[英]Fetch objects having the same value for a given property from CoreData?
[英]CoreData Fetch: Getting the objects with the maximum of one atttribute and having another attribute in common
我有一個 CoreData ManagedObject 類型的事件,其屬性名稱:Sting 和日期:Date。
我想獲取名稱中包含過濾器的所有 EventObject。 如果多個同名 object 匹配過濾器,則只返回最新日期的 object。
只是為了澄清我想要什么。 在基於表的方法中,我會在 SQL 中查詢,例如:
SELECT name, max(date) FROM Event
WHERE name contains 'filter'
GROUP BY name
在 CoreData 我試過這樣,但得到:
無法將 'NSKnownKeysDictionary1' 類型的值轉換為 'xx12.Event' 。
let fetchRequest : NSFetchRequest<Event> = Event.fetchRequest( fetchRequest.predicate = NSPredicate (format: "name contains %@", filter)
fetchRequest.sortDescriptors = [NSSortDescriptor (key: "name", ascending: true)]
// We want Event not Dictionary back
// fetchRequest.resultType = NSFetchRequestResultType.dictionaryResultType
let keypathExpression = NSExpression(forKeyPath: "date")
let maxExpression = NSExpression(forFunction: "max:", arguments: [keypathExpression])
let key = "maxDate"
let expressionDescription = NSExpressionDescription()
expressionDescription.name = key
expressionDescription.expression = maxExpression
expressionDescription.expressionResultType = .dateAttributeType
fetchRequest.propertiesToFetch = [expressionDescription]
let resultEvents = try moc.fetch(fetchRequest)
例子:
如果我有這些物品
meeting1, 1.1.2020
meeting1, 1.4.2020
meeting2, 1.1.2020
meeting2, 1.4.2020
party, 2.3.2020
和過濾器:會議
我想獲取對象:
meeting1, 1.4.2020
meeting2, 1.4.2020
我將使用基本的獲取請求,然后在 swift 代碼中進行分組部分,而不是使用 NSExpression
let fetchRequest : NSFetchRequest<Event> = Event.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "name contains %@", filter)
let result = try moc.fetch(fetchRequest)
return Dictionary(grouping: result, by: \.name).values
.compactMap { $0.max {$0.date < $1.date } }
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