Rails User model has_many activities (observer) but should also be observed

I'm running into an issue with an existing ActiveRecord::Observer model that records various Activities of a User the site. Everything was working really well, until I tried to observe the User class with the same Activity model that it uses to observe other mo开发者_运维百科dels. Consider that:

class Activity < ActiveRecord::Base
    belongs_to :user
    belongs_to :item, :polymorphic => true
end

class ActivityObserver < ActiveRecord::Observer
    observe :billing, :call, :vanity_number
end

class User < ActiveRecord::Base
    has_many :comments
    has_many :activities
end

class Comment < ActiveRecord::Base
    belongs_to :user
    has_many :activities, :as => :item
end

The above worked fine. A query for User.activities would return rows of Comment Activities. As soon as I added :user to the observe method in ActivityObserver and changed has_many :activities in the User model to has_many :activities, :as => :item, User.activities would only return rows that were activities pertaining to that User instance and not any Comments

Why is this? What can I do to get this working as expected?

When you changed

class User < ActiveRecord::Base
  has_many :activities
end

to

class User < ActiveRecord::Base
  has_many :activities, :as => :item
end

it is now looking at the item_id field in the activities table where the type field is User to pull back activities where item_id = users.id. Before it was looking at the user_id field in activities. Since the item_id field and type field cannot have two values, you made it so that an activity points at either a user or a comment. It can't be both.