Django values_list vs values

In Django, when querying the database, the QuerySet API offers two powerful methods for fetching specific fields from the database: values() and values_list(). Both methods return query results in a more efficient format compared to fetching entire model instances, which is especially useful when we're interested in only a subset of fields. However, they differ in the structure of the data they return.

In this article, we will explore both values_list() and values(), comparing their functionalities, use cases, and performance considerations.

Overview of values() and values_list()

• values(): Returns a QuerySet where each result is a dictionary, with the field names as keys and their corresponding values as the dictionary values.
• values_list(): Returns a QuerySet where each result is a tuple (or a flat list, if specified), containing the values of the fields in the specified order.

Both methods allow we to specify the fields we want to retrieve from the database, which makes them useful for optimizing performance when we don't need the entire object.

Basic Examples of values() and values_list()

Assume we have the following Django model:

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=255)
    author = models.CharField(max_length=255)
    publication_date = models.DateField()
    price = models.DecimalField(max_digits=5, decimal_places=2)
	
    def __str__(self):
      	return self.title

1. values()

The values() method returns a list of dictionaries, where each dictionary represents a record and contains key-value pairs corresponding to the fields requested.

Example:

# Fetching all books with their title and author
books = Book.objects.values('title', 'author')

Output:

>>> Book.objects.values('title', 'authors') 
<QuerySet [
{'title': 'Understanding Django URL Routing', 'author': 'Arun Kumar'},
{'title': 'A Guide to Django Models', 'author': 'Aditya Shakya'},
{'title': 'Introduction to Django Templates', 'author': 'Aman Raj'},
{'title': 'Advanced Django Querysets', 'authors': 'Uday Rana'},
{'title': 'Deploying Django Applications', 'authors': 'Samay Khan'},
{'title': 'Authenticate User while Testing', 'authors': 'Geeskforgeeks'}]>

Here, each book record is represented as a dictionary with the keys being the field names and the values being the actual data from the database.

Use Case for values()

The values() method is useful when we need to work with named fields in our Django views or templates. It is especially helpful when we want to avoid the overhead of retrieving entire

2. values_list()

The values_list() method returns a list of tuples (or flat lists, if specified). Each tuple contains the values for the fields in the order they are specified.

Example:

# Fetching all books with their title and author
books = Book.objects.values_list('title', 'author')

Output:

>>> Book.objects.values_list('title', 'authors') 
<QuerySet [
('Understanding Django URL Routing', 'Arun Kumar'),
('A Guide to Django Models', 'Aditya Shakya'),
('Introduction to Django Templates', 'Aman Raj'),
('Advanced Django Querysets', 'Uday Rana'),
('Deploying Django Applications', 'Samay Khan')]>

In this case, each book record is represented as a tuple containing the values of the specified fields. The field names are not present in the output.

Use Case for values_list()

The values_list() method is useful when we don't need field names and we just want the values in a compact format. This is particularly helpful for reducing memory usage in cases where we are dealing with large datasets.

3. Flat values_list()

By default, values_list() returns a list of tuples. However, if we are retrieving a single field, we can pass the flat=True argument to get a list of values instead of tuples.

<QuerySet [
'Understanding Django URL Routing',
'A Guide to Django Models',
'Introduction to Django Templates',
'Advanced Django Querysets',
'Deploying Django Applications',
'Authenticate User while Testing']>

The flat=True option is very useful when we need just one field and prefer to work with a simple list instead of a list of tuples.

Key Differences Between values() and values_list()

1. Return Type

• values(): Returns a list of dictionaries.
• values_list(): Returns a list of tuples (or a list if flat=True).

2. Field Naming

• values(): The field names are included as dictionary keys.
• values_list(): The field names are not included; only the values are returned in the order specified.

3. Readability and Usage

• values(): Better for scenarios where we need to reference fields by name, such as rendering data in templates or working in views where key-value pairs are needed.
• values_list(): Better for scenarios where the field names are not important, and we need to access the values in a compact and efficient format, such as performing batch operations.

4. Performance Considerations

In general, values_list() tends to be slightly more efficient than values(), especially when used with flat=True, since no dictionary objects need to be created. However, the difference in performance is usually small unless we are working with large datasets.

When to Use values() vs values_list()

Use values() when:

• We need to access fields by name (e.g., in templates, JSON APIs).
• We need to query multiple fields and work with key-value pairs.
• Readability of the result is important for our use case.

Use values_list() when:

• We don't need field names and just want the values.
• We are dealing with large datasets and need a more memory-efficient representation.
• We are working with a single field and want to use the flat=True option for a list of values.

Conclusion

Both values() and values_list() are valuable tools in Django's ORM when we want to optimize the retrieval of specific fields from the database. The choice between the two depends on our use case:

• Use values() when we need field names and key-value pairs.
• Use values_list() when we only need the values and want a more compact and memory-efficient format.

Understanding the difference between these two methods and knowing when to use each can significantly improve both the performance and readability of our Django code.