example.rb

Route.where {
  Sequel.earth_distance(:start_point, :end_point).km <= 5
}

Route.where {
  Sequel.earth_distance(:start_point, { lat: 10.234123, lng: 25.234782 }).mi >= 100
}

Route.where {
  Sequel.earth_distance(:start_point, [10.234123, 25.234782]).mi <= 1000 # [lng, lat]
}

pg_earth_distance.rb

# frozen-string-literal: true

#
# The pg_earth_distance extension adds support to Sequel's DSL to calculate
# earth distance between GPS points.
#
module Sequel
  module Postgres
    class EarthDistance < Sequel::SQL::NumericExpression
      def initialize(from, to)
        super(
          :NOOP,
          Sequel::SQL::PlaceholderLiteralString.new(
            ['(', ' <@> ', ')'].freeze,
            [_parse_argument(from), _parse_argument(to)]
          )
        )
      end

      def miles
        self
      end

      def km
        Sequel.*(self, 1.609344)
      end

      private

      def _parse_argument(arg)
        # TODO: maybe convert to sequel expression where possible?
        if arg.is_a?(Symbol)
          Sequel.lit("point (#{arg}[1], #{arg}[0])")
        elsif arg.is_a?(Array)
          Sequel.lit("point (#{arg[1]}, #{arg[0]})")
        elsif arg.is_a?(Hash)
          Sequel.lit("point (#{arg[:lng]}, #{arg[:lat]})")
        else
          Sequel.lit(arg)
        end
      end
    end
  end

  module SQL
    module Builders
      def earth_distance(from, to)
        Postgres::EarthDistance.new(from, to)
      end
    end
  end
end

XXX_add_earthdistance_extension.rb

# frozen_string_literal: true

Sequel.migration do
  up do
    run 'CREATE EXTENSION IF NOT EXISTS "cube";'
    run 'CREATE EXTENSION IF NOT EXISTS "earthdistance";'
  end

  down do
    run 'DROP EXTENSION IF EXISTS "earthdistance";'
    run 'DROP EXTENSION IF EXISTS "cube";'
  end
end