Creating a model that has a tree structure

I have categories that are in a tree structure. I am trying to link them together by defining a paren开发者_JS百科t 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.

---

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.

---

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

---

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.

---

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?

---

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