method location

I am a novice writing a simple address book app. It is just using the
simple CRUD operations, but I would like to write a function for the
index view that divides the records up by the first letter of the last
name so I can have jump links at the top to take you to a specific
group of records according to their last name. Should this function
be put in the controller, or the model?

I think I would have a Model method that provides the records as an array of arrays, each element of the outer array being an array of records with the same first letter of the name. This would be asked for by the controller and passed to the view for display as appropriate. The view would put the links in using appropriate view helper methods.

Or perhaps the Model method would take a collection of records and convert it to the array of array form. Then one could do
@records = Model.find(…).in_groups or whatever the method is called