Add a bulk_update method to models

[Tom Forbes]

Currently it's not easily or neatly possible to update multiple rows with differing values using the ORM. Most people who need to update a number of models with a value that's distinct to each model need to issue N queries. This is akin to the situation that lead to the addition of bulk_create.

Updating multiple rows with differing values in a single query is indeed possible in SQL, and Postgres has some specific syntax for it[1]. Other databases can use a CASE/WHEN:

SomeModel.object.filter(id__in=[1,2]).update(
    some_field=Case(
        When(id=1, then=Value('Field value for ID=1')),
        When(id=2, then=Value('Field value for ID=2'))
    )
)

This isn't particularly elegant and cannot take advantage of specific DB features (like pg UPDATE FROM), and batching is hard. An API similar to bulk_create would be nice:

SomeModel.objects.bulk_update(list_of_models, batch_size=100, fields=['some_field'])

There is some prior art in the django-bulk-update package[2], which has some impressive performance numbers in it's readme.

1. ​https://stackoverflow.com/questions/18797608/update-multiple-rows-in-same-query-using-postgresql

2. ​https://github.com/aykut/django-bulk-update