts.objects.values_list('id', flat=True)
# Output: [1, 2, 3, ...]Important Limitation
values_list raises an error if you try to pass in flat=True when there is more than one field.
# This will raise an error
Posts.objects.values_list('id', 'title', flat=True)
Important Points
- All Fields: If you don't pass any values to
values_list(), it will return all the fields in the model, in the order they were declared. - QuerySet Behavior:
values_listmethod returns aValuesListQuerySetthat behaves like a list. If you require an actual Python list object, you can calllist()on it. - Getting Specific Values: A common requirement is to get the specific field value of a certain model instance. Use
values_list()and then call theget()method.
Example of getting a specific value:
# Code
Posts.objects.values_list('title', flat=True).get(pk=1)
# Output
'First entry'values_list() vs values()
| Feature | values_list() | values() |
|---|---|---|
| Return Type | Tuples | Dictionaries |
| Memory Usage | Lower (more efficient) | Higher |
| Best For | Getting specific field values | Getting labeled data |
Summary
values_list() is a powerful and efficient method for fetching specific columns from your database. Use it when you need specific field values, especially with flat=True for single fields to get clean, flat lists of values.
Remember: flat=True only works with single fields, and calling values_list() without arguments returns all fields in declaration order.
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