ActiveRecord Query Union

Here's a quick little module I wrote that allows you to UNION multiple scopes. It also returns the results as an instance of ActiveRecord::Relation.

module ActiveRecord::UnionScope  def self.included(base)    base.send :extend, ClassMethods  end  module ClassMethods    def union_scope(*scopes)      id_column = "#{table_name}.id"      sub_query = scopes.map { |s| s.select(id_column).to_sql }.join(" UNION ")      where "#{id_column} IN (#{sub_query})"    end  endend

Here's the gist: https://gist.github.com/tlowrimore/5162327

Edit:

As requested, here's an example of how UnionScope works:

class Property < ActiveRecord::Base  include ActiveRecord::UnionScope  # some silly, contrived scopes  scope :active_nearby,     -> { where(active: true).where('distance <= 25') }  scope :inactive_distant,  -> { where(active: false).where('distance >= 200') }  # A union of the aforementioned scopes  scope :active_near_and_inactive_distant, -> { union_scope(active_nearby, inactive_distant) }end

I also have encountered this problem, and now my go-to strategy is to generate SQL (by hand or using to_sql on an existing scope) and then stick it in the from clause. I can't guarantee it's any more efficient than your accepted method, but it's relatively easy on the eyes and gives you a normal ARel object back.

watched_news_posts = Post.joins(:news => :watched).where(:watched => {:user_id => id})watched_topic_posts = Post.joins(:post_topic_relationships => {:topic => :watched}).where(:watched => {:user_id => id})Post.from("(#{watched_news_posts.to_sql} UNION #{watched_topic_posts.to_sql}) AS posts")

You can do this with two different models as well, but you need to make sure they both "look the same" inside the UNION -- you can use select on both queries to make sure they will produce the same columns.

topics = Topic.select('user_id AS author_id, description AS body, created_at')comments = Comment.select('author_id, body, created_at')Comment.from("(#{comments.to_sql} UNION #{topics.to_sql}) AS comments")

Based on Olives' answer, I did come up with another solution to this problem. It feels a little bit like a hack, but it returns an instance of ActiveRelation, which is what I was after in the first place.

Post.where('posts.id IN       (        SELECT post_topic_relationships.post_id FROM post_topic_relationships          INNER JOIN "watched" ON "watched"."watched_item_id" = "post_topic_relationships"."topic_id" AND "watched"."watched_item_type" = "Topic" WHERE "watched"."user_id" = ?      )      OR posts.id IN      (        SELECT "posts"."id" FROM "posts" INNER JOIN "news" ON "news"."id" = "posts"."news_id"         INNER JOIN "watched" ON "watched"."watched_item_id" = "news"."id" AND "watched"."watched_item_type" = "News" WHERE "watched"."user_id" = ?      )', id, id)

I'd still appreciate it if anybody has any suggestions to optimize this or improve the performance, because it's essentially executing three queries and feels a little redundant.