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
Django Subdomains to do advanced things

We always struggle to give users customization's even before they login to the system like abc.micropyramid.com and django don't know how to handle that out ...

Continue Reading...
Understanding Checkout flow in Django Oscar.

Explaining Django Oscar checkout flow.

Continue Reading...
Understanding middleware functionality in django2.0

Understanding new style of middleware in Django2.0, the difference between old-style & new-style. How to write custom middlewares in new-style and upgrading from old-style middleware.

Continue Reading...

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