How to model people and companies with single table inheritance in Rails?

Question

In my Invoicing app invoices can be sent to either a company or a person. As I understand it, this is a good use case for Rails' single table inheritance (STI). Since both types share a number of attributes and functions, I figured that a super-class Recipient might be a good way to go:

class Recipient < ActiveRecord::Base 
end

class Company < Recipient
  has_many :people
end

class Person < Recipient
  belongs_to :company
end

I also understand that I need an attribute type in the Recipient model.

The only thing that bothers me is the fact that a person may (or may not) belong to a company. How can this be modelled in Rails? Normally, I'd simply add another database field company_id to the people table. But there's only one table (recipients) here. So how can this be done?

Thanks for any help.

Answer 1

A structure could look like this:

class Recipient < ActiveRecord::Base 
  has_many :invoices
end

class Company < Recipient
  has_many :people
end

class Person < Recipient
  belongs_to :company
end

class Invoice < ActiveRecord::Base
  belongs_to :recipients
end

# Schema
create_table "invoices", force: :cascade do |t|
  t.integer "recipient_id"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
  t.index ["recipient_id"], name: "index_invoices_on_recipient_id"
end

create_table "recipients", force: :cascade do |t|
  t.integer "company_id"
  t.string "type"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

I've just tried in the console:

> Recipient.all
 => [#<Company:0x007fd55d797220
  id: 1,
  company_id: nil,
  type: "Company",
  created_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00,
  updated_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00>,
 #<Person:0x007fd55d796730
  id: 2,
  company_id: 1,
  type: "Person",
  created_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00,
  updated_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00>,
 #<Person:0x007fd55d796208
  id: 3,
  company_id: nil,
  type: "Person",
  created_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00,
  updated_at: Fri, 04 Aug 2017 10:57:41 UTC +00:00>]

> Person.last.company
  Person Load (0.2ms)  SELECT  "recipients".* FROM "recipients" WHERE "recipients"."type" IN ('Person') ORDER BY "recipients"."id" DESC LIMIT ?  [["LIMIT", 1]]
 => nil

> Person.first.company
  Person Load (0.2ms)  SELECT  "recipients".* FROM "recipients" WHERE "recipients"."type" IN ('Person') ORDER BY "recipients"."id" ASC LIMIT ?  [["LIMIT", 1]]
  Company Load (0.2ms)  SELECT  "recipients".* FROM "recipients" WHERE "recipients"."type" IN ('Company') AND "recipients"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
 => #<Company id: 1, company_id: nil, type: "Company", created_at: "2017-08-04 10:57:41", updated_at: "2017-08-04 10:57:41">