Question

I’m using DataMapper to manage a database that has points of interest (POIs) located by latitude and longitude. I would like to do a query and find all POIs within x distance of a given latitude and longitude. For example, all POIs within 1000m of latitude 45, longitude 90.

I set this up:

class POI
  include DataMapper::Resource
  property :id,                  String,  :key => true
  property :title,               String,  :required => true
  property :lat,                 Float,   :required => true
  property :lon,                 Float,   :required => true

  def distance(latitude, longitude)
    # Taken from https://github.com/almartin/Ruby-Haversine
    earthRadius = 6371 # Earth's radius in km

    # convert degrees to radians
    def convDegRad(value)
      unless value.nil? or value == 0
        value = (value/180) * Math::PI
      end
      return value
    end

    deltaLat = (self.lat - latitude)
    deltaLon = (self.lon - longitude)
    deltaLat = convDegRad(deltaLat)
    deltaLon = convDegRad(deltaLon)

    # Calculate square of half the chord length between latitude and longitude
    a = Math.sin(deltaLat/2) * Math.sin(deltaLat/2) +
      Math.cos((self.lat/180 * Math::PI)) * Math.cos((latitude/180 * Math::PI)) *
      Math.sin(deltaLon/2) * Math.sin(deltaLon/2);
    # Calculate the angular distance in radians
    c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))

    distance = earthRadius * c
    return distance
  end
end

I’d like to be able to find records with a call similar to this:

pois = POI.all(distance(45,90).lte => 1000)

But that gives an error:

./poi-provider.rb:44:in `<main>': undefined method `distance' for main:Object (NoMethodError)

I read dkubb’s answer about defining complex queries in a method but this is different because I need to pass parameters in and I’m trying to use the method as a condition.

How can I do that—or, is there a better way to use DataMapper to find points near a given latitude and longitude without breaking down and just using raw SQL?

Answer 1

1. You’re calling an instance method without instantiating. That’s what is causing your error.
2. You can’t use DataMapper methods as conditions. This means you’re going to have to write some SQL.

Something like:

class POI
  ...
  def self.all_within_distance(distance, longitude, latitude)
    repository.adapter.query "SELECT *
    FROM pois 
    WHERE " + distance.to_s + " > ** Complicated math coded in SQL that gets uses poi.lon and poi.lat to get the distance from the given longitude and latitude ** " 
  end

Then call

POI.all_within_distance(1000, 45, 90)