postgresql - 在 Rails 中，如何创建将向与该表类型相同的表添加列/外键的迁移？

Question

我正在使用 Rails 4.2.3。如何创建将向与表类型相同的表添加列的迁移？此外，如何添加外键(如果该列为 NULL 就可以)？我试过了

class AddLinkedMyObjectToMyObjects < ActiveRecord::Migration
  def change
    add_reference :my_objects, :linked_my_object, type: :my_object, index: true, foreign_key: true
  end
end

但是我在运行迁移时遇到了这个错误

== 20160616201701 AddLinkedMyObjectToMyObjects: migrating =============================
-- add_reference(:my_objects, :linked_my_object, {:type=>:my_object, :index=>true, :foreign_key=>true})
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::UndefinedObject: ERROR:  type "my_object" does not exist
LINE 1: ALTER TABLE "MyObjects" ADD "linked_my_object_id" my_object
                                                 ^
: ALTER TABLE "MyObjects" ADD "linked_my_object_id" my_object
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:155:in `async_exec'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:155:in `block in execute'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract_adapter.rb:472:in `block in log'

Answer 1

我认为你需要“手动”完成:

add_column :my_objects, :linked_my_object_id, :integer
add_index :my_objects, :linked_my_object_id
add_foreign_key :my_objects, :my_objects, column: :linked_my_object_id

然后在MyObject类中这样说:

belongs_to :linked_my_object, class_name: MyObject
has_many :my_objects, foreign_key: :linked_my_object_id