you dealing with where you have categories, and products in those categories, clothes website as an example. You may have following tree of categories: Imagine project Women Bags Clothes Shirts Jeans [SELECTED] Straight Skinny...Men Accessories Shoes... DB structure is very simple — id, parent_id, title for categories and category_id in products. Pretty straightforward, you can easily get parent and/or children of a category. But you need to consider — if you select JEANS, it means you want JEANS + Straight JEANS + Skinny JEANS and so on. To achieve that you need selected category id, and ids of its children. Its easy to get them with rails, just write a small custom scope for it. But what if children have their own children and tree is much deeper ? You will have to write advanced queries, joins, recursions to get ids of children on all levels. Or imagine you have selected last category, and you want to get all existing parents for it, or you want siblings of selected category (all items on the selected level, like Shirts, Jeans, …). By writing custom scopes and methods for all such cases, you will end up with huge and messy model pretty quickly. One of the best tools to deal with nested categories is gem Ancestry ( ), with a lot of helper scopes and methods, especially for cases mentioned above. https://github.com/stefankroes/ancestry Usage Ancestry is very easy to understand and use. Example project here: https://github.com/gioch/ancestry_test.git Add gem to Gemfile. ancestry Create model for categories (without parent_id or anything like that, ancestry will take care of it for us) Generate migration to add ancestry field to categories: rails g migration add_ancestry_to_category ancestry:string:index ### it will add new column to categories, which will handle parent/child stuff Add `has_ancestry` to Categories model class Category < ApplicationRecordhas_ancestryend Creating categories and subcategories Its easy to create subcategories, you just need to specify on create: parent women = Category.create(name: 'Women')women_bags = Category.create(name: 'Bags', parent: women)women_clothes = Category.create(name: 'Clothes', parent: women) # Check seeds.rb inside example project for more seeds. Getting all children for selected category Ancestry’s helpers are working like magic. Use method to get subcategories, or to get only an array of ids of children: children child_ids # category {Women}cat = Category.first # [{Bags}, {Clothes}]child_objects = cat.children # [2, 3]child_ids = cat.child_ids But, as mentioned above, if tree is very deep, and you want all levels of children, use instead: descendants # [{Bags}, {Clothes}, {Shirts}, {Jeans}, {Straight}, {Skinny}]all_children = cat.descendants # [2, 3, 4, 5, 6, 7]all_child_ids = cat.descendant_ids Getting all parents of selected category If you have selected last category in a tree, and you want to get all parents, use or : ancestors ancestor_ids # get category {Straight}cat = Category.find(7) # get parent category objects [{Women}, {Clothes}, {Jeans}]all_parents = cat.ancestors # get parent category ids [1, 3, 5]all_parent_ids = cat.ancestor_ids Getting products of specified category # get category {Clothes}cat = Category.find(3) # get child idsdescendant_ids # include selected category id if needed << cat.id descendant_ids Product.where(category_id: ) descendant_ids Ancestry has a lot of other helpful methods. Additional Resources Check Ancestry’s github page here - https://github.com/stefankroes/ancestry Check Railscast for ancestry — http://railscasts.com/episodes/262-trees-with-ancestry
