Add task to backfill location polygons centroid

The task will calculate and populate the centroid of the location
polygons with an area but not stored centroid info.

lib/tasks/backfill_location_polygons_centroid.rake

@@ -0,0 +1,6 @@
+namespace :location_polygons do
+  desc "Computes and set centroid on location polygons containing an area but no centroid"
+  task backfill_centroid: :environment do
+    LocationPolygon.where.not(area: nil).where(centroid: nil).update_all("centroid = ST_Centroid(area)")
+  end
+end

spec/factories/location_polygons.rb

@@ -3,5 +3,6 @@
     name { "London" }
     location_type { "cities" }
     area { "POLYGON((0 0, 1 1, 0 1, 0 0))" }
+    centroid { "POINT(0.33331264776372055 0.6666929898148579)" }
   end
 end

spec/tasks/backfill_location_polygons_centroid_spec.rb

@@ -0,0 +1,26 @@
+require "rails_helper"
+
+RSpec.describe "location_polygons:backfill_centroid" do
+  let(:area) { "POLYGON((0 0, 1 1, 0 1, 0 0))" }
+  let(:centroid) { "POINT(0.33331264776372055 0.6666929898148579)" }
+
+  it "computes and sets the centroid on location polygons containing an area but no centroid" do
+    location_polygon = create(:location_polygon, area: area, centroid: nil)
+
+    expect { task.invoke }.to change { location_polygon.reload.centroid }.from(nil).to(
+      RGeo::Geographic.spherical_factory(srid: 4326).parse_wkt(centroid),
+    )
+  end
+
+  it "does not update the centroid if it is already set" do
+    location_polygon = create(:location_polygon, area: area, centroid: centroid)
+
+    expect { task.invoke }.not_to(change { location_polygon.reload.centroid })
+  end
+
+  it "does not update the centroid if the area is nil" do
+    location_polygon = create(:location_polygon, area: nil, centroid: nil)
+
+    expect { task.invoke }.not_to(change { location_polygon.reload.centroid })
+  end
+end