Django：复制和更新查询集

Question

I have the following models:

class A(models.Model):
    pass

class B(models.Model):
   a = models.ForeignKey(A, related_name="bs")

then I have a function where I create a new record of object A and I would like to copy all the related records of B

I have done something such as

def foo(new_a, old_a):
    old_a.bs.all().update(pk=None, a=new_a)

but I get the following error ERROR - failed to write data to stream: <open file '<stdout>', mode 'w' at 0x7f207949a150>

Answer 1

Hope that help:

queryset = B.objects.filter(something="a")

for element in query_set:
    a_object = A.objects.bulk_create(**element)

Answer 2

Doing something like this

def foo(new_a, old_a):
    old_a.bs.all().update(pk=None)

will try to set pk=None on all the bs for old_a which is not what you want.

Instead you'll want to do something like:

for b in old_a.bs.all():
    b.pk = None # make sure you're not overwriting the existing record
    b.a = new_a
    b.save()

I think you should also be able to use bulk_create to do this more efficiently:

new_bs = []
for b in old_a.bs.all():
    b.pk = None # make sure you're not overwriting the existing record
    b.a = new_a
    new_bs.append(b)

B.objects.bulk_create(new_bs)