Rails - Parent/child relationships

I'm currently using a standard one-to-one relationship to handle parent/child relationships:

class Category < ActiveRecord::Base
  has_one :category
  belong开发者_StackOverflows_to :category
end

Is there a recommended way to do it or is this ok?

You will need to tweak the names you are using to get this working - you specify the name of the relationship, and then tell AR what the class is:

class Category < ActiveRecord::Base
  has_one :child, :class_name => "Category"
  belongs_to :parent, :class_name => "Category" 
end

I found that I had to make a minor change to @equivalent8's solution to make it work for Rails 5 (5.1.4):

class Category < ActiveRecord::Base
  has_many :children, :class_name => "Category", foreign_key: 'parent_id'
  belongs_to :parent, :class_name => "Category", foreign_key: 'parent_id', :optional => true
end

Without the foreign_key declaration, Rails tries to find the children by organization_id instead of parent_id and chokes.

Rails also chokes without the :optional => true declaration on the belongs_to association since belongs_to requires an instance to be assigned by default in Rails 5. In this case, you would have to assign an infinite number of parents.

has_many version:

class Category < ActiveRecord::Base
  has_many :children, :class_name => "Category"
  belongs_to :parent, :class_name => "Category" 
end

#migratio
class CreateCategories < ActiveRecord::Migration 
 def change
    create_table :categories do |t|
      t.integer :parent_id
      t.string  :title

      t.timestamps null: false
    end
 end
end

# RSpec test 
require 'rails_helper'
RSpec.describe Category do
  describe '#parent & #children' do
    it 'should be able to do parent tree' do
      c1 = Category.new.save!
      c2 = Category.new(parent: c1).save!

      expect(c1.children).to include(c2)
      expect(c2.parent).to eq c1
    end
  end
end

I think , should be foreign_key

class Product < ActiveRecord::Base
 has_many :children, :foreign_key => "parent_id" , :class_name => "Product"
 belongs_to :parent, :class_name => "Product" 
end

Console :

2.4.3 :004 > product  = Product.find_by_name "BOB"

 => #<Product id: 1, name: "BOB", parent_id: nil>
2.4.3 :005 > product.children
D, [2019-11-07T14:56:14.273606 #66632] DEBUG -- :   

Product Load (0.7ms) SELECT "products".* FROM "products" WHERE "products"."parent_id" = $1  [["parent_id", 1]]

 => #<ActiveRecord::Associations::CollectionProxy 
[#<Product id: 7, parent_id: 1>, 
#<Product id: 8,  parent_id: 1>, 
#<Product id: 9, parent_id: 1>]>

If you already have a model Category and a table categories (in schema.rb) you could do this to the model:

class Category < ApplicationRecord
    has_many :children, class_name: 'Category', foreign_key: :parent_id
    belongs_to :parent, class_name: 'Category', optional: true
end

and this as a migration:

class AddParentIdToCategories < ActiveRecord::Migration[6.0]
  def change
    add_column :categories, :parent_id, :integer
  end
end

NOTE: In newer versions of Ruby you don't need the rocket hashes =>

Since the relation is symmetric, I actually find that different than what Toby wrote, that I prefer the following:

class Category < ActiveRecord::Base 
  has_one :parent, :class_name => "Category" 
  belongs_to :children, :class_name => "Category"
end

For some reason "has one parent, many children" is the way my mind things, not "has many parents, only one child"