I've written a couple of complex queries (at least to me) with Ruby on Rail's query interface:
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})
Both of these queries work fine by themselves. Both return Post objects. I would like to combine these posts into a single ActiveRelation. Since there could be hundreds of thousands of posts at some point, this needs to be done at the database level. If it were a MySQL query, I could simply user the UNION
operator. Does anybody know if I can do something similar with RoR's query interface?
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
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