iOS Core Data获取新插入的，尚未保存的对象 [英] iOS Core Data fetch newly inserted, not yet saved objects

问题描述

我必须在批处理（导入过程）中创建一些数据在CoreData实体，我想提交结束或回滚的错误（所以保存之间将无法工作）。

问题是，我需要创建一个实体Person，稍后我需要重新使用该实体。

所以我试图用一个谓词（personId == 4711）来获取它，但是这个过程可能已经存在， 。但是，虽然我已经设置
[fetchRequest setIncludesPendingChanges：YES]; 它找不到新创建的Person对象。

我阅读了此此问题和这个回答哪个状态，它不可能？

解决方案

div>

从我所知道的CoreData，这是不可能的（如果我错了，请纠正我）。

这是可能的，你永远不想在每个对象的基础上查询您的商店。
对大型（超过几十个）导入的性能影响是巨大的。

我的建议是在导入阶段创建一个由您的唯一标识符键入的字典（从存储现有实体预取，并为不存在的实体创建新的实体）。

注意：您应该注意不要在多线程环境中从不同的上下文执行多个插入。

存储内容：1 - > P1，3 - > P3

服务回应：1 - > Data1，2 - > Data2

算法：
响应完成时，从响应中获取所有唯一的ID - > recievedIds = @ [1,2] >
创建 recievedIds 集创建personId - >数据的映射/字典：

@ {1：Data1， 2：Data2}

通过谓词从商店提取：

[NSPredicate predicateWithFormat：@personId IN％@ ，recievedIds]

从结果数组中创建字典。

在这种情况下： existingItems = @ {1：P1}

传递 recievedIds 中的所有ID： b $ b 1）如果id存在于 existingItems 中更新现有对象\\ b $ b 2）否则创建一个新人并将数据插入到新的

这只会从商店获取一次。
$ $ $ $
您只保存一次。 $ b ==>只有2次到商店，而不是每个对象的旅行

I have to create some data in CoreData entities in batch (import process) and I would like to "commit" at the end or "rollback" on an error (so saving inbetween won't work).

The problem is that I for example need to create an entity "Person" and later in that progress I need to re-use that entity. BUT it can already exists BEFORE this process or may be created WHILE this import process.

So I'm trying to fetch it with a predicate "(personId == 4711)". But although I have set [fetchRequest setIncludesPendingChanges:YES]; it doesn't find the newly created Person object.

I read this this question and this answer which state, that it is not possible? Am I right?

If so, how can I workaround / handle this?

解决方案

From what I know of CoreData, this is indeen impossible (please correct me if I'm wrong).

However, even if this was possible, you never want to query your store on a "per object" basis.
The performance impact on large (more then a few dozens) imports is huge.

My suggestion for you is to create a dictionary keyed by your unique identifier during the import stage (prefetch from the store existing entities and create new ones for ones that aren't).

Note: You should be careful not to perform multiple inserts from different contexts in a multithreaded environment. in such cases you will need a coordinator to prevent duplications.

example:

store content: 1 --> P1, 3 --> P3
service response: 1 --> Data1, 2 --> Data2

Algorithm:
on response completion, get all unique ids from response --> recievedIds = @[1,2]
during the creation of recievedIds set create a mapping/dictionary of personId --> data:
@{1 : Data1, 2 : Data2}

fetch from the store by predicate:
[NSPredicate predicateWithFormat:@"personId IN %@",recievedIds]
Create a dictionary from the resulting array.
in this case: existingItems = @{1 : P1}

Pass over all ids in recievedIds:
1) if the id exist in existingItems update the existing object
2) else create a new person and insert the data to the new record.

This will only fetch from the store once.
And you only save once.
==> only 2 trips to the store instead of a trip per object

这篇关于iOS Core Data获取新插入的，尚未保存的对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！