how to setting additional attributes of a join model?

I've spent a lot of time trying to create a new model and, at the same time, setting an attribute of the join model. I have: Company has_many :managements has_many :managers, :through => :managements

Manager has_many :managements has_many :companies, :through => :managements

Then I try to create a new manager with a associated company like this:

- args = @company.nil? ? [@manager] : [[@company, @manager], {:remote => true}] = simple_form_for *args do |f|

  = field_set_tag t('manager.manager') do           = f.input :name           = f.input :surname           = f.input :fiscal_code           = f.input :city           = f.input :zip_code           = f.input :address           = f.input :street_number           = f.input :tel           = f.input :email           = f.simple_fields_for :managements do |p|             = p.input :role

I don't see the field role in the form and I don't know how to set role when the Management model is created. How to do this? Please an advice. Thanks.