new record is created while updating

[Ayush Bajpai]

model:

class Batch(models.Model):

    batch_id = models.AutoField(primary_key=True, default=None)

    batch_name = models.CharField(max_length=256, default=None, null=True)

    ... some more fields

views:

def organisationEditBatch(request, batch_id):

    batch = Batch.objects.get(batch_id=batch_id)

    if request.method == 'POST':

        batch.batch_name = request.POST.get("batch_name")

        batch.save() 

[Ayush Bajpai]

    raise self.model.DoesNotExist(

apps.courseApp.models.Batch.DoesNotExist: Batch matching query does not exist.

actually this is the error it is giving but i do have a batch_id.., when i prefill the data of particular batch_id in edit form it gets displayed corretly there. but here it gives this error. i don't understand this issue so pls if anyone could help, i need it urgently!!