When a model has an STI enum type, the type wasn't properly applied when
disable joins was set to true. In this case we need to apply the scope
from the association in `add_constraints` so that `type` is included in
the query. Otherwise in a `has_one :through` with `type` all records
will be returned because we're not filtering on type.

Long term I think this might be in the wrong place and that we want to
do this in a new definition of `target_scope` but that's a future
refactoring that needs to be done.