Heroku memory problem

gistfile1.rb

    coordinate_map = Destination.only(:id, :coordinates).all.inject({}) {|h, d| h[d.id.to_s] = d.coordinates.reverse; h}
    user_count = User.where(destination_id: nil).count
    0.step(user_count, BATCH_SIZE) do |offset|
      User.where(destination_id: nil).limit(BATCH_SIZE).skip(offset).each do |user|
        next unless user.coordinates.present?
        # This used to be a one-liner but broke it up to see if it helped mem-usage
        distances = coordinate_map.each_with_object({}) {|(k,v), h| h[k] = user.distance_from(v)}
        destination_id = distances.min_by(&:last).first
        user.update_attribute(:destination_id, destination_id) rescue nil
        distances = nil
        destination_id = nil
      end
    end