Querying with Django Q Objects2022-07-25
Querying with 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.
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:
Q objects not only simplify complex queries, they are very handy for dynamic filtering.
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