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8. How to filter a queryset with criteria based on comparing their field values
Django ORM makes it easy to filter based on fixed values.
To get all User objects with first_name starting with 'R',
you can do User.objects.filter(first_name__startswith='R').
What if you want to compare the first_name and last name?
You can use the F object. Create some users first.
In [27]: User.objects.create_user(email="shabda@example.com", username="shabda", first_name="Shabda", last_name="Raaj")
Out[27]: <User: shabda>
In [28]: User.objects.create_user(email="guido@example.com", username="Guido", first_name="Guido", last_name="Guido")
Out[28]: <User: Guido>
Now you can find the users where first_name==last_name
In [29]: User.objects.filter(last_name=F("first_name"))
Out[29]: <QuerySet [<User: Guido>]>
F also works with calculated field using annotate. What if we wanted users whose first and last names have same letter?
You can set the first letter from a string using Substr("first_name", 1, 1), so we do.
In [41]: User.objects.create_user(email="guido@example.com", username="Tim", first_name="Tim", last_name="Teters")
Out[41]: <User: Tim>
#...
In [46]: User.objects.annotate(first=Substr("first_name", 1, 1), last=Substr("last_name", 1, 1)).filter(first=F("last"))
Out[46]: <QuerySet [<User: Guido>, <User: Tim>]>
F can also be used with __gt, __lt and other expressions.