NSFetchRequest: Query for distinct values and count other properties values

Question

I try to set up an NSFetchRequest for an entity Location with properties like countryand city:

country  | city
————————————————
Germany  | Berlin
USA      | San Francisco
USA      | New York
Germany  | Munich
Germany  | Munich
USA      | San Francisco
Germany  | Stuttgart

The NSFetchRequest should return the country (or a Location object with the appropriate country) and the number of cities.

[
    { country: 'Germany', cityCount: 3 },
    { country: 'USA', cityCount: 2 }
]

I know that I can just fetch all entries and 'count it myself' but I am interested in how to set up an appropriate fetch request (or if it's possible to do so) and would like to see how you would do it! :)

Answer 1

The correct answer to this question to refactor the data model in order to avoid redundancy.

The country strings are repeated unnecessarily in the table. Additionally, you make a simple query gratuitously complicated. The model should reflect your data, and writing out "USA" for every American city is just not smart or efficient.

Your data model should look like this

Country <----->> City

Now you can just fetch all countries and get the cities with cities.count.