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PostGIS and Rails
James R. Bracy
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PostGIS is and extension for PostgreSQL that adds support for geographic objects. Daniel Azuma has developed a suite of tools for Rails to process geographic data.

Working with geo-spacial data has gotten easier, but setting up an environment, is still a bit of work. This tutorial will walk through setting up PostGIS for Rails.

Dependencies

This tutorial assumes that you are running Mac OS X and have the following installed:

With all of the above setup the first thing that we need to install is PostgreSQL and PostGIS.

brew install postgres postgis

Geos and Proj are both dependencies of PostGIS and are installed by Homebrew with PostGIS. Both of these are needed for the RGeo gem.

Setting Up Rails

For this example we'll create a new Rails application, if you are adding PostGIS support to your current Rails app then skip this step.

rails new geo_events
cd geo_events

Open the Gemfile and add the following gems to it.

The file should contain the following lines:

gem 'pg'
gem 'rgeo'
gem 'activerecord-postgis-adapter'

If the pg gem has already been installed into your system and was build against a different version of PostgreSQL you need to reinstall it. Simply run gem install pg. The same goes for RGeo and PostGIS. Run gem install rgeo to reinstall the gem with native extensions.

The activerecord-postgis-adapter is the database adapter that will be using instead of the normal postgresql adapter. Add the following line after require 'rails/all' to config/application.rb.

require 'active_record/connection_adapters/postgis_adapter/railtie'

The config/application.rb file should now look like:

require File.expand_path('../boot', __FILE__)

require 'rails/all'
require 'active_record/connection_adapters/postgis_adapter/railtie'

if defined?(Bundler)
  Bundler.require(*Rails.groups(:assets => %w(development test)))
end

module GeoEvents
  class Application < Rails::Application
    config.encoding = "utf-8"
    config.filter_parameters += [:password]
    config.active_support.escape_html_entities_in_json = true
    config.active_record.whitelist_attributes = true
    config.assets.enabled = true
    config.assets.version = '1.0'
  end
end

Next update the database.yml file to make use of the new adapter and geo-spacial extensions.

development:
  adapter: postgis
  database: geoevents-dev
  encoding: utf8
  postgis_extension: postgis
  schema_search_path: '"$user", public, postgis'

Notice that the adapter was changed to postgis and the postgis_extension and schema_search_path lines were added. The postgis_extension tells the app to install the PostGIS extensions to the database when you run rake db:create. It will run commands similar to the following:

-- NOTE: These commands do not need to be run manually unless you are
--       installing into a existing database.
CREATE SCHEMA postgis;
CREATE EXTENSION postgis WITH SCHEMA postgis;

The PostGIS extensions are namespaced in the database so that they aren't exported to the db/schema.rb file. Because the extensions are stored in a different namespace the schema_search_path line is added so that anytime a query is run it will look for tables or views in the postgis path in addition to the default "$user" and public path.

Now bundle the app and create the database.

bundle
rake db:create

Verify that the extensions are installed with psql.

psql geoevents-dev
geoevents-dev# \d
No relations found.
geoevents-dev# show search_path;
  search_path   
----------------
 "$user",public
(1 row)


geoevents-dev# set search_path = "$user", public, postgis;
SET
geoevents-dev# \d
                  List of relations
 Schema  |        Name        |   Type   |   Owner   
---------+--------------------+----------+-----------
 postgis | geography_columns  | view     | waratuman
 postgis | geometry_columns   | view     | waratuman
 postgis | raster_columns     | view     | waratuman
 postgis | raster_overviews   | view     | waratuman
 postgis | spatial_ref_sys    | table    | waratuman
(5 rows)

The PostGIS views and table are installed.

Creating a Model

Now create a model that will have a geo-spacial feature.

rails g model earthquake

Open the database migration for the model and add the columns given below.

class CreateEarthquakes < ActiveRecord::Migration
  def change
    create_table :earthquakes do |t|
      t.point :center, :srid => 4326, :null => false
      t.decimal :magnitude, :null => false

      t.timestamps
    end
  end
end

The column center is a geometric column with an SRID of 4326. You don't need to worry much about weather the column is geometric or geography, for more info on this read this. If you don't know stick with geometric and an SRID 4326. SRID specified the spacial reference system that is used. This is the one used by Google maps. There are other types of geometric columns such as line, polygon,and multi-polygon. This will tell you about all of the options.

Now migrate the database.

rake db:migrate

And add the RGeo factory for creating the point to the Earthquake model (app/models/earthquake.rb).

class Earthquake < ActiveRecord::Base
  self.rgeo_factory_generator = RGeo::Geos.factory_generator(:srid => 4326)

end

And try to create a model from the console.

rails c
> e = Earthquake.new
> e.center = 'POINT(112.5 5655.6)'
> e.magnitude = 5
> e.save
> e.center.x
> e.center.y

The center is inputed as as a string in WKT format.

Notice that the center isn't validated to be within ±180 for longitude and ±90 for latitude. Also calling center.latitude or center.longitude results in an error. Only center.x or center.y work. Lets fix that by monkey patching RGeo. Create a file namelib/rgeo.rb and type the following.

class RGeo::Geos::CAPIPointImpl

  alias_method :lat, :y
  alias_method :latitude, :y

  alias_method :lon, :x
  alias_method :lng, :x
  alias_method :longitude, :x

end

This simple creates aliases for x and y so that latitude and longitude can be called. Add the following line to the end of application.rb so that our extension gets loaded.

require "#{Rails.root}/lib/rgeo"

Now add validation to the Earthquake model.

class Earthquake < ActiveRecord::Base
  self.rgeo_factory_generator = RGeo::Geos.factory_generator(:srid => 4326)

  delegate :latitude, :to => :center, :allow_nil => true
  delegate :longitude, :to => :center, :allow_nil => true

  validates :latitude, :presence => true, :numericality => { :greater_than_or_equal_to => -90, :less_than_or_equal_to => 90 }
  validates :longitude, :presence => true, :numericality => { :greater_than_or_equal_to => -180, :less_than_or_equal_to => 180 }
end

Now try to create the same model as before in the console and it should have errors on the latitude and longitude.

rails c
> e = Earthquake.new
> e.center = 'POINT(112.5 5655.6)'
> e.magnitude = 5
> e.save
=> false
> e.errors.to_a
=> ["Latitude must be less than or equal to 90"]

And now a valid model:

rails c
> e = Earthquake.new
> e.center = 'POINT(12.5 5.6)'
> e.magnitude = 5
> e.save
> e.center.x
> e.center.y
> e.center.latitude
> e.center.longitude

Conclusion

These are the basics of setting up Rails for geo-spacial processing. There are many more data types and all kinds of uses for web applications. The reference manual for PostGIS is a great resource. If you are looking for some data to play around with TIGER has some great data from the the U.S. Census Bureau.

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