GROUP BY equivalent for Core Data

Question

I know that I can use @distinctUnionOfObjects to find something like the following in SQL:

SELECT a_value
FROM my_table
GROUP BY a_value;

W

Answer 1

I find this method (roughly similar to the accepted answer) to be a little cleaner and easier to understand. This is the SQL equivalent to:

SELECT COUNT(*), a_value FROM my_table GROUP BY a_value;

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:[MyTable className]];

// create expression for grouped column
NSExpressionDescription *aValueDescription = [[NSExpressionDescription alloc] init];
aValueDescription.name = @"aValue"; // name of key in result dictionary
aValueDescription.expression = [NSExpression expressionWithFormat:@"aValue"];
aValueDescription.expressionResultType = NSObjectIDAttributeType;

// create expression for count
NSExpressionDescription *countDescription = [[NSExpressionDescription alloc] init];
countDescription.name = @"count"; // name of dictionary key in result dictionary
countDescription.expression = [NSExpression expressionWithFormat:@"aValue.@count"];
countDescription.expressionResultType  = NSInteger32AttributeType;

// fill out fetch request
fetchRequest.propertiesToGroupBy = @[@"aValue"];
fetchRequest.propertiesToFetch = @[aValueDescription, countDescription];
//fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"count" ascending:NO]]; // not sure why this crashes
fetchRequest.resultType = NSDictionaryResultType; // required for "group by" requests

NSError *error = nil;
NSArray *results = [managedObjectContext executeFetchRequest:fetchRequest error:&error];

The returned results is an array of NSDictionary. Note that the description name properties can be anything you want - they are just the names of the keys in the returned dictionaries. One can add a predicate to the fetch request to filter rows from the table; this code returns all rows.

Bonus points to anyone who can tell me how to make the sort descriptor work...