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.

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Posted On 22 January 2016 By MicroPyramid


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