union_module.rb

# Create UNION qeuries between ActiveRecord 3.x Relation objects.
# inspired by: https://coderwall.com/p/9hohaa

module Union
  def union *relations
    opts  = relations.extract_options!
    model = ((is_a?(ActiveRecord::Base) || is_a?(ActiveRecord::Relation)) && self) || relations.first.klass

    query =  'SELECT'
    query << ' DISTINCT' if opts[:distinct]
    query << ' * FROM (('
    query << relations.map { |r| r.ast.to_sql }.join(') UNION (')
    query << ')) AS t'

    query << " ORDER BY #{opts[:order]}" if opts[:order]
    query << " LIMIT #{opts[:limit]}"    if opts[:limit]

    model.find_by_sql query
  end
end