Fix migrations task for country petition journals

A `TRUNCATE` followed by an `INSERT ... FROM` doesn't really work that
well on a busy site like the petitions website because new journals will
start to appear before the `INSERT` completes. Fix this by doing it in
Ruby and locking the journals before calculating the signatures.

To do this in a performant manner we need an index on the petition_id
and location_code for the signatures table. We need to do this
concurrently otherwise it will block writes to the table, preventing
anyone from signing any petitions.

db/migrate/20160217192016_add_location_code_index_to_country_petition_journal.rb

@@ -0,0 +1,15 @@
+class AddLocationCodeIndexToCountryPetitionJournal < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def up
+    unless index_exists?(:signatures, [:petition_id, :location_code])
+      add_index :signatures, [:petition_id, :location_code], algorithm: :concurrently
+    end
+  end
+
+  def down
+    if index_exists?(:signatures, [:petition_id, :location_code])
+      remove_index :signatures, [:petition_id, :location_code]
+    end
+  end
+end

db/structure.sql

@@ -1240,6 +1240,13 @@ CREATE UNIQUE INDEX index_signatures_on_email_and_petition_id_and_name ON signat
 CREATE INDEX index_signatures_on_petition_id ON signatures USING btree (petition_id);
 
 
+--
+-- Name: index_signatures_on_petition_id_and_location_code; Type: INDEX; Schema: public; Owner: -; Tablespace: 
+--
+
+CREATE INDEX index_signatures_on_petition_id_and_location_code ON signatures USING btree (petition_id, location_code);
+
+
 --
 -- Name: index_signatures_on_updated_at; Type: INDEX; Schema: public; Owner: -; Tablespace: 
 --
@@ -1497,3 +1504,5 @@ INSERT INTO schema_migrations (version) VALUES ('20160211003703');
 
 INSERT INTO schema_migrations (version) VALUES ('20160211020341');
 
+INSERT INTO schema_migrations (version) VALUES ('20160217192016');
+

lib/tasks/migrations.rake

@@ -239,7 +239,20 @@ namespace :epets do
         signature.save(validate: false)
       end
 
-      CountryPetitionJournal.reset!
+      Petition.find_each do |petition|
+        locations = petition.signatures.validated.distinct.pluck(:location_code).compact
+
+        locations.each do |location|
+          journal = CountryPetitionJournal.for(petition, location)
+
+          journal.with_lock do
+            signature_count = petition.signatures.validated.where(location_code: location).count
+            journal.update(signature_count: signature_count)
+          end
+        end
+
+        petition.touch
+      end
     end
   end
 end