Cannot determine comparator for Sortfield

Question

I have a DataGrid that gets its data from an HttpService. If
the first row of the data from the http request has a null value
for a given field, I get the "Cannot determine comparator for
Sortfield" error.
I think the simplest workaround would be to specify the data
type for each DataGridColumn. I see that the DataGridColumn objects
can have a sortCompareFunction. I don't want to write my own custom
comparators. I assume that the built-in comparators will do just
fine.
Suppose my MXML code looks something like this...
<mx:DataGrid>
<mx:columns>
<mx:DataGridColumn headerText="My String"
dataField="name" sortCompareFunction="???"/>
<mx:DataGridColumn headerText="My Integer" dataField="ss"
sortCompareFunction="???"/>
<mx:DataGridColumn headerText="My Float" dataField="gpa"
sortCompareFunction="???"/>
<mx:DataGridColumn headerText="My Date"
dataField="birthdate" sortCompareFunction="???"/>
</mx:columns>
</mx:DataGrid>
What values would I use for the sortCompareFunctions in order
to use the built-in comparators for the string, int, float and date
types, respectively?

Answer 1

One trick that I've used in the past (which wont really give
you type specific ability) is to check the returned data prior to
assigning it as a dataProvider. In my case the data is xml based
son I don't get nodes, but possible missing elements all together.
I check for their existance for each row (in your case test for
null) then assign a default value (-1 for int, 6/6/2006 for date
etc...). After populating the 'voids' in my data I assign it as the
dataProvider in ActionScript.
To get more type specifc, you may want to look into using a
data model and check out the validator classes.