iPhone核心数据迁移日期到字符串
我有一个最初以日期属性开头的实体。现在，除了保留原始字段之外，我还添加了一个属性，它是一个我想要保存日期的字符串。 我已经制作了一个映射模型，但我不确定在“值表达式”中放入什么内容，以便在迁移过程中用日期字段中的数据填充新的字符串字段，格式为“2010-10-25”。
任何帮助将不胜感激。
另外，我需要它是一个真实的属性而不是瞬态的，因为我想对其进行排序。我没有意识到你无法对瞬态属性进行排序，直到为时已晚。
I have an entity that originally started with a date attribute. I have now added a attribute that is a string that I want to hold the date, in addition to keeping the original field.
I've made a mapping model, but I'm not sure what to put into "value expression" to get the new string field filled with data from the date field in the format "2010-10-25" during migration.
Any help would be appreciated.
Also I need it to be a real attribute not a transient because I want to sort on it. I didn't realize you couldn't sort on transient attribute until too late.
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将属性从日期转换为字符串需要您为该迁移构建自己的
NSEntityMigrationPolicy并重写-createDestinationInstancesForSourceInstance:entityMapping: manager: error:方法处理翻译。这应该是非常直接的代码，因为您只需要操作几个属性，并且实体映射仍然是一对一的。
Translating the property from a date to a string is going to require that you build your own
NSEntityMigrationPolicyfor that migration and override the-createDestinationInstancesForSourceInstance: entityMapping: manager: error:method to handle the translation.This should be very straight forward code since you are only going to be manipulating a couple of properties and the entity mapping is still one to one.