Re: Bulk update instances to different values

ttps://github.com/saxix/django-adminactions

This app allows for what is termed "mass updates".

On Thursday, 5 October 2017 05:50:45 UTC+2, David Foster wrote:

I would like to update many model instances at a time in the database.

If I could select all such instances using a single query and have the same new field value across all instances I could write code like:

def reset_choices(question: Question):
    Choice.objects.filter(question=question).update(votes=0)

However if I have different new field values I don't see an easy way to perform all the database updates at once:

def update_choices(choice_votes_tuples: List[Tuple[Choice, int]]):
    for (choice, votes) in choice_votes_tuples:
        choice.votes = votes
        choice.save()   # 1 query per Choice!

Here's a hard way of performing all database updates at once with raw SQL:

from django.db import connection

def update_choices(choice_votes_tuples: List[Tuple[Choice, int]]):
    query = 'UPDATE polls_choice SET votes=%s WHERE id=%s'
    values = [(votes, choice.id) for (choice, votes) in choice_votes_tuples]
    with connection.cursor() as c:
        c.executemany(query, values)  # 1 bulk query for all Choices

Is there an easy way to perform the above kind of update that I don't know about?

If there was an easier API, I'd expect it to look something like:

def update_choices(choice_votes_tuples: List[Tuple[Choice, int]]):
    for (choice, votes) in choice_votes_tuples:
        choice.votes = votes
    choices = choice_votes_tuples.keys()
    Choice.objects.bulk_update(choices, fields=['votes'])

Thanks for taking the time to read my question.

- David

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