从核心数据中获取子总和 [英] Fetching child sum from Core Data

问题描述

让我们说我有三个实体.人员:名称地址(多工资)和(多贷款)

Lets say I have three Entitys. Person: name address (to-many-salary) and (to-many-loans)

工资:收入税相对:(一个人)

Salary: income tax Rel: (to-one-Person)

账单数量相对:(一对一)

Bills amount Rel: (to-one-person)

如何执行具有以下结果的提取:

How do I perform a fetch that have a result like this:

John Doe，SUM>收入，SUM>金额Eva Doe，SUM>收入，SUM>金额

John Doe, SUM>income, SUM>amount Eva Doe, SUM>income, SUM>amount

使用Swift

推荐答案

使用键值编码集合运算符"可能最容易做到这一点(请参见

This is probably most easily done using Key Value Coding "collection operators" (see here) rather than a fetch. For each person:

let name = person.name
let totalIncome = person.valueForKeyPath("[e-mail]")
let totalLoans = person.valueForKeyPath("[e-mail]")

如果性能是一个问题，则可以通过修改获取请求(针对Person对象)来预取"相关的 Salary 和 Bills 对象:/p>

If performance is an issue, you might improve things by amending the fetch request (for the Person objects) to "prefetch" the related Salary and Bills objects:

fetchRequest.relationshipKeyPathsForPrefetching = ["salary", "loans"]

或者，可以一次取回所有必需的信息，尽管我不建议这样做，因为它需要更改fetchRequest以返回字典数组而不是 NSManagedObjects 数组.这使得后续处理(例如，填充表格视图)更加困难.

Alternatively, it is possible to retrieve the required information all in one fetch, though I do not recommend it because it requires changing the fetchRequest to return an array of dictionaries rather than an array of NSManagedObjects. This makes subsequent processing (eg. populating your table view) more difficult.

// Define NSExpression and NSExpressionDescription for the total income
let incomeED = NSExpressionDescription()
incomeED.expression = NSExpression(forKeyPath: "[e-mail]")
incomeED.name = "totalIncome"
incomeED.expressionResultType = .Integer64AttributeType
// Define NSExpression and NSExpressionDescription for the total amount
let amtED = NSExpressionDescription()
amtED.expression = NSExpression(forKeyPath: "[e-mail]")
amtED.name = "totalLoans"
amtED.expressionResultType = .Integer64AttributeType
// specify attributes and expressions to fetch
fetchRequest.propertiesToFetch = ["name", incomeED, amtED]
// specify dictionary return type (required to process NSExpressions)
fetchRequest.resultType = .DictionaryResultType

提取的结果将是一个字典数组；每个字典将具有键名称"，"totalIncome"和"totalLoans"(具有相应的值).

The result of the fetch will be an array of dictionaries; each dictionary will have keys "name", "totalIncome" and "totalLoans" (with corresponding values).

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