如何在 GROUP BY 之前对 Core Data 记录进行排序？

Question

Imagine I have this Core Data entity

@objc(SomeEntity)
class SomeEntity: NSManagedObject {
  @NSManaged var date: Date
  @NSManaged var timeZoneIdentifier: String
  @NSManaged var name: String
  @NSManaged var brand: String
}

I want to fetch all rows with distinct name and brand fields, but for each one, I want the latest date and the timeZoneIdentifier for that latest date row.

Here's some example data:

date	timeZoneIdentifier	name	brand
2020-01-01	America/Los_Angeles	Name 1	Brand 1
2020-01-02	America/Tiajuana	Name 1	Brand 1
2020-01-03	America/Denver	Name 1	Brand 2
2020-01-04	America/Phoenix	Name 2	Brand 1

I essentially want to fetch .dictionaryResultType results that will look like this:

lastDate	lastDateTimeZoneIdentifier	name	brand
2020-01-02	America/Tiajuana	Name 1	Brand 1
2020-01-03	America/Denver	Name 1	Brand 2
2020-01-04	America/Phoenix	Name 2	Brand 1

Which you could do with this SQL query:

SELECT * FROM (SELECT * FROM SomeEntity ORDER BY date DESC) as sub GROUP BY sub.name, sub.brand;

Can I achieve the same thing with Core Data with minimal queries? I can't find any examples or documentation online trying to do something like this.

I can write a NSFetchRequest that essentially executes this:

SELECT date, timeZoneIdentifier, name, brand FROM SomeEntity GROUP BY name, brand ORDER BY date;

But that orders after the GROUP BY, not before.

I can also write a NSFetchRequest that essentially executes this:

SELECT max(date), name, brand FROM SomeEntity GROUP BY name, brand;

Which almost works only because I wanted the latest date, but of course I lose the timeZoneIdentifier that the row came from. I have written up this example as a gist that you can throw into a Swift playground.

Thanks!

Answer 1

As you've probably discovered, you cannot specify any propertiesToFetch other than the properties you group by, and aggregate functions. Hence you can't include the timeZoneIdentifier .

However, you can include an NSExpression that represents the evaluated object (see NSExpression.expressionForEvaluatedObject ). This will give you the objectID for the object with the max(date) - which you can then fetch to get the other property values.