Add optimistic lock to avoid race conditions when handling votes (#10196)

* Add optimistic lock to avoid race conditions when handling votes

* Force-reload polls when getting `ActiveRecord::StaleObjectError`

2 files changed, 8 insertions, 1 deletions

diff --git a/db/migrate/20190306145741_add_lock_version_to_polls.rb b/db/migrate/20190306145741_add_lock_version_to_polls.rb
new file mode 100644
index 00000000000..cf506404da6
--- /dev/null
+++ b/db/migrate/20190306145741_add_lock_version_to_polls.rb
@@ -0,0 +1,6 @@
+class AddLockVersionToPolls < ActiveRecord::Migration[5.2]
+  def change
+    add_column :polls, :lock_version, :integer, null: false, default: 0
+  end
+end
+
diff --git a/db/schema.rb b/db/schema.rb
index 161619dcf59..3d526027093 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_03_04_152020) do
+ActiveRecord::Schema.define(version: 2019_03_06_145741) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -464,6 +464,7 @@ ActiveRecord::Schema.define(version: 2019_03_04_152020) do
     t.datetime "last_fetched_at"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "lock_version", default: 0, null: false
     t.index ["account_id"], name: "index_polls_on_account_id"
     t.index ["status_id"], name: "index_polls_on_status_id"
   end