Querying with Django Q objects

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.

To Know more about our Django CRM(Customer Relationship Management) Open Source Package. Check Code

Posted On 22 January 2016 By MicroPyramid


Need any Help in your Project?Let's Talk

Latest Comments
Related Articles
Django Template language Intro

We can write the required logics based on programming[python] syntax in models and views but, when we want to write simple logics we should follow ...

Continue Reading...
Django Single Sign On(SSO) to multiple applications

Single sign on is a way for users to issue a security token for the first time login, login into multiple applications using one set ...

Continue Reading...
Preserve file names with sorl for better SEO

We use sorl-thumbnail for scaling images by keeping the original one intact. Sorl proven to be great tool for generating different sized images throughout website. ...

Continue Reading...
open source packages

Subscribe To our news letter

Subscribe and Stay Updated about our Webinars, news and articles on Django, Python, Machine Learning, Amazon Web Services, DevOps, Salesforce, ReactJS, AngularJS, React Native.
* We don't provide your email contact details to any third parties