I used an attr_accessor
for now:
class Service
attr_accessor :modifier
attr_accessible ..., :modifier, as: :admin
And I send it from the form:
ActiveAdmin.register Service do
form do |f|
f.input :modifier, as: :hidden, input_html: { value: :admin }
...
Then I could use it like this:
validate :something
def something
if modifier == 'admin'
# some code here