Merge branch 'cascade_drop' into 'master'

Add if exists to drop command

Add `IF EXISTS` as a precaution. Related to gitlab-org/gitlab-ce!4020

See merge request !4100

lib/tasks/gitlab/db.rake

@@ -29,10 +29,12 @@ namespace :gitlab do
       tables.delete 'schema_migrations'
       # Truncate schema_migrations to ensure migrations re-run
       connection.execute('TRUNCATE schema_migrations')
+
       # Drop tables with cascade to avoid dependent table errors
       # PG: http://www.postgresql.org/docs/current/static/ddl-depend.html
       # MySQL: http://dev.mysql.com/doc/refman/5.7/en/drop-table.html
-      tables.each { |t| connection.execute("DROP TABLE #{t} CASCADE") }
+      # Add `IF EXISTS` because cascade could have already deleted a table.
+      tables.each { |t| connection.execute("DROP TABLE IF EXISTS #{t} CASCADE") }
     end
   end
 end