def block in rake task

Update: Gotcha

This potentially adds the method to global scope and will conflict with any other method with the same name. Look at @Hula_Zell’s answer https://stackoverflow.com/a/44294243/584440 for a better way.

Original answer

You are defining the method inside the rake task. For getting the function, you should define outside the rake task (outside the task block). Try this:

include Geokit::Geocoders

namespace :geocode do
  desc "Geocode to get latitude, longitude and address"
  task :all => :environment do
    @spot = Spot.find(:first)
    if @spot.latitude.blank? && !@spot.address.blank?
      puts address_geo
    end
  end

  def address_geo
    arr = []
    arr << address if @spot.address
    arr << city if @spot.city
    arr << country if @spot.country
    arr.reject{|y|y==""}.join(", ")
  end
end