Ознакомьтесь с документацией MongoDB и документацией по индексированию Mongoid .
class Zip
include Mongoid::Document
field :code
field :location, :type => Array
# make sure to rake db:mongoid:create_indexes
index [[ :location, Mongo::GEO2D ]], :min => 200, :max => 200
end
# at the console
Zip.create(:code => 1001, :location => [0, 0])
Zip.create(:code => 1002, :location => [1, 1])
Zip.create(:code => 1003, :location => [2, 1])
Zip.create(:code => 1004, :location => [-1, -1])
Zip.where(:location => { '$near' => [0, 0] } ).count
# 4
Zip.where(:location => { '$near' => [0, 0], '$maxDistance' => 1.5 } ).count
# 3
Zip.where(:location => { '$near' => [0, 0], '$maxDistance' => 1 } ).first
# 1