Unions multiple scopes on a model, and returns an instance of ActiveRecord::Relation.

union_scope.rb

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

I guess I should mention that this is a somewhat naive implementation; however, it solves my needs more than 80% of the time.

Naive but effective. Thanks !

Example usage for beginners?

I found this solution in SO: http://stackoverflow.com/a/15413611/406438

"#{table_name}.id" => "#{table_name}.#{primary_key}"

Or like this with Concern

module UnionScope
  extend ActiveSupport::Concern

  class_methods do
    def union_scope(*scopes)
      id_column = "#{table_name}.#{primary_key}"
      sub_query = scopes.map { |s| s.select(id_column).to_sql }.join(' UNION ')
      where "#{id_column} IN (#{sub_query})"
    end
  end
end

@araslanov-e Thanks for the feedback. I've updated the gist to include your edit.

Hi everyone, it's been a while!

I've added another GIST that extends the functionality provided in union_scope.rb, to also provide INTERSECT and EXCEPT operations. Inspired by @sekrett, I've structured this one as a Concern.

https://gist.github.com/tlowrimore/f0e3eca483395de3d373

Cheers!
Tim

id_column = "#{table_name}.#{primary_key}"
sub_query = scopes.map { |s| s.select(id_column).to_sql }.join(" UNION ")
where "#{id_column} IN (#{sub_query})"

1. Is this secure against XSS or SQLi?
2. I'm new with ruby on rails. Can you use
   where('? IN (?)', id_column, sub_query)
   instead of
   where "#{id_column} IN (#{sub_query})

Cheers