how to optimize the query here with select_related?

Question

i have

class A(models.Model):
    field_a = models.OneToOneField(B, on_delete=models.CASCADE)

class B(models.Model):
    field_b = models.charField()

how to write the most optimum query here using select_related or something else? In my use case i need to use both, field_a of model A and field_b of model B somewhere in my code. So can i fetch both the objects in one db hit?

queryset_b = B.objects.get(field_b="some name")
queryset_a = A.objects.get(b=queryset_b).field_a
field_a_of_model = queryset_a
field_b_of_model = queryset_b.field_b
# then some manipulation with both fields

it hits the db twice. Can i do this thing in one db hit using select_related so that it fetches both object at once?

iklinac · Accepted Answer · 2020-04-20T11:23:52.543

0

You could use lookup that span relationship

a = A.objects.get(b__field_b="some name")

This would result in single query to database

If you want single query to access fields from b too use select_related()

a = A.objects.select_related().get(b__field_b="some name")
a.field_a.field_b

edited Apr 20 '20 at 11:23

answered Apr 20 '20 at 10:22

iklinac

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Sorry for the incomplete question. I also want to access fields of model B. I have updated the question. – coderelliot Apr 20 '20 at 11:03
what if in `model B` i have another `foreignkey field` `field_c` so...how can i fetch that object also in this query? because now if i do `a.field_a.field_c` will again make a db query. – coderelliot Apr 20 '20 at 11:49
what do you mean by fetch – iklinac Apr 20 '20 at 11:50
by fetch i mean that only one query can get the object of field_c also in cache. like i said..`a.field_a.field_c` should not make another db query – coderelliot Apr 20 '20 at 11:53
select_related('field_b__field_c') – iklinac Apr 20 '20 at 11:58
you have following in documentation i have attached regarding select_related – iklinac Apr 20 '20 at 11:59

how to optimize the query here with select_related?

1 Answers1