Rails: getting data from a table for each iteraction of a loop

I have a loop (for item in @dataset) and I want, in each iteration, to get different data from another table and make some operations that will be printed in the view. I cant't get this data from the dataset used in the loop.

How can I do this according to MVC? I can put the code into the loop, in the view, but I think it's horrible.

Must I use a helper for do this, and call the function from the view?