rails generate migration AddReferenceToExistingModel existing_model_id:references

class AddReferenceToExistingModel < ActiveRecord::Migration[6.1]
  def change
    add_reference :existing_models, :related_model, foreign_key: true
    # 或者明确指定列名
    # add_column :existing_models, :related_model_id, :integer
    # add_foreign_key :existing_models, :related_models
  end
end

rails db:migrate

# 在ExistingModel模型中
class ExistingModel < ApplicationRecord
  belongs_to :related_model
end

# 在RelatedModel模型中
class RelatedModel < ApplicationRecord
  has_one :existing_model
  # 或者如果是has_many关系
  # has_many :existing_models
end

# 在迁移中使用null: true允许空值
add_reference :existing_models, :related_model, foreign_key: true, null: true

class ExistingModel < ApplicationRecord
  belongs_to :owner, class_name: "User", foreign_key: "user_id"
end

class User < ApplicationRecord
  has_many :owned_models, class_name: "ExistingModel", foreign_key: "user_id"
end

# 迁移
add_reference :existing_models, :commentable, polymorphic: true

# 模型
class ExistingModel < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

class Article < ApplicationRecord
  has_many :existing_models, as: :commentable
end

class Photo < ApplicationRecord
  has_many :existing_models, as: :commentable
end

class Comment < ApplicationRecord
  belongs_to :article, touch: true
end