module ActiveRecord::UnionScope

gistfile1.rb

#https://gist.github.com/tlowrimore/5162327
#http://stackoverflow.com/a/15413611/270511

module ActiveRecord::UnionScope
  def self.included(base)
    base.send :extend, ClassMethods
  end
  
  module ClassMethods
    def union_scope(*scopes)
      id_column = "#{table_name}.id"
      
      if (sub_query = scopes.reject { |sc| sc.count == 0 }.map { |s| s.select(id_column).to_sql }.join(" UNION ")).present?
        where "#{id_column} IN (#{sub_query})"
      else
        none
      end
    end
  end
end

This fixes a problem I encountered when I integrated https://gist.github.com/tlowrimore/5162327 into my RoR app. The problem happens when one of the scopes returns no values. Apparently, to_sql() is able to evaluate when a query will return nothing and it returns an empty string. So the final query begins " UNION SELECT ... " which is invalid SQL of course.

There are other ways to fix this, like checking for /^ UNION / and prepending "select 1 where 1=0". The main thing is that anyone using https://gist.github.com/tlowrimore/5162327 this has to be aware of the problem.

Just fixed this by counting the scopes that actually return something. The previous way failed sometimes.

Even better!

If my scopes accept arguments, will it work ??

this is correct but needs just small change:

This code:

 map { |s| s.select(id_column).to_sql }

should be replaced with this:

.map { |s|   "(#{s.select(id_column).to_sql})" }