Hack to allow unions on ActiveRecord models

README.txt

Add union_hack.rb to your project, for example at lib folder and ensure you're loading it on your application.rb file:

    # You'll need to add something like that
    config.autoload_paths += %W(#{config.root}/lib)

Then extend your favourite model with the module and you'll be able to do unions with unique records, sorted by any fields and limited in number of records (see my_timeline_method):

    class Profile < ActiveRecord::Base
      extend UnionHack

      has_many :events
      has_many :friends
      has_many :friend_events, :through => :friends, :source => :events

      def my_timeline
        Profile.union([events, friend_events], :distinct => true, :order => 'created_at DESC', :limit => 20)
      end
    end

union_hack.rb

module UnionHack
  def union(relations, opts={})
    query = 'SELECT '+ (opts[:distinct] ? 'DISTINCT ' : '' ) +'* FROM ((' + relations.map { |r| r.ast.to_sql }.join(') UNION (') + ')) AS t'
    query << " ORDER BY #{opts[:order]}" if opts[:order]
    query << " LIMIT #{opts[:limit]}" if opts[:limit]
    find_by_sql(query)
  end
end

what does 'ast' means on

relations.map { |r| r.ast.to_sql }

ast is "Abstract Syntax Tree"