So I am trying to update my model by running the following:

FooBar.objects.filter(something=True).update(foobar=F('foo__bar'))

but I get the following error:

FieldError: Joined field references are not permitted in this query

if this is not allowed with F expressions...how can I achieve this update?

ticket

given the information in this ticket, I now understand that this is impossible and will never be implemented in django, but is there any way to achieve this update? maybe with some work around? I do not want to use a loop because there are over 10 million FooBar objects, so SQL is much faster than python.

Django 1.11 adds supports for subqueries. You should be able to do:

from django.db.models import Subquery, OuterRef

FooBar.objects.filter(something=True).update(
    foobar=Subquery(FooBar.objects.filter(pk=OuterRef('pk')).values('foo__bar')[:1])
)

Why don't use raw sql here: Based on this, it will be something like

from django.db import connection

raw_query = '''
update app_foobar set app_foobar.foobar = 
(select app_foo.bar from app_foo where app_foo.id = app_foobar.foo_id) 
where app_foobar.something = 1; 
'''

cursor = connection.cursor()
cursor.execute(raw_query)

This is the implementation of Georgi Yanchev's answer for two models:

class Foo(models.Model):
    bar = models.ForeignKey(Bar)

Foo.objects \
    .filter(foo_field_1=True) \
    .update(foo_field_2=Subquery(
        Bar.objects \
            .filter(id=OuterRef('bar_id')) \
            .values('bar_field_1')[:1]))