values_list django如何工作？

Question

I have interesting experience with value_list and I don't know why is it acting in this way.

I want to update any value in TestObject has value_1 to value_2 and any value_2 to value_1. Where value_1 and value_2 from type Value and TestObject has a foreign key to Value.

This is my code:

def _swap(value_1, value_2):
    from_values_ids = TestObject.objects.filter(value=value_1).values_list('id', flat=True)

    to_values_ids = TestObject.objects.filter(value=value_2).values_list('id', flat=True)

    TestObject.objects.filter(id__in=from_values_ids).update(value=value_2)
    TestObject.objects.filter(id__in=to_values_ids).update(value=value_1)

I tested with TestObject has value_1 but does not have any value_2 . The end result was nothing happened after running this function. After investigating I found the TestObject got updated to value_2 after running:

TestObject.objects.filter(id__in=from_values_ids).update(value=value_2)

but it returned back after running

TestObject.objects.filter(id__in=to_values_ids).update(value=value_1)

I thought may be to_values_ids has lazy load that is why I added print to_values_id .

def _swap(value_1, value_2):
    from_values_ids = TestObject.objects.filter(value=value_1).values_list('id', flat=True)

    to_values_ids = TestObject.objects.filter(value=value_2).values_list('id', flat=True)

    print to_values_ids

    TestObject.objects.filter(id__in=from_values_ids).update(value=value_2)
    TestObject.objects.filter(id__in=to_values_ids).update(value=value_1)

But I got the same result. Although my print for to_values_ids has [] .

The way I fix it I created new lists with the ids and it worked but still needs to understand the core python how is it work? Any good explanation.

Answer 1

Your understanding is correct. Django querysets are lazy .

from_values_ids = TestObject.objects.filter(value=value_1).values_list('id', flat=True) # doesn't hit the db

to_values_ids = TestObject.objects.filter(value=value_2).values_list('id', flat=True) # doesn't hit the db

Now when you do:

TestObject.objects.filter(id__in=from_values_ids).update(value=value_2)
                                   |                  
                                   |__> will fetch from db 

Now all the values that matched value_1 got updated to value_2 . Now next line is executed:

TestObject.objects.filter(id__in=to_values_ids).update(value=value_1)
                                      |
                                      |__> Will actually execute the query you assigned it
                 # TestObject.objects.filter(value=value_2).values_list('id', flat=True)

At this moment all the objects that match value_2 are fetched and updated to value_1

But you see no difference because you have all value_1 in the database before starting. Hence from_values_ids fetches all the objects and updates them to value_2 and then back to value_1 . See having a mix of value_1 and value_2 records in the database. The difference will be evident.

Answer 2

The issue you're seeing is likely due to the fact that Django QuerySet values_lists return generators. Since 1.9, QuerySet.values_list implements an iterable class like FlatValuesListIterable Prior to 1.9, QuerySet.values_list returned an instance of ValuesListQuerySet ... Both of these return a generator, so the query is executed each time you access the variable (hence the behavior you were seeing when calling print).

The resulting object will behave a lot like a list, which is confusing, but if you need proof it's not a list, try this:

from_values_ids = TestObject.objects.filter(value=value_1).values_list('id', flat=True)
from_list_ids = [obj.id for obj in TestObject.objects.filter(value=value_1)]
combined_list = from_values_ids + from_list_ids

... this will result in:

TypeError: unsupported operand type(s) for +: 'ValuesListQuerySet' and 'list'

The solution is simply to cast your from_values_ids variable as a list:

from_values_ids = list(TestObject.objects.filter(value=value_1).values_list('id', flat=True))

or just generate the list yourself:

from_values_ids = [obj.id for obj in TestObject.objects.filter(value=value_1)]