ruby on rails - How to set additional variables upon form submit, based on value of one of the submitted fields? -

i have form , upon create save additional variables not included in form itself. therefore in create method have line params[:invitation][:person_two_id] = @person_two.id (see below). calling line before .save thought include person_two_id when saving invitation. however, not case. debugger (see code below placed debugger) shows:

• params[:invitation][:person_two_id] 101
• @invitation has nil value person_two_id: #<invitation id: 171, person_one_id: nil, person_two_id: nil, organization_id: 87, email: "example@example.com", message: "dsfsd", created_at: "2015-07-07 14:36:35", updated_at: "2015-07-07 14:36:35">

hence, when saving @invitation, has not automatically saved person_two_id value @invitation. why not? should differently?

my controller method:

def create   @invitation = invitation.new(new_params)   @person_two = user.find_by(email: params[:invitation][:email].downcase)   params[:invitation][:person_two_id] = @person_two.id   @invitation.save   debugger   invitationmailer.invitation(@invitation).deliver_now end 

i needed replace params[:invitation][:person_two_id] = @person_two.id @invitation.person_two_id = @person_two.id , worked.