## Controller action code

@user = User.new(params[:user])
if current_user.create_vacant_user(@user)
  # do successful actions
else
  # do not successful actions
end

## User model code

class User < ActiveRecord::Base
#...

def create_vacant_user(user_instance)
  if self.is_procoach? && self.can_add_client?
    user_instance.vacant = true
    transaction do
      user_instance.save!
      CoachingRelationship.create!(:coach => self,
                                   :coachee => user_instance,
                                   :is_professional => true)
    end
    true
  else
    false
  end
rescue ActiveRecord::RecordInvalid => e
  user_instance.errors.add_to_base(e.message)
  false
end

#...
end