Fetching child sum from Core Data

问题

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

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

Bills amount Rel: (to-one-person)

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

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

Using Swift

回答1:

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("salary.@sum.income")
let totalLoans = person.valueForKeyPath("loans.@sum.amount")

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"]

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: "salary.@sum.income")
incomeED.name = "totalIncome"
incomeED.expressionResultType = .Integer64AttributeType
// Define NSExpression and NSExpressionDescription for the total amount
let amtED = NSExpressionDescription()
amtED.expression = NSExpression(forKeyPath: "loans.@sum.amount")
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

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

来源：https://stackoverflow.com/questions/34556561/fetching-child-sum-from-core-data