Count all unique objects in CoreData?

Question

How would the following SQL-query be implemented in CoreData?

-- table-definition
CREATE TABLE my_numbers (number INTEGER NOT NULL)

-- query
SELECT number, COUNT(1) as n FROM my_numbers GROUP BY number

So if I have a similar entity in CoreData called MyNumberEntity with an attribute called value of type Int16. Is there some way of using a predicate (or something else?) to achieve the same as the SQL-query above in a few lines?

The only way I can think of is to first fetch all MyNumberEntity into a set, then iterate over the set and using the iterator to get a count of respective value, storing it in an array/dictionary.

Is there an easier/neater way of doing this in CoreData?

Answer 1

You can use NSExpression

// Create a fetch request for the entity you want to group and count
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "YourEntity")

// Create an expression to perform the count
let countExpression = NSExpression(format: "count:(yourAttribute)")

// Create an expression description to define the result
let countExpressionDescription = NSExpressionDescription()
countExpressionDescription.name = "count"
countExpressionDescription.expression = countExpression
countExpressionDescription.expressionResultType = .integer32AttributeType

// Set the properties to fetch in the fetch request
fetchRequest.propertiesToFetch = ["yourAttribute", countExpressionDescription]

// Set the grouping properties
fetchRequest.propertiesToGroupBy = ["yourAttribute"]

// Specify that the result type should be a dictionary
fetchRequest.resultType = .dictionaryResultType

// Execute the fetch request
do {
    let results = try context.fetch(fetchRequest) as? [NSDictionary]
    if let results = results {
        for result in results {
            let attributeValue = result["yourAttribute"] as? String
            let count = result["count"] as? Int
            print("Attribute: \(attributeValue ?? "") Count: \(count ?? 0)")
        }
    }
} catch {
    print("Fetch failed: \(error)")
}