如何核对<代码> 主动查询 物体是新的,还是已经存在?
活性核物体生命周期:
1. <>新纪录
item = Item.new
item.new_record? #=> true
2.persisted
item.save
item.persisted? #=> true
3.changed
item.name = "other"
item.changed? #=> true
4.destroyed
item.destroy
item.destroyed? #=> true
#new_record? 只是:
object.new_record?
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