Creating a model that has a tree structure - ruby-on-rails

Question

I have categories that are in a tree structure. I am trying to link them together by defining a parent for each one. (I couldn't figure out how to call the property parent so it's just category for now, but it means the parent).
class Category < ActiveRecord::Base
has_one :category # the parent category
end
But the relationship ends up the wrong way around.
The getter function is on the child category (correctly) but the category_id is stored on the parent:
parent = Category.create(:name => "parent")
child = Category.create(:name => "child", :category => parent)
parent.id # 1
child.id # 2
child.category_id # nil
parent.category_id # 2
child.category.name # "parent" (!!)
The parent needs to be able to have multiple children so this isn't going to work.

Answer 1

What you're looking for is self joins. Check this section of the Rails guide out: http://guides.rubyonrails.org/association_basics.html#self-joins
class Category < ActiveRecord::Base
  has_many :children, class_name: "Category", foreign_key: "parent_id"
  belongs_to :parent, class_name: "Category"
end
Every Category will belong_to a parent, even your parent categories. You can create a single category parent that your highest level categories all belong to, then you can disregard that information in your application.

Answer 2

You can use acts_as_tree gem to achieve this, find below example and link.
https://github.com/amerine/acts_as_tree/tree/master
class Category < ActiveRecord::Base
include ActsAsTree
acts_as_tree order: "name"
end
root = Category.create("name" => "root")
child1 = root.children.create("name" => "child1")
subchild1 = child1.children.create("name" => "subchild1")
root.parent # => nil
child1.parent # => root
root.children # => [child1]
root.children.first.children.first # => subchild1

Answer 3

You should take a look at the ancestry gem: https://github.com/stefankroes/ancestry
It provides all the functionality you need and is able to get all descendants, siblings, parents, etc with a single SQL query by using a variant of materialized paths so it'll have better performance than the self-joins and acts_as_tree answers above.

Answer 4

Category should have many categories, and the foreign key of each category should be the parent_id. So, when you do parent.children it lists all the categories which have parent_id=parent.id.
Have you read on Single Table Inheritance?

Answer 5

Full Article - https://blog.francium.tech/best-practices-for-handling-hierarchical-data-structure-in-ruby-on-rails-b5830c5ea64d
A Simple table
Table Emp
id: Integer
name: String
parent_id: Integer
Associations
app/models/emp.rb
class Emp < ApplicationRecord
has_many :subs, class_name: 'Emp', foreign_key: :parent_id
belongs_to :superior, class_name: 'Emp', foreign_key: :parent_id
end
Scope Definition
class Emp < ApplicationRecord
----
----
scope :roots, -> { where(parent_id: nil) }
end
Fetching data
def tree_data
output = []
Emp.roots.each do |emp|
output << data(emp)
end
output.to_json
end
def data(employee)
subordinates = []
unless employee.subs.blank?
employee.subs.each do |emp|
subordinates << data(emp)
end
end
{name: employee.name, subordinates: subordinates}
end
Eager Loading
def tree_data
output = []
Emp.roots.includes(subs: {subs: {subs: subs}}}.each do |emp|
output << data(emp)
end
output.to_json
end