pools: store post_ids as array instead of string (fix #3979)

db/migrate/20181108162204_change_post_ids_to_array_on_pools.rb

@@ -0,0 +1,17 @@
+class ChangePostIdsToArrayOnPools < ActiveRecord::Migration[5.2]
+  def up
+    Pool.without_timeout do
+      change_column_default :pools, :post_ids, nil
+      change_column :pools, :post_ids, "integer[]", using: "string_to_array(post_ids, ' ')::integer[]"
+      change_column_default :pools, :post_ids, "{}"
+    end
+  end
+
+  def down
+    Pool.without_timeout do
+      change_column_default :pools, :post_ids, nil
+      change_column :pools, :post_ids, :text, using: "array_to_string(post_ids, ' ')"
+      change_column_default :pools, :post_ids, ""
+    end
+  end
+end