How to combine two scopes with OR

Question

I have a tag feed and a friend feed. I want to combine these two and build the ultimate "all" feed.

For friend feed:

class Post < ActiveRecord::Base
  scope :friendfeed, lambda{|x| followed_by}

  def self.followed_by(user)
    where("user_id IN (?) OR user_id = ?", user.watched_ids, user.id)
  end
end

For tag feed:

class Post < ActiveRecord::Base
  scope :tagfeed, lambda{|x| infatuated_with}

  def self.infatuated_with(user)
    joins(:attachments).where("attachments.tag_id IN (?)", user.tags).select("DISTINCT pages.*")
  end
end

And I would call something like this from the controller (I'm using Kaminari gem for pagination):

@tag_feed = Post.tagfeed(current_user).page(params[:page]).per(21)
@friend_feed = Post.friendfeed(current_user).page(params[:page]).per(21)

Now I want to have a universal feed, but I'm lost. Scopes are meant for narrowing down, but in this case I'm trying to do an OR operation. Doing stuff like

@mother_of_all_feed = @tag_feed + @friend_feed

would be redundant, and I wouldn't be able to control the number of posts appearing on a single page. How can I go about doing this? Thanks!

By the way, for tags I have association set up like this:

class Post < ActiveRecord::Base
  has_many :attachments
  has_many :tags, :through => :attachments
end

class Tag < ActiveRecord::Base
  has_many :attachments
  has_many :posts, :through => :attachments
end

class Attachment < ActiveRecord::Base
  belongs_to :tag
  belongs_to :post
end

Answer 1

There's a rails pull request for this feature (https://github.com/rails/rails/pull/9052), but in the meantime, some one has created a monkey patch that you can include in your initializers that will allow you to or scopes and where clauses in one query and still give you an ActiveRecord::Relation:

https://gist.github.com/j-mcnally/250eaaceef234dd8971b

With that, you'd be able to OR your scopes like this

Post.tagfeed(current_user).or.friendfeed(current_user)

or write a new scope

scope :mother_of_all_feed, lambda{|user| tagfeed(user).or.friendfeed(user)}