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
end
end
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")