Problem with mysql & Rails for stats

Hi everybody

I have a ROR application which generates some stats taking data from a db table. To get the results I before create an array with all the values involved in the stats - each data stack label - ... then calculate the time interval splited for week or month... and for each splitted interval I have an Activerecord calculte(:count) method in order to create - for each data stack - an array of values...

This seems to work done, but sometimes something goes wrong, couse I return empty or nil array of results. I fixed it with a control to avoid system errors. But Anyway it happens... And I loose some data in my stats.

Looking at the mysql query log, all the query are executed... How can I menage this situation? Is it possible to observ the ActiveRecord::calculate feedback to understand exactly what randomly happens with these query?

here is an abstract of the code