Fetch multiple rows from multiple models Django

Question

I have this project where, I need to fetch multiple objects from multiple models in a view. I could do that by for loop but I think I shouldn't hit database in each loop. Should I use prefetch_related. or Should I know some other way to retrieve them.

for example:

class A(models.Model):
     title_name=models.CharField(...)
     id=models.AutoField(pk=True)

class B(models.Model):
     user=models.ForeignKey(User,models.ON_CASCADE=True)
     user_status=models.CharField(...)
     id=models.ForeignKey(A, models.ON_CASCADE=True)

I need to display user_status, user and associated title_name. I get multiple objects, select_related will not be useful. Any suggestions.

Yes, you should use prefetch_related. There is no better way than that — Sardorbek Imomaliev
– Sardorbek Imomaliev, Commented Jun 22, 2017 at 5:57
@SardorbekImomaliev I have tried prefetch_related , i couldn't exactly do what i wanted to, I've made a query something like B.objects.get(user=request.user).prefetch_related('id') — joelvarma
– joelvarma, Commented Jun 22, 2017 at 6:07
see this one: stackoverflow.com/questions/3046398/… you can implement function in class which you want to call in view and in property you can use function to fetch related values to your model — Gahan
– Gahan, Commented Jun 22, 2017 at 6:08

Artem Sudoma · Accepted Answer · 2017-06-22 06:10:15Z

1

You need make this queryset:

B.objects.all().select_related('user', 'id')

This queryset will generate sql that join user and A data from db

Next, in the model B to make a property:

@property
def title_name(self):
    return self.id.title_name

Finally you'll get queryset that makes one SQL request to database and returns all data you need.

By the way, I would rename attribute "id" in the model B to "a".

answered Jun 22, 2017 at 6:10

Artem Sudoma

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3 Comments

joelvarma Over a year ago

I would like to filter B objects, all() method could be costly.

Artem Sudoma Over a year ago

@joelvarma select_related method in this case will work because id is foreign key. If you were to filter A model so you would need to use prefetch_related

joelvarma Over a year ago

Yea, i want to filter A, so how should i use prefetch_related

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