this can't be removed if we want associations to be updated by the association_id setter.
we get this:

1) Failure:
DomainTest#test_0010_should update hosts_count on domain_id change [/Volumes/proj/foreman/test/unit/domain_test.rb:87]:
"domain.hosts_count" didn't change by 1.
Expected: 1
  Actual: 0

when removing this patch.

An issue remains in Rails 4 (4.1.14.1, 4.2.5 and master at the time of writing) where counters are double-counted on object reassignment, preventing this workaround from being removed still. Filed as https://github.com/rails/rails/issues/23265.

Unfortunately this means I think that counters are being updated three times on reassignment - twice by Rails and once by our workaround.