how to store a field in the database after querying - django

Question

views.py:
q3=KEBReading.objects.filter(datetime_reading__month=a).filter(datetime_reading__year=selected_year).values("signed")
for item in q3:
item["signed"]="signed"
print item["signed"]
q3.save()
How do I save a field into the database? I'm trying to save the field called "signed" with a value. If I do q3.save() it gives a error as it is a queryset. I'm doing a query from the database and then, based on the result, want to set a value to a field and save it.
prevdate=KEBReading.objects.filter(datetime_reading__lt=date)
i am getting all the rows from the database less than the current date. but i want only the latest record. if im entering 2012-06-03. wen i query i want the date less than this date i.e the date just previous to this. can sumbody help?

Answer 1

q3 = KEBReading.objects.filter(datetime_reading__month=a,
datetime_reading__year=selected_year)
for item in q3:
item.signed = True
item.save()

Answer 2

q3=KEBReading.objects.filter(...)
will return you a list of objects. Any instance of a Django Model is an object and all fields of the instance are attributes of that object. That means, you must use them using dot (.) notation.
like:
item.signed = "signed"
If your object is a dictionary or a class derived from dictionary, then you can use named-index like:
item["signed"] = "signed"
and in your situation, that usage is invalid (because your object's type is not dictionary based)
You can either call update query:
KEBReading.objects.filter(...).update(selected="selected")
or set new value in a loop and then save it
for item in q3:
item.signed="signed"
q3.save()
but in your situation, update query is a better approach since it executes less database calls.

Answer 3

Try using update query:
If signed is a booleanfield:
q3 = KEBReading.objects.filter(datetime_reading__month = a).filter(datetime_reading__year = selected_year).update(signed = True)
If it is a charfield:
q3 = KEBReading.objects.filter(datetime_reading__month = a).filter(datetime_reading__year = selected_year).update(signed = "True")
Update for comments:
If you want to fetch records based datetime_reading month, you can do it by providing month as number. For example, 2 for February:
q3 = KEBReading.objects.filter(datetime_reading__month = 2).order_by('datetime_reading')
And if you to fetch records with signed = True, you can do it by:
q3 = KEBReading.objects.filter(signed = True)
If you want to fetch only records of previous date by giving a date, you can use:
prevdate = KEBReading.objects.filter(datetime_reading = (date - datetime.timedelta(days = 1)))