There is some code that adds the host or hostgroup id parameters to ajax requests refreshing parts of the host[group] form:

if (url.match('hostgroups')) {
  data = data + '&hostgroup_id=' + host_id
} else {
  data = data + '&host_id=' + host_id
}

(for example https://github.com/theforeman/foreman/blob/develop/app/assets/javascripts/host_edit.js#L190)
However, the preceding line is:

var data = $("form").serialize().replace('method=put', 'method=post');

which means that the host or hostgroup_id, if existing, is already present in the data variable. Therefor, this code can be removed from many places in the codebase with some minor changes to the relevant controllers, reducing code complexity.