Fixed JoinTableLanguageLearningObject migration

db/migrate/20190227115926_create_join_table_language_learning_object.rb

@@ -6,11 +6,58 @@ class CreateJoinTableLanguageLearningObject < ActiveRecord::Migration[5.0]
       # t.index [:learning_object_id, :language_id]
     end
 
-    LearningObject.with_deleted.all.each do |lo|
+    reversible do |migration|
+      start = 1
+      finish = 999
+      batch_size = 1000
+      last_id = LearningObject.with_deleted.last.id
+
+      migration.up do
+        loop do
+          begin
+            LearningObject.with_deleted.find_each(start: start, finish: finish) do |lo|
               if (lo.language_id)
                 lo.update_attributes(language_ids: lo.language_id)
           	  end
             end
+          rescue PG::ConnectionBad => e
+            logger.warn 'Database error, going to sleep'
+            logger.error e
+            # Sleeps for a while to wait database's recovery
+            sleep(10.seconds)
+            # Goes to next iteration to retry
+            next
+          else
+            start += batch_size - 1
+            finish += batch_size
+            break if start >= last_id
+          end
+        end
+      end
+
+      migration.down do
+        loop do
+          begin
+            LearningObject.with_deleted.find_each(start: start, finish: start) do |lo|
+              if (lo.language_id)
+                lo.update_attributes(language_id: lo.language_ids.first)
+          	  end
+            end
+          rescue PG::ConnectionBad => e
+            logger.warn 'Database error, going to sleep'
+            logger.error e
+            # Sleeps for a while to wait database's recovery
+            sleep(10.seconds)
+            # Goes to next iteration to retry
+            next
+          else
+            start += batch_size - 1
+            finish += batch_size
+            break if start >= last_id
+          end
+        end
+      end
+    end
 
     remove_column :learning_objects, :language_id
   end