Rails how to sum columns?

You can summarize directly on the database by creating the respective SQL like this:

Pakke.sum(:prismd, :conditions => {:id => [4,5]})

On newer Rails versions (i.e. Rails >= 4.0), you can use an ActiveRecord queries more intuitively, such as:

Pakke.where(id: [4,5]).sum(:prismd)

See ActiveRecord::Calculations for more usage examples and general documentation.