postgis-rails-3.2-experiments

This is a Rails 3.2.x proof-of-concept application.

Its purpose is to demonstrate how to use the PostGIS extensions for Postgresql through a simple example of business locations.

Most of this guide was sourced from the activerecord-postgis-adapter guide process

Setup

Using the PostGIS extension requires a little extra work. But it's not difficult.

Database & Rails Configuration

Add to the Gemfile

gem 'activerecord-postgis-adapter'

If you don't have a non-admin Postgres database user, now is the perfect time to create one

$ psql -c "CREATE ROLE rails_user WITH PASSWORD 'rails_user' LOGIN CREATEDB;"

Change the adapter setting on database.yml

development: &postgres
  adapter:   postgis
  database:  postgis-ft-dev
  host:      localhost
  user:      rails_user
  password:  rails_user
  port:      5432
  pool:      25
  timeout:   5000
  schema_search_path: public

Note: When I was working on my local OSX machine, I had to add the PostGIS extension to Postgres' template1. This will then include the PostGIS extension in every new database.

For me this was necessary to silence Postgres type errors when running migrations with the new GEOGRAPHY type. Postgres did not have knowledge of the GIS types.

The work around:

psql -c 'CREATE EXTENSION postgis;' -d template1

Create the current database and enable the PostGIS extension

$ rake db:create
$ psql -c 'CREATE EXTENSION IF NOT EXISTS postgis;' -d postgis-ft-dev
$ psql -c 'CREATE EXTENSION IF NOT EXISTS postgis;' -d postgis-ft-test

Generate Model for Businesses

Use the Rails generators to create the basics

$ rails g scaffold Business name:string latlong:point

Edit the new migration to add additional constraints and an index

class CreateBusinesses < ActiveRecord::Migration
  def change
    create_table :businesses do |t|
      t.string :name
      t.point :latlon, geographic: true

      t.timestamps
    end
    add_index :businesses, :latlon, using: :gist
  end
end

Migrate the database and (optionally) load seed data

$ rake db:migrate
$ rake db:seed

---

Finding Businesses that are within a given radius

Now this gets fun. Let's customize the Business class to filter by distance from a given point.

class Business < ActiveRecord::Base
  attr_accessible :latlong, :name

  set_rgeo_factory_for_column(:latlong,
    RGeo::Geographic.spherical_factory(:srid => 4326))

  delegate :latitude, :to => :latlong
  delegate :longitude, :to => :latlong

  scope :within_range_from_lat_long, ->(range_in_meters, lat, long) { 
    where("ST_Distance(latlong, 'POINT(#{long} #{lat})') < #{range_in_meters}") }
end

Now we can call Business.within_range_from_lat_long to easily find all businesses that are within a specific range from any given point!

Testing

RSpec tests exist to verify functionality and demonstrated use.

More reading

Daniel Azuma's 9-part tutorial on working with geospatial operations in Rails