how to do multiple selects and arithmetic using Active Record

[eluser]ginger_tosser[/eluser]
hi,

not sure if this is possible in AR but I'm trying to convert the following SQL into AR

SELECT value_SI, description from `values` where value_SI < (SELECT 1/(SELECT convert2SI FROM `units` WHERE unit='hands')) ORDER BY value_SI DESC LIMIT 4;

I'm using the arithmetic functions within MySQL so maybe trying to use the AR abstraction is just wrong because I suppose not all DB's support maths.

Anyone???