Multiple Foreign Keys for the Same Relationship in Rails 6

As we all know, it is quite easy to create database tables with Ruby On Rails. However, some cases require personalizations to reproduce real world situations and the relationships between them. Here, we have two models: User and Meeting, with a 1..N relation where one Meeting has two User (yes, a two-person meeting.) The problem doesn’t rely on this relationship but in the generated migration, because we cannot have two t.references :user_id. Also, we want to identify which user is available for the meeting (the attendee) and which user requested the meeting (the requestee).

How can I generate a model in Rails?

Before we start with the solution, let’s understand the command rails generate or its shorthand, rails g.

This command is one of the many options that follows the rails command. If you open your terminal and run rails --help, you’ll see a list of options:

$ rails --help
The most common rails commands are:
 generate     Generate new code (short-cut alias: "g")
 console      Start the Rails console (short-cut alias: "c")
 server       Start the Rails server (short-cut alias: "s")
 test         Run tests except system tests (short-cut alias: "t")
 test:system  Run system tests
 dbconsole    Start a console for the database specified in config/database.yml (short-cut alias: "db")
 new          Create a new Rails application. "rails new my_app" creates a new application called MyApp in "./my_app"

And if we run rails generate --help, another list of options will be displayed:

$ rails g --help                                
Spring preloader in process 70748
Usage: rails generate GENERATOR [args] [options]
...
Please choose a generator below.
Rails:
  application_record
  assets
  channel
  controller
  generator
  helper
  integration_test
  jbuilder
  job
  mailbox
  mailer
  migration
  model
  resource
  responders_controller
  scaffold
  scaffold_controller
  system_test
  task
...

In this article, we will use the rails generate model and rails db:migrate commands to create our models and save them to db/schema.rb, the file that represents our database.

The solution

First, we will create the User and Meeting models.

$ rails generate model User name:string email:string 
$ rails generate model Meeting starts_at:datetime ends_at:datetime attendee:references requestee:references

This will generate the migrations under db/migrate and the models under app/models. The migration for Meeting should look like this:

class CreateMeetings < ActiveRecord::Migration[6.0]
  def change
    create_table :meetings do |t|
      t.datetime :starts_at
      t.datetime :ends_at
      t.references :requestee, null: false, foreign_key: true
      t.references :attendee, null: false, foreign_key: true

      t.timestamps
    end
  end
end

But if you try to run rails db:migrate to create the tables in the database, an error will be raised on your terminal since there aren’t any tables called attendee and requestee to make reference as Foreign Key. With that in mind, our next step is to remove the foreign_key: true option and create the foreign key reference with the correct columns by adding two lines to the body of the change method:

class CreateMeetings < ActiveRecord::Migration[6.0]
  def change
    create_table :meetings do |t|
      t.datetime :starts_at, null: false
      t.datetime :ends_at, null: false
      t.references :requestee, null: false # Remove foreign_key: true
      t.references :attendee, null: false # Remove foreign_key: true

      t.timestamps
    end

    # Add these two lines
    add_foreign_key :meetings, :users, column: :requestee_id
    add_foreign_key :meetings, :users, column: :attendee_id
  end
end

Now we can safely run rails db:migrate and see the changes on db/schema.rb

ActiveRecord::Schema.define(version: 2020_06_13_202030) do

  create_table "meetings", force: :cascade do |t|
    t.datetime "starts_at"
    t.datetime "ends_at"
    t.integer "requestee_id", null: false
    t.integer "attendee_id", null: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.index ["requestee_id"], name: "index_meetings_on_requestee_id"
    t.index ["attendee_id"], name: "index_meetings_on_attendee_id"
  end

  create_table "users", force: :cascade do |t|
    t.string "name"
    t.string "email"
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
  end

  add_foreign_key "meetings", "users", column: "requestee_id"
  add_foreign_key "meetings", "users", column: "attendee_id"
end

In order to tell Rails what table will be referenced by the Meeting model, we should make some changes to app/models/meetings.rb and app/models/users.rb. They should look like this:

class Meeting < ApplicationRecord
  belongs_to :requestee, class_name: 'User'
  belongs_to :attendee, class_name: 'User'
end

class User < ApplicationRecord
  has_many :requestee_meetings, class_name: 'Meeting', foreign_key: 'requestee_id'
  has_many :attendee_meetings, class_name: 'Meeting', foreign_key: 'attendee_id'
end

Now the relationship is done and we can create meetings and users in this way:

irb(main):001:0> user1 = User.create!(name: 'John Snow', email: 'night_watcher@thegreatwall.com')
=> #<User id: 1, name: "John Snow", email: "night_watcher@thegreatwall.com", created_at: "2020-06-12 20:34:41", updated_at: "2020-06-12 20:34:41">
irb(main):002:0> user2 = User.create!(name: 'Daenerys Targaryen', email: 'stormborn@dragonstone.com')
=> #<User id: 2, name: "Daenerys Targaryen", email: "stormborn@dragonstone.com", created_at: "2020-06-12 20:36:10", updated_at: "2020-06-12 20:36:10">
irb(main):003:0> starting_time = Time.current
irb(main):004:0> ending_time = Time.current + 2.hour
irb(main):005:0> meeting_about_family = Meeting.create!(starts_at: starting_time, ends_at: ending_time, attendee_id: user2.id, requestee_id: user1.id)
=> #<Meeting id: 1, starts_at: "2020-06-12 20:40:45", ends_at: "2020-06-12 22:41:04", attendee_id: 2, requestee_id: 1, created_at: "2020-06-12 20:41:41", updated_at: "2020-06-12 20:41:41">
irb(main):006:0> meeting_about_family.attendee
=> #<User id: 2, name: "Daenerys Targaryen", email: "stormborn@dragonstone.com", created_at: "2020-06-12 20:36:10", updated_at: "2020-06-12 20:36:10">
irb(main):007:0> meeting_about_family.requestee
=> #<User id: 1, name: "John Snow", email: "night_watcher@thegreatwall.com", created_at: "2020-06-12 20:34:41", updated_at: "2020-06-12 20:34:41">

But there’s a better way!

Instead of writing two new lines in our migration, we could simply replace foreign_key: true with foreign_key: { to_table: :users }. The final solution should look like this:

class CreateMeetings < ActiveRecord::Migration[6.0]
  def change
    create_table :meetings do |t|
      t.datetime :starts_at, null: false
      t.datetime :ends_at, null: false
      t.references :requestee, null: false,  foreign_key: { to_table: :users }
      t.references :attendee, null: false,  foreign_key: { to_table: :users }

      t.timestamps
    end
  end
end

Easier, isn’t it?

Conclusion

I hope this post was useful to you, and remember: always look up the docs!

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