Accessing PostGIS spatial data from Rails
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20-09-2019 - |
Question
I need to use an existing PostGIS database from my Rails application. So far I am able to access the DB just fine, GeoRuby nicely converts the 'geom' column into a Point object.
What I am looking for is an easy way to execute ActiveRecord-like queries on those tables, e.g.
Poi.find_within_radius(...)
or similar spatial queries like distance calculations et. al.
I tried several combinations of geokit, accompanying rails plugins but I'm quite sure there must be something better out there in the ruby/rails universe. Any hints?
Solution
Since you've got GeoRuby, put in your Poi model
SRID = 4326 #WGS84
# geometry column is geom
# pt is a GeoRuby Point
def self.find_within_radius(pt, dist = 0.01)
self.all :conditions => "ST_DWithin(geom, ST_GeomFromText('POINT(#{pt.x} #{pt.y})', #{SRID}), #{dist})"
end
For distance calculations you can use methods on the point objects
dist_circle = poi1.geom.spherical_distance(poi2.geom)
dist_projected = poi1.geom.euclidean_distance(poi2.geom)
OTHER TIPS
Adding to synecdoche's answer, in Rails 3 you can use ActiveRelation
scopes, which would allow you to chain queries.
Untested:
# Poi.rb
scope :within, proc { |point, distance|
p = "POINT(#{point.x} #{point.y})"
where("ST_DWithin(geom, ST_GeomFromText('#{p}', #{SRID}), #{distance})")
}
scope :rated, proc { |rating| where(rating: rating) }
Usage:
home = Point.new(5,5)
Poi.rated(5).within(home, 25)
Even if you don't chain scopes, most of the time it's better than using class methods.
ST_Distance() doesn't means you probably don't have postgis installed or you didn't create a postgis template... or you didn't create your database using the postgis template if the template exists.