The host common #lookup_values_attributes= method uses an internal Active Record method value_to_boolean to parse the _destroy parameter from the nested attributes supplied by the UI.

https://github.com/theforeman/foreman/blob/1.11.0-RC1/app/models/concerns/host_common.rb#L54-L58

This method has been moved in Rails 4.2 and since it's really rather internal, it should be replaced by Foreman::Cast.to_bool or similar.