如何使用多个外键制作 Rails 模型 belongs_to

Question

recently I have a migration that adds a user_id column to the watch_events tables. and thus I want to change the watch_event models to handle belongs_to but with multiple approach

  create_table 'users', force: :cascade do |t|
    t.integer 'id'
    t.integer 'customer_id'
  end

  create_table 'watch_events', force: :cascade do |t|
    t.integer 'customer_id'
    t.integer 'user_id'
  end

previously

class WatchEvent < ApplicationRecord
  belongs_to :user, foreign_key: :customer_id, primary_key: :customer_id
end

what I want:

• if watch_event.customer_id is present, i want to use belongs_to:user, foreign_key: :customer_id, primary_key: :customer_id
• if watch_event.customer_id is not present, i want to use normal belongs_to:user

how can I achieve this on the watch_event model?

Answer 1

If I understand your question correctly, then what you are looking for is a Polymorphic association .

If you see the code below, what it basically does is create two columns in the watch_events table, watcher_type and watcher_id . And the belongs_to:watcher then uses the watcher_type column to identify which model it should associate to,

create_table 'watch_events', force: :cascade do |t|
  t.references 'watcher', polymorphic: true, null: false
end

class WatchEvent < ApplicationRecord
  belongs_to :watcher, polymorphic: true
end

Answer 2

I do not think that Rails supports 'fallback foreign keys' on associations. However, you can write a simple wrapper for your problem. First, relate your WatchEvent class twice to the user model, using your two keys and two 'internal' association names ( :user_1 and :user_2 ). Then, add a 'virtual association reader' ( user ) and a 'virtual association setter' ( user=(new_user) ):

class WatchEvent < ApplicationRecord
  
  belongs_to :user_1,
             class_name: 'User',
             foreign_key: :customer_id

  belongs_to :user_2,
             class_name: 'User',
             foreign_key: :user_id
  
  def user
    user_1 || user_2
  end

  def user=(new_user)
    self.user_1 = new_user
  end

end

With this solution, the requirements "use customer_id to find user" and "use user_id as fallback foreign key if customer_id is nil or doesn't yield a result" is satisfied. It happens in the association reader method user . When there is a reader method, you'll need a setter method, which is user=() . Feel free to design the setter's internals as required, mine is just a suggestion.

BTW: You may need to re-add the declaration of the foreign primary_key . I omitted that for clarity.