How to get NSFormatter subclass to work with NSTableColumn sort key and selector?

My setup:

• I have a sqlite database from which I populate a NSMutableArray of NSDictionary objects this is the DataSource for my NSTableView.
• One of the columns holds "time", the time is a float that holds seconds.
• I would like to display the values in this column as minutes:seconds. For i开发者_高级运维nstance my data would be 123.4329387 I want to display 2:03 which I have no problem doing with a subclass of NSFormatter (or NSNumberFormatter) applied to my NSTextField in the column.
• I have sorting set up by using the Table Column Attributes in IB, I just have the sort key set to "time" and the selector set to "compare:" which works fine without the formatter.

Currently this gives me something like this when I sort (descending)

1:37, 1:31, 0:10, 0:10, 0:09, 1:30, 1:30, 1:26, 0:09

and similar nonsense, it looks like something is going on but it's definitely not sorted.

How do I get the sort to look at the underlying data instead of the formatted value? Alternately, how do I specify a custom sort method and where do I put the code for said method? I have searched around quite a bit and have not found anything to help me out with this problem, any help with this is most appreciated.

It turns out that when it was sorting it was using compare: from the NSString class but it was using the underlying numeric data. I'm not exactly sure what all is going on under the hood here but I managed to fix this by creating my own compare method in a category of NSString called "timeCompare". Then in interface builder I just put "timeCompare:" in the selector field for the the table column. This gets it to call my custom compare method from which I just converted the strings given to NSNumbers and call compare: on them.