rails 3 habtm只删除关联
问题描述：
class Company
has_and_belongs_to_many :users
end
class User
has_and_belongs_to_many :companies
end
当我删除公司时，最好的(推荐)方法是仅删除该公司中用户的关联?(我的意思不是实际用户，只有关联)
when i delete a company, what's the best (recommended) way to delete ONLY the associations of the users from that company? (i mean not the actual users, only the associations)