smtlaissezfaire smtlaissezfaire - 4 years ago 248
Objective-C Question

GROUP BY equivalent for Core Data

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;

What I'm looking for is all of the data returned in an array, not just the array of values that matches the group by expression. Essentially, I'm looking for the equivalent of the following SQL query for core data:

FROM my_table
GROUP BY a_value;

Answer Source

It's analog

SELECT 'Status', COUNT(*) FROM 'Records' GROUP BY 'Status':

NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:@"Record"];
NSEntityDescription* entity = [NSEntityDescription entityForName:@"Record"
NSAttributeDescription* statusDesc = [entity.attributesByName objectForKey:@"status"];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: @"url"]; // Does not really matter
NSExpression *countExpression = [NSExpression expressionForFunction: @"count:"
                                                          arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: @"count"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
[fetch setPropertiesToFetch:[NSArray arrayWithObjects:statusDesc, expressionDescription, nil]];
[fetch setPropertiesToGroupBy:[NSArray arrayWithObject:statusDesc]];
[fetch setResultType:NSDictionaryResultType];
NSError* error = nil;
NSArray *results = [myManagedObjectContext executeFetchRequest:fetch

Found here

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