How to associate has_one relationships when using a generic id field?

Question

I have an Activity model that will show a list of events that occurred (post created, event created, etc.).

In activity.rb, I have:

class Activity < ApplicationRecord
  has_one :post
  has_one :event
end

For my Activity migration file, I have it set up like so:

class CreateActivities < ActiveRecord::Migration[7.1]
  def change
    create_table :activities do |t|
      t.string :action, null: false
      t.string :action_model, null: false
      t.string :action_model_id, null: false

      t.timestamps
    end
  end
end

I'm using a generic action_model_id for reusability between different models (instead of needing to store a field for every association, for example: post_id, event_id, etc).

How can I call activity.post or activity.event while keeping this generic?

Answer 1

Models changes

class Activity < ApplicationRecord
  belongs_to :action_model, polymorphic :true
end


class Post < ApplicationRecord
  has_many :activities, as: :action_model
end


class Event < ApplicationRecord
  has_many :activities, as: :action_model
end

And you need to change migration

class CreateActivities < ActiveRecord::Migration[7.1]
  def change
    create_table :activities do |t|
      t.bigint :action_model_id, null: false
      t.string :action_model_type, null: false

      t.timestamps
    end
    add_index :activities, [:action_model_id, :action_model_type]
  end
end