Question

I am trying to search my Core Data database, returning a very specific set of records.

My model looks something like this:

firstName (NSString)
lastName (NSString)
dateOfBirth (NSDate)
updatedAt (NSDate)
weight (NSNumber)
height (NSNumber)

I want to get a list of users with unique firstName, lastName, and dateOfBirth values, but only return the full record of each of these users with the latest updatedAt value.

So, for database like:

firstName: Jim, lastName: Brown, dateOfBirth: 1/1/2012, updatedAt: 1/1/2012, weight: 100, height: 100
firstName: Jim, lastName: Brown, dateOfBirth: 1/1/2012, updatedAt: 2/1/2012, weight: 120, height: 100
firstName: Joe, lastName: Smith, dateOfBirth: 1/1/2012, updatedAt: 1/1/2012, weight: 100, height: 100
firstName: Joe, lastName: Smith, dateOfBirth: 2/1/2012, updatedAt: 1/1/2012, weight: 100, height: 100

I would get an array:

firstName: Jim, lastName: Brown, dateOfBirth: 1/1/2012, updatedAt: 2/1/2012, weight: 120, height: 100
firstName: Joe, lastName: Smith, dateOfBirth: 1/1/2012, updatedAt: 1/1/2012, weight: 100, height: 100
firstName: Joe, lastName: Smith, dateOfBirth: 2/1/2012, updatedAt: 1/1/2012, weight: 100, height: 100

I know I could run around performing a whole bunch of different filtering operations, but I was wondering if there were any “shortcuts” that could help this. I could use [fetchRequest setReturnsDistinctResults:YES] if I wasn’t interested in getting the updatedAt value, but this would return essentially all of the records if I include updatedAt in the properties to fetch. I’m worried about loading the entire database into an NSArray because it could be very large (>10,000 records).

Answer 1

First, I think you should revise your data model. First name, last name and birthdate are just not enough to substitute unique ids (out of 10.000s of records) reliably.

Second, you should therefore create a Person entity and use the same record of this entity to associate with some kind of Event that carries the updated date attribute.

With this much more robust structure in place, your query becomes trivial. You just fetch all persons. For each person you select the most recent event if needed.

Event *latestEvent = [person.events valueForKeyPath@"@max.updated"];