Install the GeoKit gem and the Rails plugin:
```ruby
sudo gem install geokit
script/plugin install git://github.com/andre/geokit-rails.git
```
Edit config/environment.rb:
```ruby
Rails::Initializer.run do |config|
config.gem "geokit"
end
```
Generate a model and migration file with script/generate model Place:
```ruby
class CreatePlaces < ActiveRecord::Migration
def self.up
create_table :places do |t|
t.string :name
t.string :external_id
t.string :external_type
t.decimal :latitude, :precision => 15, :scale => 10
t.decimal :longitude, :precision => 15, :scale => 10
t.timestamps
end
add_index :places, :latitude
add_index :places, :longitude
add_index :places, [:latitude, :longitude]
end
def self.down
drop_table :places
end
end
```
Configure GeoKit:
```ruby
class Place < ActiveRecord::Base
acts_as_mappable :default_units => :kms,
:default_formula => :sphere,
:distance_field_name => :distance,
:lat_column_name => :latitude,
:lng_column_name => :longitude
end
```
Use your GeoKit enabled model:
```ruby
Place.find_within(105, :origin => Place.last)
```