tag implications: convert descendant_names to array (#3987).

db/migrate/20181114180205_change_descendant_names_to_array_on_tag_implications.rb

@@ -0,0 +1,13 @@
+class ChangeDescendantNamesToArrayOnTagImplications < ActiveRecord::Migration[5.2]
+  def up
+    TagImplication.without_timeout do
+      change_column :tag_implications, :descendant_names, "text[]", using: "string_to_array(descendant_names, ' ')::text[]", default: "{}"
+    end
+  end
+
+  def down
+    TagImplication.without_timeout do
+      change_column :tag_implications, :descendant_names, "text", using: "array_to_string(descendant_names, ' ')", default: nil
+    end
+  end
+end