From 5fadd12be6bb2fef1da4caec37723c44a5011852 Mon Sep 17 00:00:00 2001
From: Akinori MUSHA <knu@idaemons.org>
Date: Tue, 9 May 2017 14:40:22 +0900
Subject: [PATCH] Protect the latest event from automatic deletion when using
 MySQL

Detecting the database type by the name this time, because
ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter is defined only
if the mysql2 gem is loaded.

This works around #1940 for the most typical cases.
---
 app/models/event.rb       | 17 ++++++++++++++---
 spec/models/event_spec.rb | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/app/models/event.rb b/app/models/event.rb
index 024c7f5e..3f2f1cc9 100644
--- a/app/models/event.rb
+++ b/app/models/event.rb
@@ -28,6 +28,15 @@ class Event < ActiveRecord::Base
     where("expires_at IS NOT NULL AND expires_at < ?", Time.now)
   }
 
+  case ActiveRecord::Base.connection.adapter_name
+  when /\Amysql/i
+    # Protect the Event table from InnoDB's AUTO_INCREMENT Counter
+    # Initialization by always keeping the latest event.
+    scope :to_expire, -> { expired.where.not(id: maximum(:id)) }
+  else
+    scope :to_expire, -> { expired }
+  end
+
   scope :with_location, -> {
     where.not(lat: nil).where.not(lng: nil)
   }
@@ -72,9 +81,11 @@ class Event < ActiveRecord::Base
   # Look for Events whose `expires_at` is present and in the past.  Remove those events and then update affected Agents'
   # `events_counts` cache columns.  This method is called by bin/schedule.rb periodically.
   def self.cleanup_expired!
-    affected_agents = Event.expired.group("agent_id").pluck(:agent_id)
-    Event.expired.delete_all
-    Agent.where(:id => affected_agents).update_all "events_count = (select count(*) from events where agent_id = agents.id)"
+    transaction do
+      affected_agents = Event.expired.group("agent_id").pluck(:agent_id)
+      Event.to_expire.delete_all
+      Agent.where(id: affected_agents).update_all "events_count = (select count(*) from events where agent_id = agents.id)"
+    end
   end
 
   protected
diff --git a/spec/models/event_spec.rb b/spec/models/event_spec.rb
index e5458c4e..92d34b70 100644
--- a/spec/models/event_spec.rb
+++ b/spec/models/event_spec.rb
@@ -118,6 +118,20 @@ describe Event do
       Event.cleanup_expired!
       expect(Event.find_by_id(event.id)).not_to be_nil
     end
+
+    it "always keeps the latest Event regardless of its expires_at value only if the database is MySQL" do
+      Event.delete_all
+      event1 = agents(:jane_weather_agent).create_event expires_at: 1.minute.ago
+      event2 = agents(:bob_weather_agent).create_event expires_at: 1.minute.ago
+
+      Event.cleanup_expired!
+      case ActiveRecord::Base.connection.adapter_name
+      when /\Amysql/i
+        expect(Event.all.pluck(:id)).to eq([event2.id])
+      else
+        expect(Event.all.pluck(:id)).to be_empty
+      end
+    end
   end
 
   describe "after destroy" do