Querying with Django Q objects

Reading Time : ~ .

Django Q objects:

     Q object encapsulates a SQL expression in a Python object that can be used in database-related operations. Using Q objects we can make complex queries with less and simple code.

For example, this Q object filters whether the question starts wiht 'what':

from django.db.models import Q
Q(question__startswith='What')

Q objects are helpfull for complex queries because they can be combined using logical operators and(&), or(|), negation(~)

For example, this statement returns if the question starts with 'who' or with 'what'.

Q(question__startswith='Who') | Q(question__startswith='What')

Note:
    If the operator is not included then by default 'AND' operator is used

The following code is source of Q class:

class Q(tree.Node):
    AND = 'AND'
    OR = 'OR'
    default = AND

    def __init__(self, *args, **kwargs):
        super(Q, self).__init__(children=list(args) + list(six.iteritems(kwargs)))

    def _combine(self, other, conn):
        if not isinstance(other, Q):
            raise TypeError(other)
        obj = type(self)()
        obj.connector = conn
        obj.add(self, conn)
        obj.add(other, conn)
        return obj

    def __or__(self, other):
        return self._combine(other, self.OR)

    def __and__(self, other):
        return self._combine(other, self.AND)

    def __invert__(self):
        obj = type(self)()
        obj.add(self, self.AND)
        obj.negate()
        return obj

As you can interpret from above code, we have three operators 'or', 'and' and invert(negation) and the default operator is AND.

Dynamic querying with Q objects:

    This is an interesting feature as we can use the operator module to create dynamic queries. 

import operator
from django.db.models import Q
from your_app.models import your_model_object


q_list = [Q(question__startswith='Who'), Q(question__startswith='What')]
your_model_object.objects.filter(reduce(operator.or_, q_list))

We are performing the or operation using operator.or_

To use and operations simply execute:

your_model_object.objects.filter(reduce(operator.and_, q_list))

Q objects not only simplify complex queries, they are very handy for dynamic filtering.

 

 

 

 

 

 

 

 

 

 

 

 

 

    By Posted On
SENIOR DEVELOPER at MICROPYRAMID

Need any Help in your Project?Let's Talk

Latest Comments
Related Articles
What's great about Django girls to inspire women into programming Vinisha Naladala

Django girls is a non-profit organization, that helps women to learn Django programming language and to inspire them into programming. They are organizing workshops all ...

Continue Reading...
Retrieve average, minimum, maximum values from model field using Django Aggregation Ravi Kumar Gadila

To retrieve maximum, minimum or average values from group of rows we can use django Aggregation. For example to retrieve max price or avg price ...

Continue Reading...
How To Write Custom migrations in Django Shirisha Gaddi

This blog post will help you to get deeper into Django Migrations - how Migrations works and how to create or add our custom migrations. ...

Continue Reading...

Subscribe To our news letter

Subscribe to our news letter to receive latest blog posts into your inbox. Please fill your email address in the below form.
*We don't provide your email contact details to any third parties