关联两张表时，会有生成以下的方法，更新orders_users也是通过以下方法操作

user.orders<<(object, ...)
user.orders.delete(object, ...)
user.orders.destroy(object, ...)
user.orders=objects
user.orders_singular_ids
user.orders_singular_ids=ids
user.orders.clear
user.orders.empty?
user.orders.size
user.orders.find(...)
user.orders.where(...)
user.orders.exists?(...)
user.orders.build(attributes = {})
user.orders.create(attributes = {})
user.orders.create!(attributes = {})

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