Reset primary keys after swapping events tables

This is required as otherwise newly created events will start with the
wrong ID.

db/migrate/20170830131015_swap_event_migration_tables.rb

@@ -7,6 +7,10 @@ class SwapEventMigrationTables < ActiveRecord::Migration
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
+  class Event < ActiveRecord::Base
+    self.table_name = 'events'
+  end
+
   def up
     rename_tables
   end
@@ -19,5 +23,25 @@ class SwapEventMigrationTables < ActiveRecord::Migration
     rename_table :events, :events_old
     rename_table :events_for_migration, :events
     rename_table :events_old, :events_for_migration
+
+    # Once swapped we need to reset the primary key of the new "events" table to
+    # make sure that data created starts with the right value. This isn't
+    # necessary for events_for_migration since we replicate existing primary key
+    # values to it.
+    if Gitlab::Database.postgresql?
+      reset_primary_key_for_postgresql
+    else
+      reset_primary_key_for_mysql
+    end
+  end
+
+  def reset_primary_key_for_postgresql
+    reset_pk_sequence!(Event.table_name)
+  end
+
+  def reset_primary_key_for_mysql
+    amount = Event.pluck('COALESCE(MAX(id), 1)').first
+
+    execute "ALTER TABLE #{Event.table_name} AUTO_INCREMENT = #{amount}"
   end
 end