gistfile1.m

AWSDynamoDBAttributeValue *dbav = [AWSDynamoDBAttributeValue new];
dbav.S = _userID;
update.key = @{@"UserID": dbav};
    
// can be boiled down to:

update.key = @{@"UserID": [AWSDynamoDBAttributeValue withString:_userID]};

/* ----------------- */

@interface AWSDynamoDBAttributeValue (Helpers)

+(AWSDynamoDBAttributeValue*) withNumber:(NSNumber*)value;
+(AWSDynamoDBAttributeValue*) withString:(NSString*)value;

@end

@implementation AWSDynamoDBAttributeValue (Helpers)

+(AWSDynamoDBAttributeValue*) withNumber:(NSNumber*)value {
    AWSDynamoDBAttributeValue *dbav = [AWSDynamoDBAttributeValue new];
    dbav.N = [value stringValue];
    return dbav;
}

+(AWSDynamoDBAttributeValue*) withString:(NSString*)value {
    AWSDynamoDBAttributeValue *dbav = [AWSDynamoDBAttributeValue new];
    dbav.S = value;
    return dbav;
}

@end