How do I add migration with multiple references to the same model in one table? Ruby/Rails

问题

How do I create a migration with two fields that reference the same table? I have tables A, and image. A.image1_id will reference image, and A.image2_id will reference image also. There are only 2 images, not many. If I use

class AddFields < ActiveRecord::Migration
   def change
    change_table(:ticket) do |t|
        t.references :image1_id
        t.references :image2_id
    end
  end
end

I don't think that will work because it will add another _id to the end and probably won't know to use the 'image' model. I also thought about

change_table(:ticket) do |t|
    t.references :image

But then how do I add two of those? I also thought about adding

create_table :images do |t|
  t.belongs_to :ticket
  t.string :file

But I only want 2, not many, and this doesn't appear to allow getting to the image from the ticket, like ticket.image1 or ticket.image2.

According to this documentation http://apidock.com/rails/v3.2.8/ActiveRecord/ConnectionAdapters/SchemaStatements/change_table which is all I could find, t.references doesn't appear to take any arguments either.

change_table(:suppliers) do |t|
  t.references :company
end

回答1:

You can do this simply with the add_column method in your migrations and set up the proper associations in your classes:

class AddFields < ActiveRecord::Migration
  def change
    add_column :tickets, :image_1_id, :integer
    add_column :tickets, :image_2_id, :integer
  end
end

class Ticket < ActiveRecord::Base
  belongs_to :image_1, :class_name => "Image"
  belongs_to :image_2, :class_name => "Image"
end

class Image < ActiveRecord::Base
  has_many :primary_tickets, :class_name => "Ticket", :foreign_key => "image_1_id"
  has_many :secondary_tickets, :class_name => "Ticket", :foreign_key => "image_2_id"
end

This blog post, Creating Multiple Associations with the Same Table, goes into more detail.

回答2:

In Rails 5.1 or greater you can do it like this:

Migration

class AddFields < ActiveRecord::Migration
   def change
    change_table(:tickets) do |t|
        t.references :image1, foreign_key: { to_table: 'images' }
        t.references :image2, foreign_key: { to_table: 'images' }
    end
  end
end

This will create the fields image1_id, and image2_id and make the database level references to the images table

Models

as in rossta's asnwer

class Ticket < ActiveRecord::Base
  belongs_to :image_1, class_name: "Image"
  belongs_to :image_2, class_name: "Image"
end

class Image < ActiveRecord::Base
  has_many :primary_tickets, class_name: "Ticket", foreign_key: "image_1_id"
  has_many :secondary_tickets, class_name: "Ticket", foreign_key: "image_2_id"
end

FactoryBot

If you uses FactoryBot then your factory might look something like this:

FactoryBot.define do
  factory :ticket do
    association :image1, factory: :image
    association :image2, factory: :image
  end
end

来源：https://stackoverflow.com/questions/14867981/how-do-i-add-migration-with-multiple-references-to-the-same-model-in-one-table