How do i cast char to integer while querying in django ORM?

Question

Recently started using Django ORM.I want to execute this query

 select student_id from students where student_id like \"%97318%\" order by CAST(student_id a         


        

Answer 1

I have tried extra() and annotate() to CAST, but they did not work well with related fields and generates JOINS resulting unexpected queryset sometimes.

What I ended up was to create a Custom Lookup.

The documentation is few but can be found at here and here

Here is my example:

@Field.register_lookup
class IntegerValue(Transform):
    # Register this before you filter things, for example in models.py
    lookup_name = 'int'  # Used as object.filter(LeftField__int__gte, "777")
    bilateral = True  # To cast both left and right

    def as_sql(self, compiler, connection):
        sql, params = compiler.compile(self.lhs)
        sql = 'CAST(%s AS UNSIGNED)' % sql
        return sql, params

Then below should work:

students.objects.filter(student_id__int__gte="97318").order('-student_id')

Answer 2

An updated alternative without requiring the use of extra is the cast function (new in Django 1.10):

>>> from django.db.models import FloatField
>>> from django.db.models.functions import Cast
>>> Value.objects.create(integer=4)
>>> value = Value.objects.annotate(as_float=Cast('integer', FloatField())).get()> 
>>> print(value.as_float)
4.0

From https://docs.djangoproject.com/en/1.10/ref/models/database-functions/#cast

Answer 3

Use queryset's extra() method:

students.objects.filter(student_id__contains="97318") \
                .extra({'stident_id_uint': "CAST(student_id as UNSIGNED)"}) \
                .order_by('-student_id_uint')