ADD a field 'ancestry' to project_categories table

app/models/project_category.rb

@@ -1,2 +1,3 @@
 class ProjectCategory < ApplicationRecord
+  has_ancestry
 end

db/migrate/20191218021144_add_ancestry_to_project_categories.rb

@@ -0,0 +1,6 @@
+class AddAncestryToProjectCategories < ActiveRecord::Migration[5.2]
+  def change
+    add_column :project_categories, :ancestry, :string
+    add_index :project_categories, :ancestry
+  end
+end