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

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Querying with Django Q objects

Django Q objects:

The following code is source of Q class:

Dynamic querying with Q objects:

To use and operations simply execute:

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