gistfile1.rb

[50, 100, 200, 500, 1000, 5000].each do |buffer|
  puts "Calculating population within #{buffer} m of each shape_geometry"
  Shape.find_each do |s|
    execute "UPDATE shapes SET pop#{buffer}m = 
               (SELECT SUM((ST_SummaryStats(ST_Clip(rast,1,ST_Buffer(geom::geography, #{buffer})::geometry))).sum) 
                FROM shapes, population_tiles 
                WHERE id = #{s.id} AND ST_Intersects(ST_Buffer(geom::geography, #{buffer})::geometry,rast) 
                GROUP BY id) 
             WHERE id = #{s.id}"
  end
end