Easy, high performance geospatial nearest-neighbor searches with ActiveRecord, leveraging PostGIS.

Dependencies:

1. PostgreSQL.

2. ActiveRecord.

3. PostGIS ActiveRecord Adapter gem.

In your Gemfile:

  gem 'active_record_nearest_neighbor'

The central feature of this gem is a scope method for your Active Record models, close_to. With close_to, you can perform blazing fast and accurate nearest neighbor searches with PostGIS, without writing a line of geospatial SQL. All you need to do is add the appropriate geospatial columns, include a module, and you're set:

Active Record Nearest Neighbor provides you with several helpful rake tasks to get you started:

1. If you don't yet have the PostGIS extension added to your PostgreSQL database:

  rake db:gis:setup

2. In your config/database.yml file, change the adapater of your database to postgis:

     database: postgis
   

3. Generate a rake task to create a table or add columns:

4. To generate a migration to create a new table with geospatial columns:

```
  rake nearest_neighbor:create[table_name]
```

2. To generate a migration to add geospatial columns and point index to a table that you already have:

```
  rake nearest_neighbor:add_columns[table_name]
```

4. Run the generated migration with $ rake db:migrate

5. To add close_by to your model class, include NearestNeighbor:

   class Building < ActiveRecord::Base
     include NearestNeighbor

   end

6. Now you're set!

  # Buildings close to the Empire State Building
  latitude = 40.748441
  longitude = -73.985664
  Building.close_to(longitude, latitude)

You can use close_to in different ways to perform the nearest neighbor query that you want. By default, close_to will use a bounding box of 10 kilometers. Set the :distance option (uses meters) if you want to change the size of this bounding box.

  # Buildings within 500 meters from the Empire State Building
  latitude = 40.748441
  longitude = -73.985664
  Building.close_to(longitude, latitude, distance: 500)

If you want to know what's close to your geospatial Active Record objects, simply pass the object to close_to instead of longitude and latitude!

  # Buildings within 500 meters from the Space Needle 
  space_needle = Building.find_by_name("Space Needle")
  Building.close_to(space_needle, distance: 500)

Maybe you need to avoid a bounding box? No sweat! Provide the :k_nearest_neighbor option to close_to and close_to will avoid a bounding box. This is great for nearest neighbor searches with data of greatly varying distances and/or datasets that aren't significantly large. NOTE: This will have slower performance with larger datasets.

  # Volcanoes, ordered by proximity to Mount Rainier 
  mount_rainier = Volcanoe.find_by_name("Mount Rainier")
  Volcano.close_to(mount_rainier, method: :k_nearest_neighbor)

You can provide close_to with a limit option if you know a limit ahead of time:

  # The 5 closest roller coasters to Los Angeles 
  RollerCoaster.close_to(-118.243685, 34.052234, method: :k_nearest_neighbor, limit: 5)